@article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/107459, title ="Calibration and Uncertainty Quantification of Convective Parameters in an Idealized GCM", author = "Dunbar, Oliver R. A. and Garbuno-Inigo, Alfredo", month = "January", year = "2021", url = "https://resolver.caltech.edu/CaltechAUTHORS:20210113-143919927", note = "License: Attribution 4.0 International. \n\nPublished Online: Mon, 4 Jan 2021. \n\nThis work was supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by the Hopewell Fund, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, award AGS1835860). A.M.S. was also supported by the Office of Naval Research (award N00014-17-1-2079). We thank Emmet Cleary for his preliminary work underlying some of the results shown here. \n\nData Availability: All computer code used in this paper is open source. The code for the idealized GCM, the Julia code for the CES algorithm, the plot tools, and the slurm/bash\nscripts to run both GCM and CES are available at https://doi.org/10.5281/zenodo.4393029.", revision_no = "12", abstract = "Parameters in climate models are usually calibrated manually, exploiting only small subsets of the available data. This precludes an optimal calibration and quantification of uncertainties. Traditional Bayesian calibration methods that allow uncertainty quantification are too expensive for climate models; they are also not robust in the presence of internal climate variability. For example, Markov chain Monte Carlo (MCMC) methods typically require O(10⁵) model runs, rendering them infeasible for climate models. Here we demonstrate an approach to model calibration and uncertainty quantification that requires only O(10²) model runs and can accommodate internal climate variability. The approach consists of three stages: (i) a calibration stage uses variants of ensemble Kalman inversion to calibrate a model by minimizing mismatches between model and data statistics; (ii) an emulation stage emulates the parameter-to-data map with Gaussian processes (GP), using the model runs in the calibration stage for training; (iii) a sampling stage approximates the Bayesian posterior distributions by using the GP emulator and then samples using MCMC. We demonstrate the feasibility and computational efficiency of this calibrate-emulate-sample (CES) approach in a perfect-model setting. Using an idealized general circulation model, we estimate parameters in a simple convection scheme from data surrogates generated with the model. The CES approach generates probability distributions of the parameters that are good approximations of the Bayesian posteriors, at a fraction of the computational cost usually required to obtain them. Sampling from this approximate posterior allows the generation of climate predictions with quantified parametric uncertainties.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/102274, title ="Calibrate, emulate, sample", author = "Cleary, Emmet and Garbuno-Inigo, Alfredo", journal = "Journal of Computational Physics", volume = "424", pages = "Art. No. 109716", month = "January", year = "2021", doi = "10.1016/j.jcp.2020.109716", issn = "0021-9991", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200402-140348174", note = "© 2020 Published by Elsevier Inc. \n\nReceived 11 January 2020, Revised 1 July 2020, Accepted 8 July 2020, Available online 13 July 2020. \n\nAll authors are supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, The Paul G. Allen Family Foundation, and The National Science Foundation (NSF, award AGS-1835860). A.M.S. is also supported by NSF (award DMS-1818977) and by the Office of Naval Research (award N00014-17-1-2079). \n\nCRediT authorship contribution statement: Emmet Cleary: Methodology. Alfredo Garbuno-Inigo: Methodology, Software, Visualization, Writing - original draft, Writing - review & editing. Shiwei Lan: Methodology. Tapio Schneider: Conceptualization, Funding acquisition, Writing - original draft. Andrew M. Stuart: Conceptualization, Funding acquisition, Writing - original draft, Writing - review & editing. \n\nThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", revision_no = "20", abstract = "Many parameter estimation problems arising in applications can be cast in the framework of Bayesian inversion. This allows not only for an estimate of the parameters, but also for the quantification of uncertainties in the estimates. Often in such problems the parameter-to-data map is very expensive to evaluate, and computing derivatives of the map, or derivative-adjoints, may not be feasible. Additionally, in many applications only noisy evaluations of the map may be available. We propose an approach to Bayesian inversion in such settings that builds on the derivative-free optimization capabilities of ensemble Kalman inversion methods. The overarching approach is to first use ensemble Kalman sampling (EKS) to calibrate the unknown parameters to fit the data; second, to use the output of the EKS to emulate the parameter-to-data map; third, to sample from an approximate Bayesian posterior distribution in which the parameter-to-data map is replaced by its emulator. This results in a principled approach to approximate Bayesian inference that requires only a small number of evaluations of the (possibly noisy approximation of the) parameter-to-data map. It does not require derivatives of this map, but instead leverages the documented power of ensemble Kalman methods. Furthermore, the EKS has the desirable property that it evolves the parameter ensemble towards the regions in which the bulk of the parameter posterior mass is located, thereby locating them well for the emulation phase of the methodology. In essence, the EKS methodology provides a cheap solution to the design problem of where to place points in parameter space to efficiently train an emulator of the parameter-to-data map for the purposes of Bayesian inversion.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106905, title ="An Improved Perturbation Pressure Closure for Eddy-Diffusivity Mass-Flux Schemes", author = "He, Jia and Cohen, Yair", month = "December", year = "2020", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201204-110354763", note = "© 2020. California Institute of Technology. Government sponsorship acknowledged. \n\nThis research was made possible by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, award AGS-1835860). We would like to thank the Resnick Sustainability Institute at Caltech for fellowship support. Parts of the research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and funded through the internal Research and Technology Development program. The PyCLES code used to generate LES results is available at climate-dynamics.org/software/#pycles. The SCM code is available at https://doi.org/10.5281/zenodo.4291143.", revision_no = "10", abstract = "Convection parameterizations such as eddy-diffusivity mass-flux (EDMF) schemes require a consistent closure formulation for the perturbation pressure, which arises in the equations for vertical momentum and turbulence kinetic energy (TKE). Here we derive an expression for the perturbation pressure from approximate analytical solutions for 2D and 3D rising thermal bubbles. The new closure combines a modified pressure drag and virtual mass effects with a new momentum advection term. This momentum advection is an important source in the lower half of the thermal bubble and at cloud base levels in convective systems. It represents the essential physics of the perturbation pressure, that is, to ensure the 3D non-divergent properties of the flow. Moreover, the new formulation modifies the pressure drag to be inversely proportional to updraft depth. This is found to significantly improve simulations of the diurnal cycle of deep convection, without compromising simulations of shallow convection. It is thus a key step toward a unified scheme for a range of convective motions. By assuming that the pressure only redistributes TKE between plumes and the environment, rather than vertically, a closure for the velocity pressure-gradient correlation is obtained from the perturbation pressure closure. This novel pressure closure is implemented in an extended EDMF scheme and is shown to successfully simulate a rising bubble test case as well as shallow and deep convection cases in a single column model.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106693, title ="Solar geoengineering may not prevent strong warming from direct effects of CO₂ on stratocumulus cloud cover", author = "Schneider, Tapio and Kaul, Colleen M.", journal = "Proceedings of the National Academy of Sciences of the United States of America", volume = "117", number = "48", pages = "30179-30185", month = "December", year = "2020", doi = "10.1073/pnas.2003730117", issn = "0027-8424", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201117-081328944", note = "© 2020 National Academy of Sciences. Published under the PNAS license. \n\nEdited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 7, 2020 (received for review February 27, 2020). PNAS first published November 16, 2020. \n\nWe thank Clare Singer for assistance with data processing. This research was made possible by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, Charles Trimble, and NSF Grant AGS-1835860. The computations were performed on the California Institute of Technology’s (Caltech’s) High Performance Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, Caltech, under a contract with NASA. C.M.K. and K.G.P. were at Caltech while carrying out this research. \n\nData Availability: All data needed to evaluate the conclusions in the paper are present in the paper. The source code for the simulations is available at climate-dynamics.org/software/#pycles. \n\nAuthor contributions: T.S. and C.M.K. designed research; T.S., C.M.K., and K.G.P. performed research; T.S., C.M.K., and K.G.P. analyzed data; and T.S. wrote the paper. \n\nThe authors declare no competing interest. \n\nThis article is a PNAS Direct Submission.", revision_no = "15", abstract = "Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO₂ can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106101, title ="A Generalized Mixing Length Closure for Eddy-Diffusivity Mass-Flux Schemes of Turbulence and Convection", author = "Lopez‐Gomez, Ignacio and Cohen, Yair", journal = "Journal of Advances in Modeling Earth Systems", volume = "12", number = "11", pages = "Art. No. e2020MS002161", month = "November", year = "2020", doi = "10.1029/2020ms002161", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201015-152734310", note = "© 2020 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nIssue Online: 29 October 2020; Version of Record online: 29 October 2020; Accepted manuscript online: 14 October 2020; Manuscript accepted: 07 October 2020; Manuscript revised: 02 September 2020; Manuscript received: 01 May 2020. \n\nThis research was made possible by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, Award AGS1835860)). I. L. would like to thank the Resnick Sustainability Institute at Caltech for fellowship support. Parts of the research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and funded through the Internal Research and Technology Development Program. We thank Gregory L. Wagner and two anonymous reviewers for helpful comments on an earlier version of this paper. © 2020. California Institute of Technology. Government sponsorship acknowledged. \n\nData Availability Statement: The PyCLES code used to generate LES results is available online (climate‐dynamics.org/software/#pycles). The SCM code is available at https://doi.org/10.5281/zenodo.3789011 website. All LES and SCM data are publicly available online (https://doi.org/10.5281/zenodo.3996252).", revision_no = "22", abstract = "Because of their limited spatial resolution, numerical weather prediction and climate models have to rely on parameterizations to represent atmospheric turbulence and convection. Historically, largely independent approaches have been used to represent boundary layer turbulence and convection, neglecting important interactions at the subgrid scale. Here we build on an eddy‐diffusivity mass‐flux (EDMF) scheme that represents all subgrid‐scale mixing in a unified manner, partitioning subgrid‐scale fluctuations into contributions from local diffusive mixing and coherent advective structures and allowing them to interact within a single framework. The EDMF scheme requires closures for the interaction between the turbulent environment and the plumes and for local mixing. A second‐order equation for turbulence kinetic energy (TKE) provides one ingredient for the diffusive local mixing closure, leaving a mixing length to be parameterized. Here, we propose a new mixing length formulation, based on constraints derived from the TKE balance. It expresses local mixing in terms of the same physical processes in all regimes of boundary layer flow. The formulation is tested at a range of resolutions and across a wide range of boundary layer regimes, including a stably stratified boundary layer, a stratocumulus‐topped marine boundary layer, and dry convection. Comparison with large eddy simulations (LES) shows that the EDMF scheme with this diffusive mixing parameterization accurately captures the structure of the boundary layer and clouds in all cases considered.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106265, title ="Top-of-atmosphere albedo bias from neglecting three-dimensional radiative transfer through clouds", author = "Singer, Clare E. and Lopez-Gomez, Ignacio", month = "October", year = "2020", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201023-133020582", note = "Published Online: Fri, 16 Oct 2020. \n\nC.E.S. acknowledges support from NSF Graduate Research Fellowship under Grant No. DGE-1745301. I.L. is supported by a fellowship from the Resnick Sustainability Institute at Caltech. This research was additionally supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program and by Mountain Philanthropies. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. \n\nAll code or data used in this paper are freely available online. The LES were run using the PyCLES code (https://climate-dynamics.org/software/#pycles). The radiative transfer computations were done using the libRadtran code (http://www.libradtran.org). Post-processed LES 3D fields used as input files for libRadtran computations are available in Singer et al. (2020). The ISCCP data were downloaded from the GEWEX database (https://climserv.ipsl.polytechnique.fr/gewexca/).", revision_no = "12", abstract = "Clouds cover on average nearly 70% of Earth’s surface and are important for the global albedo. The magnitude of the shortwave reflection by clouds depends on their location, optical properties, and 3D structure. Earth system models are unable to perform 3D radiative transfer calculations and thus partially neglect the effect of cloud morphology on albedo. We show how the resulting radiative flux bias depends on cloud morphology and solar zenith angle. Using large-eddy simulations to produce 3D cloud fields, a Monte Carlo code for 3D radiative transfer, and observations of cloud climatology, we estimate the effect of this flux bias on global climate. The flux bias is largest at small zenith angles and for deeper clouds, while the albedo bias is largest (and negative) for large zenith angles. Globally, the radiative flux bias is estimated to be 1.6 W m⁻² and locally can be on the order of 5 W m⁻².", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/104645, title ="Pattern Recognition Methods to Separate Forced Responses from Internal Variability in Climate Model Ensembles and Observations", author = "Wills, Robert C. J. and Battisti, David S.", journal = "Journal of Climate", volume = "33", number = "20", pages = "8693-8719", month = "October", year = "2020", doi = "10.1175/JCLI-D-19-0855.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200729-145256584", note = "© 2020 American Meteorological Society. \n\nManuscript received 25 November 2019, in final form 16 July 2020. \n\nR.C.J.W. and D.S.B. acknowledge support from the National Science Foundation (Grant AGS-1929775) and the Tamaki Foundation. R.C.J.W. and K.C.A. acknowledge support from the National Science Foundation (Grant AGS-1752796). R.C.J.W. is also supported by the University of Washington eScience Institute. T.S. is supported by Eric and Wendy Schmidt by recommendation of the Schmidt Futures program and by the Earthrise Alliance. The CESM project is supported primarily by the National Science Foundation (NSF). This material is based on work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the NSF under Cooperative Agreement no. 1852977. We thank Dennis Hartmann, Cristian Proistosescu, Flavio Lehner, Elizabeth Maroon, Mingfang Ting, and David Bonan for valuable input on this work. The code for S/NP filtering is available at github.com/rcjwills/forced806 patterns. The code for LFCA is available at github.com/rcjwills/lfca.\n", revision_no = "27", abstract = "Ensembles of climate model simulations are commonly used to separate externally forced climate change from internal variability. However, much of the information gained from running large ensembles is lost in traditional methods of data reduction such as linear trend analysis or large-scale spatial averaging. This paper demonstrates how a pattern recognition method (signal-to-noise-maximizing pattern filtering) extracts patterns of externally forced climate change from large ensembles and identifies the forced climate response with up to ten times fewer ensemble members than simple ensemble averaging. It is particularly effective at filtering out spatially coherent modes of internal variability (e.g., El Niño, North Atlantic Oscillation), which would otherwise alias into estimates of regional responses to forcing. This method is used to identify forced climate responses within the 40-member Community Earth System Model (CESM) large ensemble, including an El-Niño-like response to volcanic eruptions and forced trends in the North Atlantic Oscillation. The ensemble-based estimate of the forced response is used to test statistical methods for isolating the forced response from a single realization (i.e., individual ensemble members). Low-frequency pattern filtering is found to skillfully identify the forced response within individual ensemble members and is applied to the HadCRUT4 reconstruction of observed temperatures, whereby it identifies slow components of observed temperature changes that are consistent with the expected effects of anthropogenic greenhouse gas and aerosol forcing.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/103819, title ="Sensitivity of idealized mixed-phase stratocumulus to climate perturbations", author = "Zhang, Xiyue and Schneider, Tapio", journal = "Quarterly Journal of the Royal Meteorological Society", volume = "146", number = "732", pages = "3285-3305", month = "October", year = "2020", doi = "10.1002/qj.3846", issn = "0035-9009", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200610-102248992", note = "© 2020 Royal Meteorological Society. \n\nIssue Online: 09 November 2020; Version of Record online: 20 July 2020; Accepted manuscript online: 09 June 2020; Manuscript accepted: 04 June 2020; Manuscript revised: 15 May 2020; Manuscript received: 06 August 2019.\n\nFunding: National Science Foundation. Grant Number: CCF‐1048575.", revision_no = "13", abstract = "Large eddy simulations (LES) that explicitly resolve boundary layer (BL) turbulence and clouds are used to explore the sensitivity of idealized Arctic BL clouds to climate perturbations. The LES focus on conditions resembling springtime, when surface heat fluxes over sea ice are weak, and the cloud radiative effect is dominated by the longwave effect. In the LES, the condensed water path increases with BL temperature and free‐tropospheric relative humidity, but it decreases with inversion strength. The dependencies of cloud properties on environmental variables exhibited by the LES can largely be reproduced by a mixed‐layer model. Mixed‐layer model analysis shows that the liquid water path increases with warming because the liquid water gradient increase under warming overcompensates for geometric cloud thinning. This response contrasts with the response of subtropical stratocumulus to warming, whose liquid water path decreases as the clouds thin geometrically under warming. The results suggest that methods used to explain the response of lower‐latitude BL clouds to climate change can also elucidate changes in idealized Arctic BL clouds, although subtropical and Arctic clouds occupy different thermodynamic regimes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105044, title ="Unified Entrainment and Detrainment Closures for Extended Eddy-Diffusivity Mass-Flux Schemes", author = "Cohen, Yair and Lopez‐Gomez, Ignacio", journal = "Journal of Advances in Modeling Earth Systems", volume = "12", number = "9", pages = "Art. No. e2020MS002162", month = "September", year = "2020", doi = "10.1029/2020ms002162", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200820-123726127", note = "© 2020 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nIssue Online: 14 September 2020; Version of Record online: 14 September 2020; Accepted manuscript online: 04 August 2020; Manuscript accepted: 29 July 2020; Manuscript revised: 09 July 2020; Manuscript received: 01 May 2020. \n\nThis research was made possible by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, Award AGS‐1835860). I. L. would like to thank the Resnick Sustainability Institute at Caltech for fellowship support. Parts of the research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004) and funded through the internal Research and Technology Development program. The comments provided by Nadir Jeevanjee and one more anonymous reviewer greatly improved the final form of this work. © 2020. California Institute of Technology. Government sponsorship acknowledged. \n\nData Availability Statements: The PyCLES code used to generate LES results is available at this site (climate-dynamics.org/software/#pycles). The SCM code is available at this site (https://doi.org/10.5281/zenodo.3789011).", revision_no = "27", abstract = "We demonstrate that an extended eddy‐diffusivity mass‐flux (EDMF) scheme can be used as a unified parameterization of subgrid‐scale turbulence and convection across a range of dynamical regimes, from dry convective boundary layers, through shallow convection, to deep convection. Central to achieving this unified representation of subgrid‐scale motions are entrainment and detrainment closures. We model entrainment and detrainment rates as a combination of turbulent and dynamical processes. Turbulent entrainment/detrainment is represented as downgradient diffusion between plumes and their environment. Dynamical entrainment/detrainment is proportional to a ratio of a relative buoyancy of a plume and a vertical velocity scale, that is modulated by heuristic nondimensional functions which represent their relative magnitudes and the enhanced detrainment due to evaporation from clouds in drier environment. We first evaluate the closures off‐line against entrainment and detrainment rates diagnosed from large eddy simulations (LESs) in which tracers are used to identify plumes, their turbulent environment, and mass and tracer exchanges between them. The LES are of canonical test cases of a dry convective boundary layer, shallow convection, and deep convection, thus spanning a broad rangeof regimes. We then compare the LES with the full EDMF scheme, including the new closures, in a single‐column model (SCM). The results show good agreement between the SCM and LES in quantities that are key for climate models, including thermodynamic profiles, cloud liquid water profiles, and profiles of higher moments of turbulent statistics. The SCM also captures well the diurnal cycle of convection and the onset of precipitation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105023, title ="A Continuous Record of Central Tropical Pacific Climate Since the Midnineteenth Century Reconstructed From Fanning and Palmyra Island Corals: A Case Study in Coral Data Reanalysis", author = "Sanchez, S. C. and Westphal, N.", journal = "Paleoceanography and Paleoclimatology", volume = "35", number = "8", pages = "Art. No. e2020PA003848", month = "August", year = "2020", doi = "10.1029/2020pa003848", issn = "2572-4517", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200819-095025315", note = "© 2020 American Geophysical Union. \n\nIssue Online: 22 August 2020; Version of Record online: 22 August 2020; Accepted manuscript online: 04 August 2020; Manuscript accepted: 23 July 2020; Manuscript revised: 21 July 2020; Manuscript received: 08 January 2020. \n\nWe thank the three anonymous reviewers whose constructive comments added to the clarity and quality of this work. S. C. S. was supported by a JISAO Postdoctoral Fellowship. We thank The Nature Conservancy, the U.S. Fish and Wildlife Service, and the Palmyra Atoll Research Consortium, for logistical support and access to the refuge. This research was conducted under special use Permit 12533‐16024. \n\nThe authors declare no competing interests.\n\nData Availability Statement: Data will be available on NOAA paleoclimate (https://www.ncdc.noaa.gov/paleo/study/30493) upon publication. Code and information behind the RegEM algorithm can be found at GitHub (https://github.com/tapios/RegEM).", revision_no = "22", abstract = "Accurate estimation of central tropical Pacific (CTP) climate variability on interannual to centennial time scales is required for robust projections of future global climate trends. Here we outline an approach that blends instrumental and coral proxy observations to yield a continuous, monthly resolved record of climate evolution in the CTP spanning the past 160 years. We concatenate coral oxygen isotope (δ¹⁸O) records from multiple living and fossil corals collected from Fanning Island (4°N, 160°W) and Palmyra Island (5°N; 162°W) located in the heart of the El Niño–Southern Oscillation. We use the regularized expectation maximization (RegEM) method to impute missing data across short gaps of 5 to 23 years within and beyond individual coral records. The resulting monthly resolved Fanning/Palmyra Island climate record spans continuously from 1863 to 2016 and provides an example of how extended time series can be built from shorter coral segments. The extended record highlights the strong trend toward warmer and wetter mean conditions in late twentieth century, in agreement with the majority of climate model hindcast simulations. The continuous reconstruction also enables a direct comparison of four exceptionally strong El Niño events (1877–1878, 1940–1941, 1997–1998, and 2015–2016). Three of these very strong El Niño events in the CTP featured a precursor warm event in the prior year and that may have favored the development of a strong El Niño event.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106562, title ="Ensemble Kalman Inversion for Sparse Learning of Dynamical Systems from Time-Averaged Data", author = "Schneider, Tapio and Stuart, Andrew M.", journal = "arXiv", month = "July", year = "2020", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201109-141011032", note = "We thank Melanie Bieli, Tobias Bischoff and Anna Jaruga for sharing their formulation of the moment-based coalescence equation, and for discussions about it. All authors are supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, award AGS1835860). A.M.S. is also supported by NSF (award DMS-1818977) and by the Office of Naval Research (award N00014-17-1-2079).", revision_no = "10", abstract = "Enforcing sparse structure within learning has led to significant advances in the field of data-driven discovery of dynamical systems. However, such methods require access not only to time-series of the state of the dynamical system, but also to the time derivative. In many applications, the data are available only in the form of time-averages such as moments and autocorrelation functions. We propose a sparse learning methodology to discover the vector fields defining a (possibly stochastic or partial) differential equation, using only time-averaged statistics. Such a formulation of sparse learning naturally leads to a nonlinear inverse problem to which we apply the methodology of ensemble Kalman inversion (EKI). EKI is chosen because it may be formulated in terms of the iterative solution of quadratic optimization problems; sparsity is then easily imposed. We then apply the EKI-based sparse learning methodology to various examples governed by stochastic differential equations (a noisy Lorenz 63 system), ordinary differential equations (Lorenz 96 system and coalescence equations), and a partial differential equation (the Kuramoto-Sivashinsky equation). The results demonstrate that time-averaged statistics can be used for data-driven discovery of differential equations using sparse EKI. The proposed sparse learning methodology extends the scope of data-driven discovery of differential equations to previously challenging applications and data-acquisition scenarios.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106301, title ="Seasonal cycle of idealized polar clouds: large eddy simulations driven by a GCM", author = "Zhang, Xiyue and Schneider, Tapio", month = "June", year = "2020", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201027-125955966", note = "License: Attribution-NonCommercial-NoDerivatives 4.0 International. \n\nPublished Online: Tue, 9 Jun 2020. \n\nX.Z. is supported by an Advanced Study Program postdoctoral fellowship from the National Center for Atmospheric Research. Part of this material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement No. 1852977. \n\nPart of this research was supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Mountain Philanthropies, and by the National Science Foundation (NSF grant AGS-1835860). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The simulations were performed on Caltech's High Performing Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. The GCM and LES codes are available online at http://climate-dynamics.org/software. GCM forcing and LES output files are\navailable online at https://data.caltech.edu/records/1429.", revision_no = "9", abstract = "The uncertainty in polar cloud feedbacks calls for process understanding of the cloud response to climate warming. As an initial step, we investigate the seasonal cycle of polar clouds in the current climate by adopting a novel modeling framework using large eddy simulations (LES), which explicitly resolve cloud dynamics. Resolved horizontal and vertical advection of heat and moisture from an idealized GCM are prescribed as forcing in the LES. The LES are also forced with prescribed sea ice thickness, but surface temperature, atmospheric temperature, and moisture evolve freely without nudging. A semigray radiative transfer scheme, without water vapor or cloud feedbacks, allows the GCM and LES to achieve closed energy budgets more easily than would be possible with more complex schemes; this allows the mean states in the two models to be consistently compared, without the added complications from interaction with more comprehensive radiation. We show that the LES closely follow the GCM seasonal cycle, and the seasonal cycle of low clouds in the LES resembles observations: maximum cloud liquid occurs in late summer and early autumn, and winter clouds are dominated by ice in the upper troposphere. Large-scale advection of moisture provides the main source of water vapor for the liquid clouds in summer, while a temperature advection peak in winter makes the atmosphere relatively dry and reduces cloud condensate. The framework we develop and employ can be used broadly for studying cloud processes and the response of polar clouds to climate warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106558, title ="Learning Stochastic Closures Using Ensemble Kalman Inversion", author = "Schneider, Tapio and Stuart, Andrew M.", journal = "arXiv", month = "April", year = "2020", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201109-140955956", note = "The authors thank Dr. Yvo Pokern at University College London for providing the butane dihedral angle data. All authors are supported by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, and the National Science Foundation (NSF, award AGS1835860). A.M.S. is also supported by NSF (award DMS-1818977) and by the Office of Naval Research (award N00014-17-1-2079).", revision_no = "10", abstract = "Although the governing equations of many systems, when derived from first principles, may be viewed as known, it is often too expensive to numerically simulate all the interactions within the first principles description. Therefore researchers often seek simpler descriptions that describe complex phenomena without numerically resolving all the interacting components. Stochastic differential equations (SDEs) arise naturally as models in this context. The growth in data acquisition provides an opportunity for the systematic derivation of SDE models in many disciplines. However, inconsistencies between SDEs and real data at small time scales often cause problems, when standard statistical methodology is applied to parameter estimation. The incompatibility between SDEs and real data can be addressed by deriving sufficient statistics from the time-series data and learning parameters of SDEs based on these. Following this approach, we formulate the fitting of SDEs to sufficient statistics from real data as an inverse problem and demonstrate that this inverse problem can be solved by using ensemble Kalman inversion (EKI). Furthermore, we create a framework for non-parametric learning of drift and diffusion terms by introducing hierarchical, refineable parameterizations of unknown functions, using Gaussian process regression. We demonstrate the proposed methodology for the fitting of SDE models, first in a simulation study with a noisy Lorenz 63 model, and then in other applications, including dimension reduction starting from various deterministic chaotic systems arising in the atmospheric sciences, large-scale pattern modeling in climate dynamics, and simplified models for key observables arising in molecular dynamics. The results confirm that the proposed methodology provides a robust and systematic approach to fitting SDE models to real data.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/101417, title ="Atmospheric Circulation Response to Short-Term Arctic Warming in an Idealized Model", author = "Hell, Momme C. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "77", number = "2", pages = "531-549", month = "February", year = "2020", doi = "10.1175/jas-d-19-0133.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200220-100007967", note = "© 2020 American Meteorological Society. \n\nManuscript received 15 May 2019, in final form 12 November 2019. Published online: 28 January 2020. \n\nThe idealized GCM simulations were performed on ETH Zurich’s Euler computing cluster. We thank Noel Keenlyside for facilitating this work in an early stage, and Tobias Bischoff, Farid Ait-Chaalal, Robert Wills, and Ori Adam for support with running and analyzing the GCM as well as discussions of the paper. This work was partially supported by the NFR KLIMAFORSK programme (Project 255027).", revision_no = "10", abstract = "Recent Arctic sea ice loss in fall has been posited to drive midlatitude circulation changes into winter and even spring. Past work has shown that sea ice loss can indeed trigger a weakening of the stratospheric polar vortex, which can lead to delayed surface weather changes. But the mechanisms of such changes and their relevant time scales have remained unclear. This study uses large ensembles of idealized GCM simulations to identify how and over what time scales the atmospheric circulation responds to short-term surface heat flux changes in high latitudes. The ensemble-mean response of the atmospheric circulation is approximately linear in the amplitude of the surface forcing. It is also insensitive to whether the forcing is zonally asymmetric or symmetric, that is, whether stationary waves are generated or not. The circulation response can be decomposed into a rapid thermal response and a slower dynamic adjustment. The adjustment arises through weakening of vertical wave activity fluxes from the troposphere into the stratosphere in response to polar warming, a mechanism that differs from sudden stratospheric warmings yet still results in a weakened stratospheric circulation. The stratospheric response is delayed and persists for about 2 months because the thermal response of the stratosphere is slow compared with that of the troposphere. The delayed stratospheric response feeds back onto the troposphere, but the tropospheric effects are weak compared with natural variability. The general pathway for the delayed response appears to be relatively independent of the atmospheric background state at the time of the anomalous surface forcing.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/100946, title ="Statistically Steady State Large‐Eddy Simulations Forced by an Idealized GCM: 1. Forcing Framework and Simulation Characteristics", author = "Shen, Zhaoyi and Pressel, Kyle G.", journal = "Journal of Advances in Modeling Earth Systems", volume = "12", number = "2", pages = "Art. No. e2019MS001814", month = "February", year = "2020", doi = "10.1029/2019MS001814", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200127-131857757", note = "© 2020 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nReceived 13 JUL 2019; Accepted 18 JAN 2020; Accepted article online 25 JAN 2020. \n\nWe gratefully acknowledge the generous support of Eric and Wendy Schmidt (by recommendation of Schmidt Futures), Mountain Philanthropies, EarthRise Alliance, Charles Trimble, the Paul G. Allen Family Foundation, and the National Science Foundation (Grant 1835860). The simulations were performed on Caltech's High Performance Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The GCM codes are available on the GitHub repository (https://github.com/szy21/fms_GCMForcing). The LES codes are available on the GitHub repository (https://github.com/szy21/pycles_GCM/tree/shen2020). Primary GCM and LES data that may be used to produce the plots are available online (https://data.caltech.edu/records/1337).", revision_no = "22", abstract = "Using large‐eddy simulations (LES) systematically has the potential to inform parameterizations of subgrid‐scale processes in general circulation models (GCMs), such as turbulence, convection, and clouds. Here we show how LES can be run to simulate grid columns of GCMs to generate LES across a cross section of dynamical regimes. The LES setup approximately replicates the thermodynamic and water budgets in GCM grid columns. Resolved horizontal and vertical transports of heat and water and large‐scale pressure gradients from the GCM are prescribed as forcing in the LES. The LES are forced with prescribed surface temperatures, but atmospheric temperature and moisture are free to adjust, reducing the imprinting of GCM fields on the LES. In both the GCM and LES, radiative transfer is treated in a unified but idealized manner (semigray atmosphere without water vapor feedback or cloud radiative effects). We show that the LES in this setup reaches statistically steady states without nudging to thermodynamic GCM profiles. The steady states provide training data for developing GCM parameterizations. The same LES setup also provides a good basis for studying the cloud response to global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/100575, title ="Midwinter Suppression of Storm Tracks in an Idealized Zonally Symmetric Setting", author = "Novak, Lenka and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "77", number = "1", pages = "297-313", month = "January", year = "2020", doi = "10.1175/JAS-D-18-0353.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200109-083234021", note = "© 2019 American Meteorological Society. \n\nManuscript received 8 December 2018, in final form 3 October 2019. Published online: 23 December 2019. \n\nThis work was supported by NSF (AGS-1760402). We thank Prof. Simona Bordoni, Dr. Tobias Bischoff, and Dr. Yair Cohen for useful discussions when producing the manuscript.", revision_no = "8", abstract = "The midwinter suppression of eddy activity in the North Pacific storm track is a phenomenon that has resisted reproduction in idealized models that are initialized independently of the observed atmosphere. Attempts at explaining it have often focused on local mechanisms that depend on zonal asymmetries, such as effects of topography on the mean flow and eddies. Here an idealized aquaplanet GCM is used to demonstrate that a midwinter suppression can also occur in the activity of a statistically zonally symmetric storm track. For a midwinter suppression to occur, it is necessary that parameters, such as the thermal inertia of the upper ocean and the strength of tropical ocean energy transport, are chosen suitably to produce a pronounced seasonal cycle of the subtropical jet characteristics. If the subtropical jet is sufficiently strong and located close to the midlatitude storm track during midwinter, it dominates the upper-level flow and guides eddies equatorward, away from the low-level area of eddy generation. This inhibits the baroclinic interaction between upper and lower levels within the storm track and weakens eddy activity. However, as the subtropical jet continues to move poleward during late winter in the idealized GCM (and unlike what is observed), eddy activity picks up again, showing that the properties of the subtropical jet that give rise to the midwinter suppression are subtle. The idealized GCM simulations provide a framework within which possible mechanisms giving rise to a midwinter suppression of storm tracks can be investigated systematically.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/96944, title ="Both differential and equatorial heating contributed to African monsoon variations during the mid-Holocene", author = "Adam, Ori and Schneider, Tapio", journal = "Earth and Planetary Science Letters", volume = "522", pages = "20-29", month = "September", year = "2019", doi = "10.1016/j.epsl.2019.06.019", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190709-091307243", note = "© 2019 Elsevier B.V. \n\nReceived 28 January 2019, Revised 15 May 2019, Accepted 17 June 2019, Available online 1 July 2019. \n\nThe PMIP3 data was downloaded from Earth System Grid Federation (ESGF). Ori Adam acknowledges support by the Israel Science Foundation grant 1185/17.", revision_no = "20", abstract = "The Sahara was significantly greener 11-5 kya and during multiple earlier interglacial periods. But the mechanisms related to the greening of the Sahara remain uncertain as most climate models severely underestimate past wet conditions over north Africa. The variations in the African monsoon related to the greening of the Sahara are thought to be associated with the variations in the inter-hemispheric differential heating of Earth, caused by orbital variations. However, how orbital variations affect regional climate is not well understood. Using recent theory that relates the position of the tropical rain belt to the atmospheric energy budget, we study the effect of orbital forcing during the mid-Holocene on the African monsoon in simulations provided by the third phase of the Paleo Model Intercomparison Project (PMIP3). We find that energy fluxes in the African sector are related to orbital forcing in a complex manner. Contrary to generally accepted theory, orbital modulation of seasonal differential heating alone is shown to be a weak driver of African monsoon variations. Instead, net atmospheric heating near the equator, which modulates the intensity and extent of seasonal migrations of the tropical rain belt, is an important but overlooked driver of African monsoon variations. A conceptual framework that relates African monsoon variations to both equatorial and inter-hemispheric differential solar heating is presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92140, title ="Possible climate transitions from breakup of stratocumulus decks under greenhouse warming", author = "Schneider, Tapio and Kaul, Colleen M.", journal = "Nature Geoscience", volume = "12", number = "3", pages = "163-167", month = "March", year = "2019", doi = "10.1038/s41561-019-0310-1", issn = "1752-0894", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190108-130913145", note = "© The Author(s), under exclusive licence to Springer Nature Limited 2019. \n\nReceived 07 September 2018; Accepted 16 January 2019; Published 25 February 2019. \n\nThis research was supported by C. Trimble. The computations were performed on ETH Zurich’s Euler cluster and on Caltech’s High Performance Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. We thank M. Hell for assistance with the figures. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. \n\nData availability: The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files. The raw data in the figures are available from the corresponding author upon request. \n\nCode availability: The source code for the simulations is available at climate-dynamics.org/software/#pycles.", revision_no = "28", abstract = "Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight. However, as their dynamical scales are too small to be resolvable in global climate models, predictions of their response to greenhouse warming have remained uncertain. Here we report how stratocumulus decks respond to greenhouse warming in large-eddy simulations that explicitly resolve cloud dynamics in a representative subtropical region. In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO_2 levels rise above 1,200\u2009ppm. In addition to the warming from rising CO_2 levels, this instability triggers a surface warming of about 8\u2009K globally and 10\u2009K in the subtropics. Once the stratocumulus decks have broken up, they only re-form once CO_2 concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past. Such transitions to a much warmer climate may also occur in the future if CO_2 levels continue to rise.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/89018, title ="Mechanisms Setting the Strength of Orographic Rossby Waves across a Wide Range of Climates in a Moist Idealized GCM", author = "Wills, Robert C. J. and Schneider, Tapio", journal = "Journal of Climate", volume = "31", number = "18", pages = "7679-7700", month = "September", year = "2018", doi = "10.1175/JCLI-D-17-0700.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180822-092424875", note = "© 2018 American Meteorological Society. \n\nManuscript received 21 October 2017, in final form 30 May 2018. Published online: 9 August 2018. \n\nThis work was primarily completed while both authors were at the Department of Earth Sciences, ETH Zürich, Zurich, Switzerland. This research has also been supported by NSF Grant AGS-1019211 while both authors were at the California Institute of Technology, Pasadena, California. The idealized GCM simulations for this study were performed on ETH Zürich’s EULER computing cluster. Code for the idealized GCM is available online (at https://github.com/tapios/fms-idealized). We thank Simona Bordoni, Andy Thompson, Jess Adkins, Rachel White, Momme Hell, Farid Ait Chaalal, Ori Adam, and three anonymous reviewers for useful comments and discussion during the development of this manuscript.", revision_no = "12", abstract = "Orographic stationary Rossby waves are an important influence on the large-scale circulation of the atmosphere, especially in Northern Hemisphere winter. Changes in stationary waves with global warming have the potential to modify patterns of surface temperature and precipitation. This paper presents an analysis of the forcing of stationary waves by midlatitude orography across a wide range of climates in a moist idealized GCM, where latent heating and transient eddies are allowed to feed back on the stationary-eddy dynamics. The stationary-eddy amplitude depends to leading order on the surface winds impinging on the orography, resulting in different climate change responses for mountains at different latitudes. Latent heating is found to damp orographic stationary waves, whereas transient eddies are found to reinforce them. As the climate warms, the damping by latent heating becomes more effective while the reinforcement by transient eddies becomes less effective, leading to an overall reduction in orographic stationary wave amplitude. These effects overwhelm the influences of a reduced meridional temperature gradient and increased dry static stability, both of which increase the sensitivity of the free troposphere to orographic forcing. Together with a reduction in the midlatitude meridional temperature gradient, the weakening of orographic stationary waves leads to reduced zonal asymmetry of temperature and net precipitation in warm, moist climates. While circulation changes in this idealized model cannot be expected to agree quantitatively with changes in the real world, the key physical processes identified are broadly relevant.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/87776, title ="Eddy Lifetime, Number, and Diffusivity and the Suppression of Eddy Kinetic Energy in Midwinter", author = "Schemm, Sebatian and Schneider, Tapio", journal = "Journal of Climate", volume = "31", number = "14", pages = "5649-5665", month = "July", year = "2018", doi = "10.1175/JCLI-D-17-0644.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180711-153419610", note = "© 2018 American Meteorological Society. Open Choice. \n\n(Manuscript received 26 September 2017, in final form 10 April 2018) \n\nSebastian Schemm acknowledges funding from the Swiss National Science Foundation (Grants P300P2_167745 and P3P3P2_167747). ECMWF is acknowledged for providing the ERA-Interim dataset. We are grateful to Farid Ait-Chaalal for helpful comments during the course of the analysis. We also thank three anonymous reviewers and Hisashi Nakamura for their helpful comments during the review process.", revision_no = "9", abstract = "The wintertime evolution of the North Pacific storm track appears to challenge classical theories of baroclinic instability, which predict deeper extratropical cyclones when baroclinicity is highest. Although the surface baroclinicity peaks during midwinter, and the jet is strongest, eddy kinetic energy (EKE) and baroclinic conversion rates have a midwinter minimum over the North Pacific. This study investigates how the reduction in EKE translates into a reduction in eddy potential vorticity (PV) and heat fluxes via changes in eddy diffusivity. Additionally, it augments previous observations of the midwinter storm-track evolution in both hemispheres using climatologies of tracked surface cyclones. In the North Pacific, the number of surface cyclones is highest during midwinter, while the mean EKE per cyclone and the eddy lifetime are reduced. The midwinter reduction in upper-level eddy activity hence is not associated with a reduction in surface cyclone numbers. North Pacific eddy diffusivities exhibit a midwinter reduction at upper levels, where the Lagrangian decorrelation time is shortest (consistent with reduced eddy lifetimes) and the meridional parcel velocity variance is reduced (consistent with reduced EKE). The resulting midwinter reduction in North Pacific eddy diffusivities translates into an eddy PV flux suppression. In contrast, in the North Atlantic, a milder reduction in the decorrelation time is offset by a maximum in velocity variance, preventing a midwinter diffusivity minimum. The results suggest that a focus on causes of the wintertime evolution of Lagrangian decorrelation times and parcel velocity variance will be fruitful for understanding causes of seasonal storm-track variations.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/87564, title ="Regional and seasonal variations of the double-ITCZ bias in CMIP5 models", author = "Adam, Ori and Schneider, Tapio", journal = "Climate Dynamics", volume = "51", number = "1-2", pages = "101-117", month = "July", year = "2018", doi = "10.1007/s00382-017-3909-1", issn = "0930-7575", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180705-143240657", note = "© Springer-Verlag GmbH Germany 2017. \n\nReceived: 14 May 2017 / Accepted: 23 July 2017 / Published online: 16 September 2017. \n\nWe acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (Fig. 1) for producing and making available their model output. For CMIP the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We would also like to thank our anonymous reviewers for their contributions to the presentation of this work.", revision_no = "11", abstract = "Current climate models represent the zonal- and annual-mean intertropical convergence zone (ITCZ) position in a biased way, with an unrealistic double precipitation peak straddling the equator in the ensemble mean over the models. This bias is seasonally and regionally localized. It results primarily from two regions: the eastern Pacific and Atlantic (EPA), where the ITCZ in boreal winter and spring is displaced farther south than is observed; and the western Pacific (WP), where a more pronounced and wider than observed double ITCZ straddles the equator year-round. Additionally, the precipitation associated with the ascending branches of the zonal overturning circulations (e.g., Walker circulation) in the Pacific and Atlantic sectors is shifted westward. We interpret these biases in light of recent theories that relate the ITCZ position to the atmospheric energy budget. WP biases are associated with the well known Pacific cold tongue bias, which, in turn, is linked to atmospheric net energy input biases near the equator. In contrast, EPA biases are shown to be associated with a positive bias in the cross-equatorial divergent atmospheric energy transport during boreal winter and spring, with two potential sources: tropical biases associated with equatorial sea surface temperatures (SSTs) and tropical low clouds, and extratropical biases associated with Southern Ocean clouds and north Atlantic SST. The distinct seasonal and regional characteristics of WP and EPA biases and the differences in their associated energy budget biases suggest that the biases in the two sectors involve different mechanisms and potentially different sources.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/85345, title ="Atlantic-Pacific Asymmetry in Deep-Water Formation", author = "Ferreira, David and Cessi, Paola", journal = "Annual Review of Earth and Planetary Sciences", volume = "46", pages = "327-352", month = "May", year = "2018", doi = "10.1146/annurev-earth-082517-010045", issn = "0084-6597", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180316-101647631", note = "© 2018 Annual Reviews. \n\nReview in Advance first posted online on March 15, 2018. \n\nThe authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. \n\nWe acknowledge funding from the International Meteorological Institute and the Bolin Centre for Climate Research for the workshop on “The Atlantic Meridional Overturning Circulation in a Global Perspective” held at Stockholm University in September 2015. P.C., J.F.M., and T.S. acknowledge support from the US National Science Foundation (grant OCE-1634128 to P.C., grant AGS-1635019 to J.F.M., and grant AGS-1019211 to T.S.). T.E. acknowledges support from the Research Council of Norway (project NORTH). T.S. thanks Ryan Abernathey and Laure Zanna for helpful discussions.", revision_no = "18", abstract = "While the Atlantic Ocean is ventilated by high-latitude deep water formation and exhibits a pole-to-pole overturning circulation, the Pacific Ocean does not. This asymmetric global overturning pattern has persisted for the past 2–3 million years, with evidence for different ventilation modes in the deeper past. In the current climate, the Atlantic-Pacific asymmetry occurs because the Atlantic is more saline, enabling deep convection. To what extent the salinity contrast between the two basins is dominated by atmospheric processes (larger net evaporation over the Atlantic) or oceanic processes (salinity transport into the Atlantic) remains an outstanding question. Numerical simulations have provided support for both mechanisms; observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes. A major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/86050, title ="Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications", author = "Byrne, Michael P. and Schneider, Tapio", journal = "Journal of Climate", volume = "31", number = "8", pages = "3249-3264", month = "April", year = "2018", doi = "10.1175/JCLI-D-17-0470.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180425-165415428", note = "© 2018 American Meteorological Society. \n\n(Manuscript received 13 July 2017, in final form 12 January 2018) \n\nWe thank Paulo Ceppi, Reto Knutti, Paul O’Gorman, Brian Rose, Maria Rugenstein, and the editor for helpful comments, suggestions, and discussion. M.P.B. acknowledges support from the Imperial College London Research Fellowship Scheme.", revision_no = "10", abstract = "The regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/85744, title ="Disentangling Global Warming, Multidecadal Variability, and El Niño in Pacific Temperatures", author = "Wills, Robert C. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "45", number = "5", pages = "2487-2496", month = "March", year = "2018", doi = "10.1002/2017GL076327", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180411-105304038", note = "© 2018 American Geophysical Union. \n\nReceived 6 NOV 2017; Accepted 17 JAN 2018; Accepted article online 25 JAN 2018; Published online 15 MAR 2018. \n\nWe thank T. Kohyama, K. Armour, T. Bischoff, M. Stuecker, and K.‐K. Tung for valuable conversations and feedback on this work. R. C. W. and D. L. H. acknowledge support from the National Science Foundation (grant AGS‐1549579). R. C. W. and D. S. B. acknowledge support from the Tamaki Foundation. The data sets used are documented in Smith et al. (2008) and Compo et al. (2011).", revision_no = "12", abstract = "A key challenge in climate science is to separate observed temperature changes into components due to internal variability and responses to external forcing. Extended integrations of forced and unforced climate models are often used for this purpose. Here we demonstrate a novel method to separate modes of internal variability from global warming based on differences in time scale and spatial pattern, without relying on climate models. We identify uncorrelated components of Pacific sea surface temperature variability due to global warming, the Pacific Decadal Oscillation (PDO), and the El Niño–Southern Oscillation (ENSO). Our results give statistical representations of PDO and ENSO that are consistent with their being separate processes, operating on different time scales, but are otherwise consistent with canonical definitions. We isolate the multidecadal variability of the PDO and find that it is confined to midlatitudes; tropical sea surface temperatures and their teleconnections mix in higher‐frequency variability. This implies that midlatitude PDO anomalies are more persistent than previously thought.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84566, title ="Disentangling global warming, multi-decadal variability, and El Niño in Pacific temperatures", author = "Wills, Robert C. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "45", number = "5", pages = "2487-2496", month = "March", year = "2018", doi = "10.1002/2017GL076327", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180129-142103724", note = "© 2018 American Geophysical Union. \n\nReceived 6 NOV 2017; Accepted 17 JAN 2018; Accepted article online 25 JAN 2018; Published online 15 MAR 2018. \n\nWe thank T. Kohyama, K. Armour, T. Bischoff, M. Stuecker, and K.‐K. Tung for valuable conversations and feedback on this work. R. C. W. and D. L. H. acknowledge support from the National Science Foundation (grant AGS‐1549579). R. C. W. and D. S. B. acknowledge support from the Tamaki Foundation. The data sets used are documented in Smith et al. (2008) and Compo et al. (2011).", revision_no = "23", abstract = "A key challenge in climate science is to separate observed temperature changes into components due to internal variability and responses to external forcing. Extended integrations of forced and unforced climate models are often used for this purpose. Here we demonstrate a novel method to separate modes of internal variability from global warming based on differences in time scale and spatial pattern, without relying on climate models. We identify uncorrelated components of Pacific sea surface temperature variability due to global warming, the Pacific Decadal Oscillation (PDO), and the El Niño–Southern Oscillation (ENSO). Our results give statistical representations of PDO and ENSO that are consistent with their being separate processes, operating on different time scales, but are otherwise consistent with canonical definitions. We isolate the multidecadal variability of the PDO and find that it is confined to midlatitudes; tropical sea surface temperatures and their teleconnections mix in higher‐frequency variability. This implies that midlatitude PDO anomalies are more persistent than previously thought.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84683, title ="An Extended Eddy-Diffusivity Mass-Flux Scheme for Unified Representation of Subgrid-Scale Turbulence and Convection", author = "Tan, Zhihong and Kaul, Colleen M.", journal = "Journal of Advances in Modeling Earth Systems", volume = "10", number = "3", pages = "770-800", month = "March", year = "2018", doi = "10.1002/2017MS001162", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180206-095908216", note = "© 2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the\nuse is non-commercial and no modifications or adaptations are made. \n\nReceived 1 SEP 2017; Accepted 1 FEB 2018; Accepted article online 6 FEB 2018; Published online 23 MAR 2018. \n\nThis work was supported by the U.S. National Science Foundation (grant CCF-1048575), by Caltech's Terrestrial Hazard Observation and Reporting (THOR) Center, by the Swiss National Science Foundation (grant 200021 156109), and by the President's and Director's Fund of Caltech and the Jet Propulsion Laboratory. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and funded through the internal Research and Technology Development program. JT\nacknowledges the support of the NASA MAP Program. The numerical simulations were performed on the Euler Cluster operated by the high-performance computing (HPC) team\nat ETH Züurich. The PyCLES codes used to generate results in this paper is available at climate-dynamics.org/software. We thank Wolfgang Langhans for very helpful comments on an earlier version of this paper. \n\nAuthor Contributions: T.S. and J.T. conceived the project. Z.T., T.S., and K.G.P. developed the theoretical framework. C.M.K. developed the single-column model, its numerical discretization, and some closures in it, based on a prototype by Z.T. K.G.P. carried out the LES, and C.M.K. and Y.C. carried out the single-column model tests. Z.T. and T.S. led the writing of the paper, with all authors contributing.", revision_no = "18", abstract = "Large-scale weather forecasting and climate models are beginning to reach horizontal resolutions of kilometers, at which common assumptions made in existing parameterization schemes of subgrid-scale turbulence and convection—such as that they adjust instantaneously to changes in resolved-scale dynamics—cease to be justifiable. Additionally, the common practice of representing boundary-layer turbulence, shallow convection, and deep convection by discontinuously different parameterizations schemes, each with its own set of parameters, has contributed to the proliferation of adjustable parameters in large-scale models. Here we lay the theoretical foundations for an extended eddy-diffusivity mass flux (EDMF) scheme that has explicit time-dependence and memory of subgrid-scale variables and is designed to represent all subgrid-scale turbulence and convection, from boundary layer dynamics to deep convection, in a unified manner. Coherent up- and downdrafts in the scheme are represented as prognostic plumes that interact with their environment and potentially with each other through entrainment and detrainment. The more isotropic turbulence in their environment is represented through diffusive fluxes, with diffusivities obtained from a turbulence kinetic energy budget that consistently partitions turbulence kinetic energy between plumes and environment. The cross-sectional area of up- and downdrafts satisfies a prognostic continuity equation, which allows the plumes to cover variable and arbitrarily large fractions of a large-scale grid box and to have life cycles governed by their own internal dynamics. Relatively simple preliminary proposals for closure parameters are presented and are shown to lead to a successful simulation of shallow convection, including a time-dependent life cycle.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/86311, title ="Linking Hadley Circulation and Storm Tracks in a Conceptual Model of the Atmospheric Energy Balance", author = "Mbengue, Cheikh and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "75", number = "3", pages = "841-856", month = "March", year = "2018", doi = "10.1175/JAS-D-17-0098.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180509-105945938", note = "© 2018 American Meteorological Society. \n\nManuscript received 31 March 2017, in final form 14 November 2017. Published online: 1 March 2018. \n\nWe are grateful for the financial support of the National Science Foundation (Grant AGS-1019211). We also thank Xavier Levine for several helpful discussions and for his comments on an early draft of this work.", revision_no = "8", abstract = "Midlatitude storm tracks shift in response to climate change and natural climate variations such as El Niño, but the dynamical mechanisms controlling these shifts are not well established. This paper develops an energy balance model that shows how shifts of the Hadley cell terminus and changes of the meridional energy flux out of the Hadley cell can drive shifts of storm tracks, identified as extrema of the atmospheric meridional eddy energy flux. The distance between the Hadley cell terminus and the storm tracks is primarily controlled by the energy flux out of the Hadley cell. Because tropical forcings alone can modify the Hadley cell terminus, they can also shift extratropical storm tracks, as demonstrated through simulations with an idealized GCM. Additionally, a strengthening of the meridional temperature gradient at the terminus and hence of the energy flux out of the Hadley cell can reduce the distance between the Hadley cell terminus and the storm tracks, enabling storm-track shifts that do not parallel shifts of the Hadley cell terminus. Thus, with the aid of the energy balance model and supporting GCM simulations, a closed theory of storm-track shifts emerges.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83623, title ="Earth System Modeling 2.0: A Blueprint for Models That Learn From Observations and Targeted High-Resolution Simulations\n", author = "Schneider, Tapio and Lan, Shiwei", journal = "Geophysical Research Letters", volume = "44", number = "24", pages = "12396-12417", month = "December", year = "2017", doi = "10.1002/2017GL076101", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171201-113659166", note = "© 2017 American Geophysical Union. \n\nReceived 30 AUG 2017; Accepted 23 NOV 2017; Accepted article online 30 NOV 2017. \n\nWe gratefully acknowledge financial support by Charles Trimble, by the Office of Naval Research (grant N00014-17-1-2079), and by the President's and Director's Fund of Caltech and the Jet Propulsion Laboratory. We also thank V. Balaji, Michael Keller, Dan McCleese, and John Worden for helpful discussions and comments on drafts, and Momme Hell for preparing Figure 3. The program code used in this paper is available at climate-dynamics.org/publications/. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.", revision_no = "27", abstract = "Climate projections continue to be marred by large uncertainties, which originate in processes that need to be parameterized, such as clouds, convection, and ecosystems. But rapid progress is now within reach. New computational tools and methods from data assimilation and machine learning make it possible to integrate global observations and local high-resolution simulations in an Earth system model (ESM) that systematically learns from both and quantifies uncertainties. Here we propose a blueprint for such an ESM. We outline how parameterization schemes can learn from global observations and targeted high-resolution simulations, for example, of clouds and convection, through matching low-order statistics between ESMs, observations, and high-resolution simulations. We illustrate learning algorithms for ESMs with a simple dynamical system that shares characteristics of the climate system; and we discuss the opportunities the proposed framework presents and the challenges that remain to realize it.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82787, title ="Feedback of Atmosphere-Ocean Coupling on Shifts of the Intertropical Convergence Zone", author = "Schneider, Tapio", journal = "Geophysical Research Letters", volume = "44", number = "22", pages = "11644-11653", month = "November", year = "2017", doi = "10.1002/2017GL075817", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171030-144512215", note = "© 2017 American Geophysical Union. \n\nReceived 19 JUL 2017; Accepted 24 OCT 2017; Accepted article online 30 OCT 2017. \n\nI thank Momme Hell for drawing Fig. 2, Karim Lakhani for calculating O_0 and NEI_0 from the CMIP5 models, and Ori Adam for helpful comments on a draft and help with Fig. 1. The National Center for Atmospheric Research (NCAR) provided the ERA-Interim energy budget products used in Fig. 1: https://climatedataguide.ucar.edu/climate-data/era-interim-derived-components. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.", revision_no = "23", abstract = "It is well known that the Intertropical Convergence Zone (ITCZ) shifts in response to remote perturbations in the atmospheric energy balance, with shifts roughly in proportion to changes in the cross-equatorial atmospheric energy flux. However, atmospheric and oceanic energy fluxes in low latitudes are mechanically coupled, and the oceanic energy flux dominates the atmospheric energy flux. Here a quantitative framework is derived that shows how Ekman coupling of atmospheric and oceanic energy fluxes damps the perturbation response of the atmospheric energy flux, energy flux equator (EFE), and ITCZ. To first order, Ekman coupling alone mutes the response of EFE and ITCZ in the coupled atmosphere-ocean system by a factor γ = 1+O_0/NEI_0, where O_0 is the ocean energy uptake and NEI0 is the net energy input into the atmosphere at the equator. In the current climate in the zonal and annual mean, this factor is about γ≈3.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/81878, title ="A Conceptual Model for the Response of Tropical Rainfall to Orbital Variations", author = "Bischoff, Tobias and Schneider, Tapio", journal = "Journal of Climate", volume = "30", number = "20", pages = "8375-8391", month = "October", year = "2017", doi = "10.1175/JCLI-D-16-0691.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170927-110506100", note = "© 2017 American Meteorological Society. \n\nManuscript received 23 September 2016, in final form 4 June 2017. Published online: 22 September 2017. \n\nWe thank Stacy Carolin and Jess Adkins for discussions on the model presented in this paper. We also thank Aaron Donohoe and Zhengyu Liu for helpful reviews of the manuscript. The Geophysical Observational Analysis Tool (http://www.goat-geo.org/) was used to process the 20th-century reanalysis data shown in the appendix. Support for the Twentieth Century Reanalysis Project dataset is provided by the U.S. Department of Energy, Office of Science Innovative and Novel Computational Impact on Theory and Experiment (DOE INCITE) program, and Office of Biological and Environmental Research (BER), and by the National Oceanic and Atmospheric Administration Climate Program Office. This research was partially supported by NSF Grant AGS-1049201, ERC Starting Grant 638467, and by the Bergen Research Foundation.", revision_no = "9", abstract = "Tropical rainfall to first order responds to variations in Earth’s orbit through shifts of the intertropical convergence zone (ITCZ) and changes in zonally averaged rainfall intensity. Here, a conceptual model is developed that represents both processes and their response to orbital insolation variations. The model predicts the seasonal evolution of tropical rainfall between 30°S and 30°N. Insolation variations impact seasonal rainfall in two different ways: thermodynamically, leading to variations in rainfall intensity through modulation of the water vapor content of the atmosphere; and dynamically, leading to shifts of the ITCZ through modulation of the global atmospheric energy budget. Thermodynamic and dynamic effects act together to shape the annual-mean response of tropical rainfall to changes in Earth’s orbit. The model successfully reproduces changes in annual-mean rainfall inferred from paleo-proxies across several glacial–interglacial cycles. It illuminates how orbital precession and variations of Earth’s obliquity affect tropical rainfall in distinct ways near the equator and farther away from it, with spectral signatures of precession and obliquity variations that shift with latitude. It also provides explanations for the observed different phasings of rainfall minima and maxima near the equator and away from it. For example, the model reproduces a phase shift of ~10 ka between rainfall records from caves in northern Borneo (4°N) and from China (approximately 30°N). The model suggests that such phase shifts arise through a different weighting of ITCZ shifts and variations in rainfall intensity, thus providing insight into the mechanisms that drive tropical rainfall changes on orbital time scales.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/81016, title ="Evolving perspectives on abrupt seasonal changes of the general circulation", author = "Lu, Jianhua and Schneider, Tapio", journal = "Advances in Atmospheric Sciences", volume = "34", number = "10", pages = "1185-1194", month = "October", year = "2017", doi = "10.1007/s00376-017-7068-4", issn = "0256-1530", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170831-122955794", note = "© 2017 Chinese National Committee for International Association of Meteorology and Atmospheric Sciences, Institute of Atmospheric Physics, Science Press and Springer-Verlag GmbH Germany. \n\nReceived 29 March 2017; revised 21 June 2017; accepted 28 June 2017. First Online: 17 August 2017. \n\nJ. H. LU is indebted to Professor YEH’s mentoring over more than two decades. Both authors acknowledge the Organization Committee of Professor YEH’s Centenary Symposium for the invitation to the symposium. J. H. LU is grateful for the support from the LASG during his visit to the lab. J. H. LU is supported by the National Nature Science Foundation of China (Grant No. 4157060636) and the Hundred Talents Program of Sun Yat-sen University.", revision_no = "12", abstract = "Professor Duzheng YE (Tu-cheng YEH) was decades ahead of his time in proposing a model experiment to investigate whether abrupt seasonal changes of the general circulation can arise through circulation feedbacks alone, unrelated to underlying inhomogeneities at the lower boundary. Here, we introduce Professor YEH’s ideas during the 1950s and 1960s on the general circulation and summarize the results and suggestions of Yeh et al. (1959) on abrupt seasonal changes. We then review recent advances in understanding abrupt seasonal changes arising from model experiments like those proposed by Yeh et al. (1959). The model experiments show that circulation feedbacks can indeed give rise to abrupt seasonal transitions. In these transitions, large-scale eddies that originate in midlatitudes and interact with the zonal mean flow and meridional overturning circulations in the tropics play central roles.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82113, title ="Factors controlling Hadley circulation changes from the Last Glacial Maximum to the end of the 21st century", author = "D'Agostino, Roberta and Lionello, Piero", journal = "Geophysical Research Letters", volume = "44", number = "16", pages = "8585-8591", month = "August", year = "2017", doi = "10.1002/2017GL074533", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171005-103437608", note = "© 2017 American Geophysical Union. \n\nReceived 9 JUN 2017; Accepted 7 AUG 2017; Accepted article online 14 AUG 2017; Published online 31 AUG 2017. \n\nThis study has been funded by University of Salento, joined by CMCC within the PhD school in Ecology and Climate Change and by the JPI Climate-Belmont Forum project “PaCMEDy” (http://www.jpi-climate.eu/2015projects/pacmedy). The CMIP5 and PMIP3 data have been analyzed using GOAT (Geophysical Observations Analysis Tool, www.goat-geo.org).", revision_no = "14", abstract = "The Hadley circulation (HC) extent and strength are analyzed in a wide range of simulated climates from the Last Glacial Maximum to global warming scenarios. Motivated by HC theories, we analyze how the HC is influenced by the subtropical stability, the near-surface meridional potential temperature gradient, and the tropical tropopause level. The subtropical static stability accounts for the bulk of the HC changes across the simulations. However, since it correlates strongly with global mean surface temperature, most HC changes can be attributed to global mean surface temperature changes. The HC widens as the climate warms, and it also weakens, but only robustly so in the Northern Hemisphere. On the other hand, the Southern Hemisphere strength response is uncertain, in part because subtropical static stability changes counteract meridional potential temperature gradient changes to various degrees in different models, with no consensus on the response of the latter to global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/78706, title ="Local Energetic Constraints on Walker Circulation Strength", author = "Wills, Robert C. and Levine, Xavier J.", journal = "Journal of the Atmospheric Sciences", volume = "74", number = "6", pages = "1907-1922", month = "June", year = "2017", doi = "10.1175/JAS-D-16-0219.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170629-133030446", note = "© 2017 American Meteorological Society. \n\nManuscript received 25 July 2016, in final form 20 January 2017, published online 24 March 2017. \n\nThe idealized GCM simulations for this study were performed on ETH Zürich’s EULER computing cluster. We thank Simona Bordoni, Jess Adkins, Andy Thompson, Michael Byrne, and Chris Bretherton for useful comments and discussion during the development of this manuscript.", revision_no = "13", abstract = "The weakening of tropical overturning circulations is a robust response to global warming in climate models and observations. However, there remain open questions on the causes of this change and the extent to which this weakening affects individual circulation features such as the Walker circulation. The study presents idealized GCM simulations of a Walker circulation forced by prescribed ocean heat flux convergence in a slab ocean, where the longwave opacity of the atmosphere is varied to simulate a wide range of climates. The weakening of the Walker circulation with warming results from an increase in gross moist stability (GMS), a measure of the tropospheric moist static energy (MSE) stratification, which provides an effective static stability for tropical circulations. Baroclinic mode theory is used to determine changes in GMS in terms of the tropical-mean profiles of temperature and MSE. The GMS increases with warming, owing primarily to the rise in tropopause height, decreasing the sensitivity of the Walker circulation to zonally anomalous net energy input. In the absence of large changes in net energy input, this results in a rapid weakening of the Walker circulation with global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/77086, title ="Numerics and subgrid-scale modeling in large eddy simulations of stratocumulus clouds", author = "Pressel, Kyle G. and Mishra, Siddhartha", journal = "Journal of Advances in Modeling Earth Systems", volume = "9", number = "2", pages = "1342-1365", month = "June", year = "2017", doi = "10.1002/2016MS000778", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170501-080154912", note = "© 2017 The Authors. This is an open access article under the\nterms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.\n\nReceived 16 AUG 2016; Accepted 22 APR 2017; Accepted article online 29 APR 2017; Published online 7 JUN 2017.\n\nThe authors would like to thank Jon Reisner for his helpful comments during this work. This work was supported by the Swiss National Science Foundation (grant 200021-156109) and the European Research Council (grant STG NN. 306279 SPARCCLE). The PyCLES code is freely available at www.climate-dynamics.org. LES data are available from the corresponding author upon request.", revision_no = "16", abstract = "Stratocumulus clouds are the most common type of boundary layer cloud; their radiative effects strongly modulate climate. Large eddy simulations (LES) of stratocumulus clouds often struggle to maintain fidelity to observations because of the sharp gradients occurring at the entrainment interfacial layer at the cloud top. The challenge posed to LES by stratocumulus clouds is evident in the wide range of solutions found in the LES intercomparison based on the DYCOMS-II field campaign, where simulated liquid water paths for identical initial and boundary conditions varied by a factor of nearly 12. Here we revisit the DYCOMS-II RF01 case and show that the wide range of previous LES results can be realized in a single LES code by varying only the numerical treatment of the equations of motion and the nature of subgrid-scale (SGS) closures. The simulations that maintain the greatest fidelity to DYCOMS-II observations are identified. The results show that using weighted essentially non-oscillatory (WENO) numerics for all resolved advective terms and no explicit SGS closure consistently produces the highest-fidelity simulations. This suggests that the numerical dissipation inherent in WENO schemes functions as a high-quality, implicit SGS closure for this stratocumulus case. Conversely, using oscillatory centered difference numerical schemes for momentum advection, WENO numerics for scalars, and explicitly modeled SGS fluxes consistently produces the lowest-fidelity simulations. We attribute this to the production of anomalously large SGS fluxes near the cloud tops through the interaction of numerical error in the momentum field with the scalar SGS model.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/91898, title ="Large-eddy simulation of subtropical cloud-topped boundary layers: 2. Cloud response to climate change", author = "Tan, Zhihong and Schneider, Tapio", journal = "Journal of Advances in Modeling Earth Systems", volume = "9", number = "1", pages = "19-38", month = "March", year = "2017", doi = "10.1002/2016ms000804", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181218-154520884", note = "© 2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. \n\nReceived 12 SEP 2016; Accepted 18 NOV 2016; Accepted article online 20 DEC 2016; Published online 20 JAN 2017. \n\nThis work was supported by the U.S. National Science Foundation (grant CCF‐1048575), by Caltech's Terrestrial Hazard Observation and Reporting (THOR) Center, and by the Swiss National Science Foundation. The numerical simulations are performed on the Euler Cluster operated by the high performance computing (HPC) team at ETH Zürich. We also thank Chris Bretherton and Peter Blossey for helpful discussions about the experimental design and interpretation of results. The PyCLES code and the configurations for the idealized climate change experiments are available online at climate-dynamics.org/software.", revision_no = "9", abstract = "How subtropical marine boundary layer (MBL) clouds respond to warming is investigated using large‐eddy simulations (LES) of a wide range of warmer climates, with CO_2 concentrations elevated by factors 2–16. In LES coupled to a slab ocean with interactive sea surface temperatures (SST), the surface latent heat flux (LHF) is constrained by the surface energy balance and only strengthens modestly under warming. Consequently, the MBL in warmer climates is shallower than in corresponding fixed‐SST LES, in which LHF strengthens excessively and the MBL typically deepens. The inferred shortwave (SW) cloud feedback with a closed energy balance is weakly positive for cumulus clouds. It is more strongly positive for stratocumulus clouds, with a magnitude that increases with warming. Stratocumulus clouds generally break up above 6 K to 9 K warming, or above a four to eightfold increase in CO_2 concentrations. This occurs because the MBL mixing driven by cloud‐top longwave (LW) cooling weakens as the LW opacity of the free troposphere increases. The stratocumulus breakup triggers an abrupt and large SST increase and MBL deepening, which cannot occur in fixed‐SST experiments. SW cloud radiative effects generally weaken while the lower‐tropospheric stability increases under warming—the reverse of their empirical relation in the present climate. The MBL is deeper and stratocumulus persists into warmer climates if large‐scale subsidence decreases as the climate warms. The contrasts between experiments with interactive SST and fixed SST highlight the importance of a closed surface energy balance for obtaining realizable responses of MBL clouds to warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/74380, title ="Storm-Track Shifts under Climate Change: Toward a Mechanistic Understanding Using Baroclinic Mean Available Potential Energy", author = "Mbengue, Cheikh and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "74", number = "1", pages = "93-110", month = "January", year = "2017", doi = "10.1175/JAS-D-15-0267.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170216-124419096", note = "© 2017 American Meteorological Society. \n\n(Manuscript received 4 September 2015, in final form 23 September 2016) \n\nWe are grateful for the financial support provided by the National Science Foundation (Grant AGS-1019211).", revision_no = "8", abstract = "Zonal-mean storm-track shifts in response to perturbations in climate occur even in idealized simulations of dry atmospheres with axisymmetric forcing. Nonetheless, a generally accepted theory of the mechanisms controlling the storm-track shifts is still lacking. Here, mean available potential energy (MAPE), a general measure of baroclinicity that is proportional to the square of the Eady growth rate, is used to understand storm-track shifts. It is demonstrated that, in dry atmospheres, the eddy kinetic energy (EKE) in a storm track is linearly related to the mean available potential energy, relative to a local reference state, and that maxima of the two are generally collocated in latitude. Changes in MAPE with climate are then decomposed into components. It is shown that in simulations of dry atmospheres, changes in the latitude of maximum MAPE are dominated by changes in near-surface meridional temperature gradients. By contrast, changes in the magnitude of MAPE are primarily determined by changes in static stability and in the depth of the troposphere. A theory of storm-track shifts may build upon these findings and primarily needs to explain changes in near-surface meridional temperature gradients. The terminus of the Hadley circulation often shifts in tandem with storm tracks and is hypothesized to play an important role in triggering the storm-track shifts seen in this idealized dry context, especially in simulations where increases only in the convective static stability in the deep tropics suffice to shift storm tracks poleward.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/91899, title ="Large-eddy simulation of subtropical cloud‐topped boundary layers: 1. A forcing framework with closed surface energy balance", author = "Tan, Zhihong and Schneider, Tapio", journal = "Journal of Advances in Modeling Earth Systems", volume = "8", number = "4", pages = "1565-1585", month = "December", year = "2016", doi = "10.1002/2016MS000655", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181218-155745542", note = "© 2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. \n\nReceived 17 FEB 2016; Accepted 16 SEP 2016; Accepted article online 21 SEP 2016; Published online 8 OCT 2016. \n\nThis work was supported by the U.S. National Science Foundation (grant CCF‐1048575), by Caltech's Terrestrial Hazard Observation and Reporting (THOR) Center, and by the Swiss National Science Foundation. The numerical simulations were performed on the Euler Cluster operated by the high performance computing (HPC) team at ETH Zürich. The PyCLES codes and the configurations for the new forcing framework are available online at climate-dynamics.org/software. We also thank Colleen Kaul for her contributions to the microphysics scheme in PyCLES.", revision_no = "8", abstract = "Large‐eddy simulation (LES) of clouds has the potential to resolve a central question in climate dynamics, namely, how subtropical marine boundary layer (MBL) clouds respond to global warming. However, large‐scale processes need to be prescribed or represented parameterically in the limited‐area LES domains. It is important that the representation of large‐scale processes satisfies constraints such as a closed energy balance in a manner that is realizable under climate change. For example, LES with fixed sea surface temperatures usually do not close the surface energy balance, potentially leading to spurious surface fluxes and cloud responses to climate change. Here a framework of forcing LES of subtropical MBL clouds is presented that enforces a closed surface energy balance by coupling atmospheric LES to an ocean mixed layer with a sea surface temperature (SST) that depends on radiative fluxes and sensible and latent heat fluxes at the surface. A variety of subtropical MBL cloud regimes (stratocumulus, cumulus, and stratocumulus over cumulus) are simulated successfully within this framework. However, unlike in conventional frameworks with fixed SST, feedbacks between cloud cover and SST arise, which can lead to sudden transitions between cloud regimes (e.g., stratocumulus to cumulus) as forcing parameters are varied. The simulations validate this framework for studies of MBL clouds and establish its usefulness for studies of how the clouds respond to climate change.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70677, title ="The organization of Jupiter’s upper tropospheric temperature structure and its evolution, 1996–1997", author = "Fisher, Brendan M. and Orton, Glenn S.", journal = "Icarus", volume = "280", pages = "268-277", month = "December", year = "2016", doi = "10.1016/j.icarus.2016.07.016", issn = "0019-1035", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-083859656", note = "© 2016 Elsevier Inc. \n\nReceived 29 July 2015, Revised 16 July 2016, Accepted 25 July 2016, Available online 3 August 2016. \n\nThe authors wish to thank the staff of the NASA Infrared Telescope Facility for their support of the observations reported here. \n\nBMF acknowledges support from NASA/National Research Council Associateships Program. GSO acknowledges support from the Galileo mission. This work was also supported by the NASA Planetary Astronomy, Planetary Atmospheres, and Outer Planets Research Programs (grant NNX10AQ05G). It was performed at the Jet Propulsion Laboratory/California Institute of Technology under contract with the National Aeronautics and Space Administration.", revision_no = "9", abstract = "High signal-to-noise images of Jupiter were made at wavelengths between 13.2 and 22.8 µm in five separate observing runs between 1996 June and 1997 November at the NASA Infrared Telescope Facility. Maps of Jupiter’s upper-tropospheric temperatures at pressures of 100 and 400 mbar were made from these images. We use the relatively frequent, well sampled data sets to examine in detail the short-term evolution of the temperature structure. Our 2–6 month sampling periods demonstrate that the longitudinal temperature structures evolve significantly in these short periods and exhibit wave features. Using a three-dimensional general circulation model simulation of Jupiter’s upper atmosphere, we show that the thermal structures are consistent with convectively generated Rossby waves that propagate upward from the lower to the upper atmosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/73316, title ="Narrowing of the ITCZ in a warming climate: Physical mechanisms", author = "Byrne, Michael P. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "43", number = "21", pages = "11350-11357", month = "November", year = "2016", doi = "10.1002/2016GL070396", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170106-154835521", note = "© 2016 American Geophysical Union. \n\nReceived 11 JUL 2016; Accepted 14 OCT 2016; Accepted article online 22 OCT 2016; Published online 9 NOV 2016. \n\nWe thank Paulo Ceppi, Angie Pendergrass, and Robert Wills for helpful discussions. We also thank two anonymous reviewers for their comments and suggestions.", revision_no = "15", abstract = "The Intertropical Convergence Zone (ITCZ) narrows in response to global warming in both observations and climate models. However, a physical understanding of this narrowing is lacking. Here we show that the narrowing of the ITCZ in simulations of future climate is related to changes in the moist static energy (MSE) budget. MSE advection by the mean circulation and MSE divergence by transient eddies tend to narrow the ITCZ, while changes in net energy input to the atmosphere and the gross moist stability tend to widen the ITCZ. The narrowing tendency arises because the meridional MSE gradient strengthens with warming, whereas the largest widening tendency is due to increasing shortwave heating of the atmosphere. The magnitude of the ITCZ narrowing depends strongly on the gross moist stability and clouds, emphasizing the need to better understand these fundamental processes in the tropical atmosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70725, title ="Seasonal and Interannual Variations of the Energy Flux Equator and ITCZ. Part II: Zonally Varying Shifts of the ITCZ", author = "Adam, Ori and Bischoff, Tobias", journal = "Journal of Climate", volume = "29", number = "20", pages = "7281-7293", month = "October", year = "2016", doi = "10.1175/JCLI-D-15-0710.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-151837626", note = "© 2016 American Meteorological Society. \n\nManuscript received 6 October 2015, in final form 13 July 2016. \n\nWe thank Momme Hell for his visualization advice.", revision_no = "13", abstract = "The ITCZ lies at the ascending branch of the tropical meridional overturning circulation, where near-surface meridional mass fluxes vanish. Near the ITCZ, column-integrated energy fluxes vanish, forming an atmospheric energy flux equator (EFE). This paper extends existing approximations relating the ITCZ position and EFE to the atmospheric energy budget by allowing for zonal variations. The resulting relations are tested using reanalysis data for 1979–2014. The zonally varying EFE is found as the latitude where the meridional component of the divergent atmospheric energy transport (AET) vanishes. A Taylor expansion of the AET around the equator relates the ITCZ position to derivatives of the AET. To a first order, the ITCZ position is proportional to the divergent AET across the equator; it is inversely proportional to the local atmospheric net energy input (NEI) that consists of the net energy fluxes at the surface, at the top of the atmosphere, and zonally across longitudes. The first-order approximation captures the seasonal migrations of the ITCZ in the African, Asian, and Atlantic sectors. In the eastern Pacific, a third-order approximation captures the bifurcation from single- to double-ITCZ states that occurs during boreal spring. In contrast to linear EFE theory, during boreal winter in the eastern Pacific, northward cross-equatorial AET goes along with an ITCZ north of the equator. EFE and ITCZ variations driven by ENSO are characterized by an equatorward (poleward) shift in the Pacific during El Niño (La Niña) episodes, which are associated with variations in equatorial ocean energy uptake.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70729, title ="Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection", author = "Brient, Florent and Schneider, Tapio", journal = "Journal of Climate", volume = "29", number = "16", pages = "5821-5835", month = "August", year = "2016", doi = "10.1175/JCLI-D-15-0897.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-152816638", note = "© 2016 American Meteorological Society. \n\nManuscript received 16 December 2015, in final form 11 May 2016. \n\nWe thank Jennifer Kay for making available the combined CloudSat and CALIPSO data used in Fig. 1, Davide Panosetti for drafting Fig. 1, and Tobias Bischoff and Zhihong Tan for helpful discussions. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1) for producing and making available their model output. This research was supported by the Swiss National Science Foundation (Grant 200021-156109).", revision_no = "11", abstract = "Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by −(0.96 ± 0.22)% K^(−1) (90% confidence level) for deseasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r = −0.67) with the models’ ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K; an ECS below 2.3 K becomes very unlikely (90% confidence).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69143, title ="Contrasting responses to orbital precession on Titan and Earth", author = "Liu, Junjun and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "43", number = "14", pages = "7774-7780", month = "July", year = "2016", doi = "10.1002/2016GL070065", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160720-153809020", note = "© 2016 American Geophysical Union. \n\nAccepted manuscript online: 19 July 2016; Manuscript Revised: 15 July 2016; Manuscript Accepted: 15 July 2016; Manuscript Received: 8 April 2016. \n\nThis work was supported by the NASA Outer Planets Research Program (Grant NNX10AQ05G) and by the National Science Foundation (Grant AGS-1049201). The program code for the Titan GCM is available at climate-dynamics.org. Simulation output is available from the authors upon request.", revision_no = "19", abstract = "Earth and Titan exhibit contrasting atmospheric responses to orbital precession. On Earth, most (water) precipitation falls in low latitudes, and precipitation is enhanced in a hemisphere when perihelion occurs in that hemisphere's summer. On Titan, most (methane) precipitation falls in high latitudes, and precipitation is enhanced in a hemisphere when aphelion occurs in that hemisphere's summer. We use a Titan general circulation model to elucidate the dynamical reasons for these different responses to orbital precession. They arise primarily because of the different diurnal rotation rates of Titan and Earth. The slower rotation rate of Titan leads to wider Hadley cells that transport moisture into polar regions. Changes in the length of summer, rather than in the intensity of summer insolation as in Earth's tropics, then dominate the precession response of the hydrologic cycle.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70726, title ="Relation of the double-ITCZ bias to the atmospheric energy budget in climate models", author = "Adam, Ori and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "43", number = "14", pages = "7670-7677", month = "July", year = "2016", doi = "10.1002/2016GL069465", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-152157215", note = "© 2016 American Geophysical Union. \n\nReceived 5 MAY 2016; Accepted 17 JUN 2016; Accepted article online 24 JUN 2016; Published online 18 JUL 2016. \n\nWe thank Michael Byrne for his helpful comments. We thank the National Center for Atmospheric Research (NCAR) Climate and Global Dynamics Laboratory (CGD) for providing the ERA-Interim energy budget products. This research was supported by grants from the Swiss National Science Foundation and the U.S. National Science Foundation (grant AGS-1049201).", revision_no = "23", abstract = "We examine how tropical zonal mean precipitation biases in current climate models relate to the atmospheric energy budget. Both hemispherically symmetric and antisymmetric tropical precipitation biases contribute to the well-known double-Intertropical Convergence Zone (ITCZ) bias; however, they have distinct signatures in the energy budget. Hemispherically symmetric biases in tropical precipitation are proportional to biases in the equatorial net energy input; hemispherically antisymmetric biases are proportional to the atmospheric energy transport across the equator. Both relations can be understood within the framework of recently developed theories. Atmospheric net energy input biases in the deep tropics shape both the symmetric and antisymmetric components of the double-ITCZ bias. Potential causes of these energetic biases and their variation across climate models are discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69072, title ="Energetic Constraints on the Width of the Intertropical Convergence Zone", author = "Byrne, Michael P. and Schneider, Tapio", journal = "Journal of Climate", volume = "29", number = "13", pages = "4709-4721", month = "July", year = "2016", doi = "10.1175/JCLI-D-15-0767.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160715-154332491", note = "© 2016 American Meteorological Society. \n\nManuscript received 28 October 2015, in final form 9 February 2016. \n\nWe thank Marty Singh, Adam Sobel, and two anonymous reviewers for helpful comments on this work.", revision_no = "9", abstract = "The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70707, title ="Shallowness of tropical low clouds as a predictor of climate models’ response to warming", author = "Brient, Florent and Schneider, Tapio", journal = "Climate Dynamics", volume = "47", number = "1-2", pages = "433-449", month = "July", year = "2016", doi = "10.1007/s00382-015-2846-0", issn = "0930-7575", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-135131589", note = "© 2015 Springer-Verlag Berlin Heidelberg. \n\nReceived: 11 March 2015; Accepted: 19 September 2015; Published online: 5 October 2015. \n\nThis work was supported by the Department of Energy’s Regional and Global Climate Modeling Program under the project “Identifying Robust Cloud Feedbacks in Observations and Model”. We thank Bjorn Stevens, Louise Nuijens, Steve Klein and Peter Caldwell for useful discussions on this topic and two anonymous reviewers for their insightful comments on the manuscript. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1) for producing and making available their model output.", revision_no = "11", abstract = "How tropical low clouds change with climate remains the dominant source of uncertainty in global warming projections. An analysis of an ensemble of CMIP5 climate models reveals that a significant part of the spread in the models’ climate sensitivity can be accounted by differences in the climatological shallowness of tropical low clouds in weak-subsidence regimes: models with shallower low clouds in weak-subsidence regimes tend to have a higher climate sensitivity than models with deeper low clouds. The dynamical mechanisms responsible for the model differences are analyzed. Competing effects of parameterized boundary-layer turbulence and shallow convection are found to be essential. Boundary-layer turbulence and shallow convection are typically represented by distinct parameterization schemes in current models—parameterization schemes that often produce opposing effects on low clouds. Convective drying of the boundary layer tends to deepen low clouds and reduce the cloud fraction at the lowest levels; turbulent moistening tends to make low clouds more shallow but affects the low-cloud fraction less. The relative importance different models assign to these opposing mechanisms contributes to the spread of the climatological shallowness of low clouds and thus to the spread of low-cloud changes under global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69071, title ="Thermodynamic and dynamic controls on changes in the zonally anomalous hydrological cycle", author = "Wills, Robert C. and Byrne, Michael P.", journal = "Geophysical Research Letters", volume = "43", number = "9", pages = "4640-4649", month = "May", year = "2016", doi = "10.1002/2016GL068418", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160715-153714338", note = "© 2016 American Geophysical Union. \n\nReceived 24 FEB 2016; Accepted 19 APR 2016; Accepted article online 25 APR 2016; Published online 14 MAY 2016. \n\nThe data used in this study are publicly available as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) and accessible via the Earth System Grid Federation. We thank Ori Adam for his work in developing the data portal GOAT (www.goat-geo.org), which was used to obtain the data for this study. We thank Aaron Donohoe and an anonymous reviewer for useful input on this manuscript.", revision_no = "15", abstract = "The wet gets wetter, dry gets drier paradigm explains the expected moistening of the extratropics and drying of the subtropics as the atmospheric moisture content increases with global warming. Here we show, using precipitation minus evaporation (P − E) data from climate models, that it cannot be extended to apply regionally to deviations from the zonal mean. Wet and dry zones shift substantially in response to shifts in the stationary-eddy circulations that cause them. Additionally, atmospheric circulation changes lead to a smaller increase in the zonal variance of P − E than would be expected from atmospheric moistening alone. The P − E variance change can be split into dynamic and thermodynamic components through an analysis of the atmospheric moisture budget. This reveals that a weakening of stationary-eddy circulations and changes in the zonal variation of transient-eddy moisture fluxes moderate the strengthening of the zonally anomalous hydrological cycle with global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/67513, title ="How Stationary Eddies Shape Changes in the Hydrological Cycle: Zonally Asymmetric Experiments in an Idealized GCM", author = "Wills, Robert C. and Schneider, Tapio", journal = "Journal of Climate", volume = "29", number = "9", pages = "3161-3179", month = "May", year = "2016", doi = "10.1175/JCLI-D-15-0781.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160601-080822648", note = "© 2016 American Meteorological Society. \n\nManuscript received 31 October 2015, in final form 9 February 2016. \n\nThis research has been supported by NSF Grant AGS-1019211. The idealized GCM simulations for this study were performed on ETH Zürich’s EULER computing cluster. We thank Michael Byrne, Xavier Levine, Timothy Cronin, and an anonymous reviewer for useful comments and discussion during the development of this manuscript.", revision_no = "8", abstract = "Stationary and low-frequency Rossby waves are the primary drivers of extratropical weather variations on monthly and longer time scales. They take the form of persistent highs and lows, which, for example, shape subtropical dry zones and guide extratropical storms. More generally, stationary-eddy circulations, including zonally anomalous tropical overturning circulations, set up large zonal variations in net precipitation (precipitation minus evaporation, P − E). This paper investigates the response of stationary eddies and the zonally asymmetric hydrological cycle to global warming in an idealized GCM, simulating a wide range of climates by varying longwave absorption. The stationary eddies are forced by two idealized zonal asymmetries: a midlatitude Gaussian mountain and an equatorial ocean heat source. Associated with changes in stationary eddies are changes in the zonal variation of the hydrological cycle. Particularly in the subtropics, these simulations show a nearly constant or decreasing amplitude of the zonally anomalous hydrological cycle in climates warmer than modern despite the wet gets wetter, dry gets drier effect associated with increasing atmospheric moisture content. An approximation for zonally anomalous P − E, based on zonal-mean surface specific humidity and stationary-eddy vertical motion, disentangles the roles of thermodynamic and dynamic changes. The approximation shows that changes in the zonally asymmetric hydrological cycle are predominantly controlled by changes in lower-tropospheric vertical motion in stationary eddies.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/67532, title ="Seasonal and Interannual Variations of the Energy Flux Equator and ITCZ. Part I: Zonally Averaged ITCZ Position", author = "Adam, Ori and Bischoff, Tobias", journal = "Journal of Climate", volume = "29", number = "9", pages = "3219-3230", month = "April", year = "2016", doi = "10.1175/JCLI-D-15-0512.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160601-095902123", note = "© 2016 American Meteorological Society. \n\nManuscript received 23 July 2015, in final form 4 February 2016. \n\nWe thank Momme Hell for his visualization advice.", revision_no = "11", abstract = "In the zonal mean, the ITCZ lies at the foot of the ascending branch of the tropical mean meridional circulation, close to where the near-surface meridional mass flux vanishes. The ITCZ also lies near the energy flux equator (EFE), where the column-integrated meridional energy flux vanishes. This latter observation makes it possible to relate the ITCZ position to the energy balance, specifically the atmospheric net energy input near the equator and the cross-equatorial energy flux. Here the validity of the resulting relations between the ITCZ position and energetic quantities is examined with reanalysis data for the years 1979–2014. In the reanalysis data, the EFE and ITCZ position indeed covary on time scales of seasons and longer. Consistent with theory, the ITCZ position is proportional to the cross-equatorial atmospheric energy flux and inversely proportional to atmospheric net energy input at the equator. Variations of the cross-equatorial energy flux dominate seasonal variations of the ITCZ position. By contrast, variations of the equatorial net energy input, driven by ocean energy uptake variations, dominate interannual variations of the ITCZ position (e.g., those associated with ENSO).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/67198, title ="The Equatorial Energy Balance, ITCZ Position, and Double-ITCZ Bifurcations", author = "Bischoff, Tobias and Schneider, Tapio", journal = "Journal of Climate", volume = "29", number = "8", pages = "2997-3013", month = "April", year = "2016", doi = "10.1175/JCLI-D-15-0328.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160520-084733761", note = "© 2016 American Meteorological Society. \n\nManuscript received 6 May 2015, in final form 3 August 2015. Published online 12 April 2016. \n\nThis research was supported by a grant from the National Science Foundation (AGS-1049201). The idealized GCM simulations were performed on Caltech’s Geological and Planetary Sciences CITerra and on ETH Zurich’s EULER computing clusters. We thank Simona Bordoni and Anne Laraia for helpful discussions of drafts of this paper. We also acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups for producing and making available their model output, which we used in Fig. 1. For CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We also thank John Fasullo and Kevin Trenberth from the National Center for Atmospheric Research for providing the energy flux data (retrieved from https://climatedataguide.ucar.edu/climate-data/era-interim-derived-components) we used in some of the estimates in the text.", revision_no = "14", abstract = "The intertropical convergence zone (ITCZ) migrates north–south on seasonal and longer time scales. Previous studies have shown that the zonal-mean ITCZ displacement off the equator is negatively correlated with the energy flux across the equator; when the ITCZ lies in the Northern Hemisphere, energy flows southward across the equator, and vice versa. The hemisphere that exports energy across the equator is the hemisphere with more net energy input, and it is usually the warmer hemisphere. But states with a double ITCZ straddling the equator also occur, for example, seasonally over the eastern Pacific and frequently in climate models. Here it is shown how the ITCZ position is connected to the energy balance near the equator in a broad range of circumstances, including states with single and double ITCZs. Taylor expansion of the variation of the meridional energy flux around the equator leads to the conclusion that for large positive net energy input into the equatorial atmosphere, the ITCZ position depends linearly on the cross-equatorial energy flux. For small positive equatorial net energy input, the dependence of the ITCZ position on the cross-equatorial energy flux weakens to the third root. When the equatorial net energy input or its curvature become negative, a bifurcation to double-ITCZ states occurs. Simulations with an idealized aquaplanet general circulation model (GCM) confirm the quantitative adequacy of these relations. The results provide a framework for assessing and understanding causes of common climate model biases and for interpreting tropical precipitation changes, such as those evident in records of climates of the past.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/66207, title ="Cumulant expansions for atmospheric flows", author = "Ait-Chaalal, Farid and Schneider, Tapio", journal = "New Journal of Physics", volume = "18", number = "2", pages = "Art. No. 025019", month = "February", year = "2016", doi = "10.1088/1367-2630/18/2/025019", issn = "1367-2630", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160415-082349241", note = "© 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. For paper published after November 2012, authors, their institutions and third parties all have the same rights to reuse articles published in New Journal of Physics in accordance with the Creative Commons Attribution 3.0 Unported (CC-BY) license. \n\nPublished 19 February 2016. \n\nCollection: Focus on Stochastic Flows and Climate Statistics. \n\nWe thank Kyle Pressel for helping in the development of the QL LES and Joe Skitka for sharing results about the oceanic boundary layer. We are grateful for helpful discussions with Freddy Bouchet, Greg Chini, Baylor Fox-Kemper, Cesare Nardini, and Steve Tobias. The comments of two anonymous reviewers improved the manuscript significantly. This work was supported by the US National Science Foundation under grants CCF-1048575 (FAC and TS) and CCF-1048701 (FAC and JBM).", revision_no = "10", abstract = "Atmospheric flows are governed by the equations of fluid dynamics. These equations are nonlinear, and consequently the hierarchy of cumulant equations is not closed. But because atmospheric flows are inhomogeneous and anisotropic, the nonlinearity may manifest itself only weakly through interactions of nontrivial mean fields with disturbances such as thermals or eddies. In such situations, truncations of the hierarchy of cumulant equations hold promise as a closure strategy. Here we show how truncations at second order can be used to model and elucidate the dynamics of turbulent atmospheric flows. Two examples are considered. First, we study the growth of a dry convective boundary layer, which is heated from below, leading to turbulent upward energy transport and growth of the boundary layer. We demonstrate that a quasilinear truncation of the equations of motion, in which interactions of disturbances among each other are neglected but interactions with mean fields are taken into account, can capture the growth of the convective boundary layer. However, it does not capture important turbulent transport terms in the turbulence kinetic energy budget. Second, we study the evolution of two-dimensional large-scale waves, which are representative of waves seen in Earth's upper atmosphere. We demonstrate that a cumulant expansion truncated at second order (CE2) can capture the evolution of such waves and their nonlinear interaction with the mean flow in some circumstances, for example, when the wave amplitude is small enough or the planetary rotation rate is large enough. However, CE2 fails to capture the flow evolution when strongly nonlinear eddy–eddy interactions that generate small-scale filaments in surf zones around critical layers become important. Higher-order closures can capture these missing interactions. The results point to new ways in which the dynamics of turbulent boundary layers may be represented in climate models, and they illustrate different classes of nonlinear processes that can control wave dissipation and angular momentum fluxes in the upper troposphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/62447, title ="Superrotation in Terrestrial Atmospheres", author = "Laraia, Anne L. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "72", number = "11", pages = "4281-4296", month = "November", year = "2015", doi = "10.1175/JAS-D-15-0030.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151130-112437219", note = "© 2015 American Meteorological Society. \n\nManuscript received 22 January 2015, in final form 2 July 2015. \n\nSome of the scaling results in this paper were presented at the 18th Conference on Atmospheric and Oceanic Fluid Dynamics in 2011. The research was supported by the U.S. National Science Foundation through Grant AGS-1049201 and a Graduate Research Fellowship.", revision_no = "7", abstract = "Atmospheric superrotation with prograde equatorial winds and an equatorial angular momentum maximum is ubiquitous in planetary atmospheres. It is clear that eddy fluxes of angular momentum toward the equator are necessary to generate it. But under what conditions superrotation arises has remained unclear. This paper presents simulations and a scaling theory that establish conditions under which superrotation occurs in terrestrial atmospheres. Whether superrotation arises depends on the relative importance of factors that favor or disfavor superrotation. Convection preferentially generates Rossby waves near the equator, where the Rossby number is O(1). Since the Rossby waves transport angular momentum toward their source regions, this favors superrotation. Meridional temperature gradients preferentially lead to baroclinic instability and wave generation away from the equator. Eddy transport of angular momentum toward the baroclinic source region implies transport out of low latitudes, which disfavors superrotation. Simulations with an idealized GCM show that superrotation tends to arise when the equatorial convective generation of wave activity and its associated eddy angular momentum flux convergence exceed the baroclinic eddy angular momentum flux divergence. Convective and baroclinic wave activity generation is related through scaling arguments to mean-flow properties, such as planetary rotation rates and meridional temperature gradients. The scaling arguments show, for example, that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low, as they are, for example, on Venus. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global warming simulations.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/62083, title ="Large-eddy simulation in an anelastic framework with closed water and entropy balances", author = "Pressel, Kyle G. and Kaul, Colleen M.", journal = "Journal of Advances in Modeling Earth Systems", volume = "7", number = "3", pages = "1425-1456", month = "September", year = "2015", doi = "10.1002/2015MS000496", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151112-153703322", note = "© 2015 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. \n\nReceived 9 JUN 2015; Accepted 20 AUG 2015; Accepted article online 22 AUG 2015; Published online 26 SEP 2015. \n\nThis work was supported by the U.S. National Science Foundation (grants ARC-1107795 and CCF-1048575), by Caltech’s Terrestrial Hazard Observation and Reporting (THOR) Center, and by the Swiss National Science Foundation. We thank João Teixeira for stimulating discussions and Cheikh Mbengue, Bettina Meyer, David Raymond, and Sally Zhang for their help and suggestions during the development of PyCLES. The PyCLES code is freely available at climate-dynamics.org/software.", revision_no = "10", abstract = "A large-eddy simulation (LES) framework is developed for simulating the dynamics of clouds and boundary layers with closed water and entropy balances. The framework is based on the anelastic equations in a formulation that remains accurate for deep convection. As prognostic variables, it uses total water and entropy, which are conserved in adiabatic and reversible processes, including reversible phase changes of water. This has numerical advantages for modeling clouds, in which reversible phase changes of water occur frequently. The equations of motion are discretized using higher-order weighted essentially nonoscillatory (WENO) discretization schemes with strong stability preserving time stepping. Numerical tests demonstrate that the WENO schemes yield simulations superior to centered schemes, even when the WENO schemes are used at coarser resolution. The framework is implemented in a new LES code written in Python and Cython, which makes the code transparent and easy to use for a wide user group.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/59270, title ="Baroclinic Eddies and the Extent of the Hadley Circulation: An Idealized GCM Study", author = "Levine, Xavier J. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "72", number = "7", pages = "2744-2761", month = "July", year = "2015", doi = "10.1175/JAS-D-14-0152.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150806-143422294", note = "© 2015 American Meteorological Society.\n\nManuscript received 27 May 2014, in final form 31 January 2015.\n\nWe thank Paul O’Gorman for helpful clarifications on the effective static stability, Timothy M. Merlis for discussions on both linear baroclinic wave theories and ocean–atmosphere interactions, and Tobias Bischoff for his helpful comments on this work and its relation to ENSO. We are grateful for support by the National Science Foundation (Grants AGS-1019211 and AGS-1049201) and a Yale Climate and Energy Institute Fellowship. The simulations were performed on the Division of Geological and Planetary Sciences’ Dell cluster at the California Institute of Technology (the program code for the simulations described in this paper is available at www.clidyn.ethz.ch/gcms/).", revision_no = "8", abstract = "The Hadley circulation has widened over the past 30 years. This widening has been qualitatively reproduced in general circulation model (GCM) simulations of a warming climate. Comprehensive GCM studies suggest this widening may be caused by a poleward shift in baroclinic eddy activity. Yet the limited amplitude of the climate change signals analyzed so far precludes a quantitative comparison with theories.\n\nThis study uses two idealized GCMs, one with and one without an active hydrologic cycle, to investigate changes in the extent of the Hadley circulation over a wide range of climates. The climates span global-mean temperatures from 243 to 385 K and equator-to-pole temperature contrasts from 12 to 100 K. Baroclinic eddies control the extent of the Hadley circulation across most of these climates. A supercriticality criterion that quantifies the depth of baroclinic eddies relative to that of the troposphere turns out to be a good indicator of where baroclinic eddies become deep enough to terminate the Hadley circulation. The supercriticality depends on meridional temperature gradients and an effective stability that accounts for the effect of convective heating on baroclinic eddies.\n\nAs the equator-to-pole temperature contrast weakens or the convective static stability increases, convective heating increasingly influences the thermal stratification of the troposphere and the supercriticality. Consistent with the supercriticality criterion, the Hadley circulation contracts as meridional temperature gradients increase, and it widens as the effective static stability increases. The former occurs during El Niño and may account for the observed Hadley circulation contraction then; the latter occurs during global warming.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/59009, title ="Stationary Eddies and the Zonal Asymmetry of Net Precipitation and Ocean Freshwater Forcing", author = "Wills, Robert C. and Schneider, Tapio", journal = "Journal of Climate", volume = "28", number = "13", pages = "5115-5133", month = "July", year = "2015", doi = "10.1175/JCLI-D-14-00573.1 ", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150724-110437816", note = "© 2015 American Meteorological Society. \n\nManuscript received 16 August 2014, in final form 19 February 2015. \n\nThis research has been supported by NSF Grant AGS-1019211. We thank Ori Adam for his work in developing the data portal GOAT (www.goat-geo.org), which was used to obtain the data for this study. We thank James Rae, Tobias Bischoff, and Momme Hell for useful comments and discussion during the development of this draft.", revision_no = "13", abstract = "Transport of water vapor in the atmosphere generates substantial spatial variability of net precipitation\n(precipitation minus evaporation). Over half of the total spatial variability in annual-mean net precipitation is\naccounted for by deviations from the zonal mean. Over land, these regional differences determine differences in\nsurface water availability. Over oceans, they account, for example, for the Pacific–Atlantic difference in sea\nsurface salinity, with implications for the deep overturning circulation. This study analyzes the atmospheric-water\nbudget in reanalyses from ERA-Interim and MERRA, to investigate which physical balances lead to zonal\nvariation in net precipitation. It is found that the leading-order contribution is zonal variation in stationary-eddy\nvertical motion. Transient eddies modify the pattern of zonally anomalous net precipitation by moving moisture\nfrom the subtropical and tropical oceans onto land and poleward across the Northern Hemisphere storm tracks.\nZonal variation in specific humidity and stationary-eddy horizontal advection play a secondary role. The dynamics\nleading to net precipitation via vertical motion in stationary eddies can be understood from a lower-tropospheric\nvorticity budget. The large-scale variations of vertical motion are primarily described by Sverdrup\nbalance and Ekman pumping, with some modification by transient eddies. These results suggest that it is important\nto understand changes in stationary eddies and their influence on the zonal variation of transient eddy\nfluxes, in order to understand regional changes in net precipitation. They highlight the relative importance of\ndifferent atmospheric mechanisms for the freshwater forcing of the North Pacific and North Atlantic.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57772, title ="Interannual Variability in the Large-Scale Dynamics of the South Asian Summer Monsoon", author = "Walker, Jennifer M. and Bordoni, Simona", journal = "Journal of Climate", volume = "28", number = "9", pages = "3731-3750", month = "May", year = "2015", doi = "10.1175/JCLI-D-14-00612.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150522-100044286", note = "© 2015 American Meteorological Society.\n\nReceived: September 2, 2014; Final Form: December 24, 2014.\n\nThis work was supported by a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship, the Caltech Terrestrial Hazard Observation and Reporting Center, and National Science Foundation Grants AGS-1019211 and AGS-1049201. The data analyses were conducted on Caltech’s Division of Geological and Planetary Science CITerra computing cluster.", revision_no = "7", abstract = "This study identifies coherent and robust large-scale atmospheric patterns of interannual variability of the South Asian summer monsoon (SASM) in observational data. A decomposition of the water vapor budget into dynamic and thermodynamic components shows that interannual variability of SASM net precipitation (P − E) is primarily caused by variations in winds rather than in moisture. Linear regression analyses reveal that strong monsoons are distinguished from weak monsoons by a northward expansion of the cross-equatorial monsoonal circulation, with increased precipitation in the ascending branch. Interestingly, and in disagreement with the view of monsoons as large-scale sea-breeze circulations, strong monsoons are associated with a decreased meridional gradient in the near-surface atmospheric temperature in the SASM region. Teleconnections exist from the SASM region to the Southern Hemisphere, whose midlatitude poleward eddy energy flux correlates with monsoon strength. Possible implications of these teleconnection patterns for understanding SASM interannual variability are discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/53068, title ="Martian atmospheric collapse: Idealized GCM studies", author = "Soto, Alejandro and Mischna, Michael", journal = "Icarus", volume = "250", pages = "553-569", month = "April", year = "2015", doi = "10.1016/j.icarus.2014.11.028", issn = "0019-1035", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141222-085606925", note = "© 2014 Elsevier B.V.\n\nReceived 10 June 2013, Revised 23 November 2014, Accepted 25 November 2014, Available online 4 December 2014\n\nWe have benefited from numerous conversations with Andrew Ingersoll and Aaron Wolf, of Caltech, and Itay Halevy, of the Weizmann Institute of Science. Additionally, the comments from the two anonymous reviewers greatly improved this paper. The simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster as well as the Pleiades supercomputer at the NASA Advanced Supercomputing Division at NASA’s Ames Research Center. The NASA Mars Fundamental Research program, under grant NNH07ZDA001N, funded this research.", revision_no = "18", abstract = "Global energy balance models of the Martian atmosphere predict that, for a range of total CO_2 inventories, the CO_2 atmosphere may condense until a state with a permanent polar cap is reached. This process, which is commonly referred to as atmospheric collapse, may limit the time available for physical and chemical weathering. The global energy balance models that predict atmospheric collapse represent the climate using simplified parameterizations for atmospheric processes such as radiative transfer and atmospheric heat transport. However, a more detailed representation of these atmospheric processes is critical when the atmosphere is near a transition, such as the threshold for collapse. Therefore, we use the Mars Weather Research and Forecasting (MarsWRF) general circulation model (GCM) to investigate how the explicit representation of meridional heat transport and more detailed radiative transfer affects the onset of atmospheric collapse. Using MarsWRF, we find that previous energy balance modeling underestimates the range of CO_2 inventories for which the atmosphere collapses and that the obliquity of Mars determines the range of CO_2 inventories that can collapse. For a much larger range of CO_2 inventories than expected, atmospheric heat transport is insufficient to prevent the atmospheric collapse. We show that the condensation of CO_2 onto Olympus Mons and adjacent mountains generates a condensation flow. This condensation flow syphons energy that would otherwise be transported poleward, which helps explain the large range of CO_2 inventories for which the atmosphere collapses.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57115, title ="Why Eddy Momentum Fluxes are Concentrated in the Upper Troposphere", author = "Ait-Chaalal, Farid and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "72", number = "4", pages = "1585-1604", month = "April", year = "2015", doi = "10.1175/JAS-D-14-0243.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150430-130410543", note = "© 2015 American Meteorological Society.\n\nManuscript received 26 August 2014, in final form 28 November 2014.\n\nThis work was supported by the U.S. National Science Foundation Grants CCF-1048575 and CCF-1048701. We thank Brad Marston for useful discussions about quasi-linear approaches and for suggesting investigating the role of barotropic triads (section 4e). We also thank Freddy Bouchet and Cesare Nardini for useful discussions about the Orr mechanism at the Kavli Institute for Theoretical Physics (KITP) summer 2014 program on wave–flow interaction in geophysics, climate, astrophysics, and plasmas.", revision_no = "8", abstract = "The extratropical eddy momentum flux (EMF) is controlled by generation, propagation, and dissipation of large-scale eddies and is concentrated in Earth’s upper troposphere. An idealized GCM is used to investigate how this EMF structure arises. In simulations in which the poles are heated more strongly than the equator, EMF is concentrated near the surface, demonstrating that surface drag generally is not responsible for the upper-tropospheric EMF concentration. Although Earth’s upper troposphere favors linear wave propagation, quasi-linear simulations in which nonlinear eddy–eddy interactions are suppressed demonstrate that this is likewise not primarily responsible for the upper-tropospheric EMF concentration. The quasi-linear simulations reveal the essential role of nonlinear eddy–eddy interactions in the surf zone in the upper troposphere, where wave activity absorption away from the baroclinic generation regions occurs through the nonlinear generation of small scales. In Earth-like atmospheres, wave activity that is generated in the lower troposphere propagates upward and then turns meridionally, eventually being absorbed nonlinearly in the upper troposphere. The level at which the wave activity begins to propagate meridionally appears to be set by the typical height reached by baroclinic eddies. This can coincide with the tropopause height but also can lie below it if convection controls the tropopause height. In the latter case, EMF is maximal well below the tropopause. The simulations suggest that EMF is concentrated in Earth’s upper troposphere because typical baroclinic eddies reach the tropopause.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/56558, title ="Physics of Changes in Synoptic Midlatitude Temperature Variability", author = "Schneider, Tapio and Bischoff, Tobias", journal = "Journal of Climate", volume = "28", number = "6", pages = "2312-2331", month = "March", year = "2015", doi = "10.1175/JCLI-D-14-00632.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150409-152635552", note = "© 2015 American Meteorological Society. \n\nManuscript received 11 September 2014, in final form 2 December 2014. \n\nWe thank Erich Fischer and James Screen for helpful comments on this work. The research was supported by the National Science Foundation (Grants ARC-1107795 and AGS-1049201). Many calculations presented here were performed using the Geophysical Observation Analysis Tool (GOAT), a freely available MATLAB-based tool for retrieval, analysis, and visualization of geophysical data (http://www.goat-geo.org). We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1 of this paper) for producing and making available their model output. For CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals.", revision_no = "16", abstract = "This paper examines the physical processes controlling how synoptic midlatitude temperature variability near the surface changes with climate. Because synoptic temperature variability is primarily generated by advection, it can be related to mean potential temperature gradients and mixing lengths near the surface. Scaling arguments show that the reduction of meridional potential temperature gradients that accompanies polar amplification of global warming leads to a reduction of the synoptic temperature variance near the surface. This is confirmed in simulations of a wide range of climates with an idealized GCM. In comprehensive climate simulations (CMIP5), Arctic amplification of global warming similarly entails a large-scale reduction of the near-surface temperature variance in Northern Hemisphere midlatitudes, especially in winter. The probability density functions of synoptic near-surface temperature variations in midlatitudes are statistically indistinguishable from Gaussian, both in reanalysis data and in a range of climates simulated with idealized and comprehensive GCMs. This indicates that changes in mean values and variances suffice to account for changes even in extreme synoptic temperature variations. Taken together, the results indicate that Arctic amplification of global warming leads to even less frequent cold outbreaks in Northern Hemisphere winter than a shift toward a warmer mean climate implies by itself.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/54789, title ="Scaling of Off-Equatorial Jets in Giant Planet Atmospheres", author = "Liu, Junjun and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "72", number = "1", pages = "389-408", month = "January", year = "2015", doi = "10.1175/JAS-D-13-0391.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150212-130610956", note = "© 2015 American Meteorological Society. Manuscript received 7 December 2013, in final form 3 September 2014.\n\nThis work was supported by the NASA Outer Planets Research Program (Grant NNX10AQ05G) and by the National Science Foundation (Grant AGS-1049201).", revision_no = "7", abstract = "In the off-equatorial region of Jupiter’s and Saturn’s atmospheres, baroclinic eddies transport angular momentum out of retrograde and into prograde jets. In a statistically steady state, this angular momentum transfer by eddies must be balanced by dissipation, likely produced by magnetohydrodynamic (MHD) drag in the planetary interior. This paper examines systematically how an idealized representation of this drag in a general circulation model (GCM) of the upper atmosphere of giant planets modifies jet characteristics, the angular momentum budget, and the energy budget.\n\nIn the GCM, Rayleigh drag at an artificial lower boundary (with mean pressure of 3 bar) is used as a simple representation of the MHD drag that the flow on giant planets experiences at depth. As the drag coefficient decreases, the eddy length scale and eddy kinetic energy increase, as they do in studies of two-dimensional turbulence. Off-equatorial jets become wider and stronger, with increased interjet spacing. Coherent vortices also become more prevalent. Generally, the jet width scales with the Rhines scale, which is of similar magnitude as the Rossby radius in the simulations. The jet strength increases primarily through strengthening of the barotropic component, which increases as the drag coefficient decreases because the overall kinetic energy dissipation remains roughly constant. The overall kinetic energy dissipation remains roughly constant presumably because it is controlled by baroclinic conversion of potential to kinetic energy in the upper troposphere, which is mainly determined by the differential solar radiation and is only weakly dependent on bottom drag and barotropic flow variations.\n\nFor Jupiter and Saturn, these results suggest that the wider and stronger jets on Saturn may arise because the MHD drag on Saturn is weaker than on Jupiter, while the thermodynamic efficiencies of the atmospheres are not sensitive to the drag parameters.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/51528, title ="Role of Changes in Mean Temperatures versus Temperature Gradients in the Recent Widening of the Hadley Circulation", author = "Adam, Ori and Schneider, Tapio", journal = "Journal of Climate", volume = "27", number = "19", pages = "7450-7461", month = "October", year = "2014", doi = "10.1175/JCLI-D-14-00140.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141110-141049554", note = "© 2014 American Meteorological Society. Manuscript received 21 February 2014, in final form 17 June 2014.\n\nThis research was supported by the U.S. National Science Foundation (Grant AGS-1049201) and Israeli Science Foundation Grant 1537/12. All calculations presented here were performed using the Geophysical Observation Analysis Tool (GOAT), a freely available MATLAB-based tool for retrieval, analysis, and visualization of geophysical data (http://www.goat-geo.org).", revision_no = "11", abstract = "The Hadley circulation (HC) has widened in recent decades, and it widens as the climate warms in simulations. But the mechanisms responsible for the widening remain unclear, and the widening in simulations is generally smaller than observed.\nTo identify mechanisms responsible for the HC widening and for model–observation discrepancies, this study analyzes how interannual variations of tropical-mean temperatures and meridional temperature gradients influence the HC width. Changes in mean temperatures are part of any global warming signal, whereas changes in temperature gradients are primarily associated with ENSO. Within this study, 6 reanalysis datasets, 22 Atmospheric Modeling Intercomparison Project (AMIP) simulations, and 11 historical simulations from phase 5 of the Climate Modeling Intercomparison Project (CMIP5) are analyzed, covering the years 1979–2012. It is found that the HC widens as mean temperatures increase or as temperature gradients weaken in most reanalyses and climate models. On average, climate models exhibit a smaller sensitivity of HC width to changes in mean temperatures and temperature gradients than do reanalyses. However, the sensitivities differ substantially among reanalyses, rendering the HC response to mean temperatures in climate models not statistically different from that in reanalyses.\nWhile global-mean temperatures did not increase substantially between 1997 and 2012, the HC continued to widen in most reanalyses. The analysis here suggests that the HC widening from 1979 to 1997 is primarily the result of global warming, whereas the widening of the HC from 1997 to 2012 is associated with increased midlatitude temperatures and hence reduced temperature gradients during this period.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/49323, title ="Migrations and dynamics of the intertropical convergence zone", author = "Schneider, Tapio and Bischoff, Tobias", journal = "Nature", volume = "513", number = "7516", pages = "45-53", month = "September", year = "2014", doi = "10.1038/nature13636", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140908-094122453", note = "© 2014 Macmillan Publishers Limited. Received 25 November 2013. Accepted 01 July 2014. Published online 03 September 2014. J. Fasullo and K. Trenberth from the National Center for Atmospheric Research provided the energy flux data we used in Figs 1b and 5 and in some of the estimates in the text. The top-of-atmosphere radiative flux estimates in the\ntext are based on NASA Clouds and the Earth’s Radiant Energy System (CERES) data, version CERES EBAF-TOA Ed2.7. S. Marcott provided the temperature reconstructions in Fig. 3, and M. Hell drew Figs 4 and 5. We are grateful for discussions with D. Sigman and N.Meckler and for comments on drafts by F. Ait-Chaalal, A.Donohoe, R. Ferrari, and\nJ.-E. Lee. The research underlying this paper was supported by grants from the US National Science Foundation (numbers AGS-1019211, AGS-1049201 and AGS-1003614).", revision_no = "14", abstract = "Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Niño events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/49045, title ="Constraining the depth of Saturn’s zonal winds by measuring thermal and gravitational signals", author = "Liu, Junjun and Schneider, Tapio", journal = "Icarus", volume = "239", pages = "260-272", month = "September", year = "2014", doi = "10.1016/j.icarus.2014.05.036", issn = "0019-1035", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140829-083755173", note = "© 2014 Elsevier Inc. Received 9 October 2013. Revised 6 March 2014. Accepted 23 May 2014. Available online 16 June 2014. This work was supported by the NASA Outer Planets Research\nProgram (Grant NNX10AQ05G), by the National Science Foundation\n(Grant AGS-1049201), and by a Royal Society Fellowship at\nthe University of Oxford.", revision_no = "11", abstract = "Based on straightforward dynamical considerations, we show how available and upcoming measurements of Saturn’s thermal and gravitational signals can be used to constrain the depth to which its zonal winds penetrate. The dynamical considerations issue from the facts that Saturn has a strong intrinsic heat flux, rotates rapidly, and has negligible atmospheric viscosity. As a result, convective motions align with surfaces of constant specific angular momentum, which are, away from the equator, approximately cylinders concentric with the planet’s spin axis. Convective motions in the interior therefore tend to homogenize entropy in the direction of the spin axis, but not necessarily perpendicular to it. Using the assumption of interior entropy homogenization in the direction of the spin axis, we determine the zonal winds and their associated thermal and gravitational signals by combining thermal wind balance, the equation of state, the observed zonal winds at the cloud level, and estimates of the strength of the magnetohydrodynamic (MHD) drag that zonal winds experience in the deep interior.\n\nWe find zonal winds likely extend deeply into Saturn, to a depth between about 0.630.63 and 0.83R_S(with Saturn’s radius R_S), or to pressures between 1.4 and 0.3 Mbar. The equation of state of hydrogen constrains zonal winds with strengths similar to the cloud level winds to be confined within the outer few percent of Saturn’s radius, with substantially weaker winds below, irrespective of where in the range of plausible estimates Saturn’s imprecisely known rotation rate falls. Depending on the rotation rate and the precise depth to which zonal winds penetrate, we estimate that the meridional equator-to-pole temperature contrasts in thermal wind balance with the inferred zonal winds increase with depth and reach 1–2 K at 1 bar and 2–4 K at 5 bar. They would be much larger if the cutoff radii of the zonal winds were much shallower than we estimate, but thermal observations by the Cassini Composite Infrared Spectrometer (CIRS) already rule out very shallow flows: Zonal winds must extend deeper than 5000 bar (0.965R_S) because otherwise the associated equator-to-pole contrast would be inconsistent with Cassini CIRS retrievals of the temperature field.\n\nUpcoming Cassini gravitational measurements can further constrain the penetration depth of zonal winds, as gravitational zonal harmonics associated with deep zonal winds are much larger than those associated with shallow zonal winds. The even gravitational zonal harmonics associated with zonal winds that penetrate to megabar levels start to become distinguishable from the planetary solid body rotation at zonal harmonic degree n≳8n≳8. The low-order odd gravitational zonal harmonics, which do not depend on the planetary solid body rotation, will be detectable within likely Cassini measurement errors for cutoff radii r_c≲0.98R_S (≳1000 bar). Combining thermal and gravitational signals of the zonal winds, the penetration depths of the zonal winds relative to different rotation rates can thus be constrained.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47513, title ="Energetic Constraints on the Position of the Intertropical Convergence Zone", author = "Bischoff, Tobias and Schneider, Tapio", journal = "Journal of Climate", volume = "27", number = "13", pages = "4937-4951", month = "July", year = "2014", doi = "10.1175/JCLI-D-13-00650.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-091222072", note = "© 2014 American Meteorological Society. \n\nReceived: October 22, 2013; Final Form: March 12, 2014. \n\nWe thank Gerald Haug for stimulating discussions during the course of this research and Simona Bordoni and Robert Wills for helpful comments on drafts of the paper. Comments by two anonymous reviewers greatly helped to improve the quality of this paper. This work was supported by NSF Grants AGS-1019211, AGS-1049201, and AGS-1003614. Idealized\nGCM simulations were performed on Caltech’s Division\nof Geological and Planetary Science CITerra computing\ncluster.", revision_no = "20", abstract = "The intertropical convergence zone (ITCZ) can shift meridionally on seasonal and longer time scales. Previous studies have shown that the latitude of the ITCZ is negatively correlated with cross-equatorial atmospheric energy transport. For example, the ITCZ shifts southward as the Northern Hemisphere cools and the northward cross-equatorial energy transport strengthens in response. It has remained unclear what controls the sensitivity of the ITCZ position to cross-equatorial energy transport and what other factors may lead to shifts of the ITCZ position. Here it is shown that the sensitivity of the ITCZ position to cross-equatorial energy transport depends on the net energy input to the equatorial atmosphere: the net radiative energy input minus any energy uptake by the oceans. Changes in this energy input can also lead to ITCZ shifts. The cross-equatorial energy transport is related through a series of approximations to interhemispheric asymmetries in the near-surface temperature distribution. The resulting theory of the ITCZ position is tested in idealized general circulation model simulations with a slab ocean as lower boundary condition. In the simulations, cross-equatorial energy transport increases under global warming (primarily because extratropical latent energy fluxes strengthen), and this shifts the ITCZ poleward. The ITCZ shifts equatorward if primarily the tropics warm in response to an increased net energy input to the equatorial atmosphere. The results have implications for explaining the varied response of the ITCZ to global or primarily tropical changes in the atmospheric energy balance, such as those that occur under global warming or El Niño.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43227, title ="Storm Track Shifts under Climate Change: What Can Be Learned from Large-Scale Dry Dynamics", author = "Mbengue, Cheikh and Schneider, Tapio", journal = "Journal of Climate", volume = "26", number = "24", pages = "9923-9930", month = "December", year = "2013", doi = "10.1175/JCLI-D-13-00404.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140106-112443566", note = "© 2013 American Meteorological Society.\n\nManuscript received 10 July 2013, in final form 2 October 2013.\n\nWe are grateful for the financial support provided by the National Science Foundation\n(Grant AGS-1019211) and for helpful discussions with Xavier Levine.", revision_no = "9", abstract = "Earth’s storm tracks are instrumental for transporting heat, momentum, and moisture and thus strongly influence the surface climate. Climate models, supported by a growing body of observational data, have demonstrated that storm tracks shift poleward as the climate warms. But the dynamical mechanisms responsible for this shift remain unclear. To isolate what portion of the storm track shift may be accounted for by large-scale dry dynamics alone, disregarding the latent heat released in phase changes of water, this study investigates the storm track shift under various kinds of climate change in an idealized dry general circulation model (GCM) with an adjustable but constant convective stability. It is found that increasing the mean surface temperature or the convective stability leads to poleward shifts of storm tracks, even if the convective stability is increased only in a narrow band around the equator. Under warming and convective stability changes roughly corresponding to a doubling of CO_2 concentrations from a present-day Earthlike climate, storm tracks shift about 0.8° poleward, somewhat less than but in qualitative agreement with studies using moist GCMs. About 63% (0.5°) of the poleward shift is shown to be caused by tropical convective stability variations. This demonstrates that tropical processes alone (the increased dry static stability of a warmer moist adiabat) can account for part of the poleward shift of storm tracks under global warming. This poleward shift generally occurs in tandem with a poleward expansion of the Hadley circulation; however, the Hadley circulation expansion does not always parallel the storm track shift.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41051, title ="The Role of Stationary Eddies in Shaping Midlatitude Storm Tracks", author = "Kaspi, Yohai and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "70", number = "8", pages = "2596-2613", month = "August", year = "2013", doi = "10.1175/JAS-D-12-082.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130903-102819440", note = "© 2013 American Meteorological Society. \n\nManuscript received 7 March 2012, in final form 28 February 2013.\n\nThis research has been supported by the NOAA Climate and Global Change Postdoctoral\nFellowship administered by the University Corporation\nfor Atmospheric Research, by a David and Lucile\nPackard Fellowship, by NSF Grant AGS-1019211, and\nby a Marie Curie Career Integration Grant CIG-304202.\nWe thank Simona Bordoni, Xavier Levine, Tim Merlis,\nand Adam Sobel for very helpful discussions during the\npreparation of this manuscript. The simulations were\nperformed on Caltech’s Division of Geological and\nPlanetary Sciences Dell cluster.", revision_no = "13", abstract = "Transient and stationary eddies shape the extratropical climate through their transport of heat, moisture, and momentum. In the zonal mean, the transports by transient eddies dominate over those by stationary eddies, but this is not necessarily the case locally. In particular, in storm-track entrance and exit regions during winter, stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport. Here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity. Simulations with an idealized aquaplanet GCM show that zonally localized surface heating alone (e.g., ocean heat flux convergence) gives rise to storm tracks, which have a well-defined length scale that is similar to that of Earth's storm tracks. The storm tracks terminate downstream of the surface heating even in the absence of continents, at a distance controlled by the stationary Rossby wavelength scale. Stationary eddies play a dual role: within about half a Rossby wavelength downstream of the heating region, stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track; farther downstream, enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability. Transports both of sensible and latent heat (water vapor) play important roles in determining where storm tracks terminate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39902, title ="Wind driven capillary-gravity waves on Titan’s lakes: Hard to detect or non-existent?", author = "Hayes, A. G. and Lorenz, R. D.", journal = "Icarus", volume = "225", number = "1", pages = "403-412", month = "July", year = "2013", doi = "10.1016/j.icarus.2013.04.004", issn = "0019-1035", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130813-150947854", note = "© 2013 Elsevier Inc. Received 3 March 2012. Revised 12 March 2013. Accepted 10 April 2013. Available online 19 April 2013. A.G.H. would like to the thank the Miller Institute for Basic Research in Science for partially funding this work. R.L., J.I.L., and P.E. were partially supported by the Cassini Project (for R.L. via NASA\nGrant NNX13AH 14G). T.S. and S.D.G. acknowledge support from\na NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship. We would especially like to thank and acknowledge H.E. Schlichting of the Massachusetts Institute of Technology for her advice and contributions to this work and Richard Lovelace of Cornell University, whose inquiries supplied the motivation for this project. All authors would also like to thank the Cassini engineering team, without whom this manuscript would not have been possible.", revision_no = "15", abstract = "Saturn’s moon Titan has lakes and seas of liquid hydrocarbon and a dense atmosphere, an environment conducive to generating wind waves. Cassini observations thus far, however, show no indication of waves. We apply models for wind wave generation and detection to the Titan environment. Results suggest wind speed thresholds at a reference altitude of 10 m of 0.4–0.7 m/s for liquid compositions varying between pure methane and equilibrium mixtures with the atmosphere (ethane has a threshold of 0.6 m/s), varying primarily with liquid viscosity. This reduced threshold, as compared to Earth, results from Titan’s increased atmosphere-to-liquid density ratio, reduced gravity and lower surface tension. General Circulation Models (GCMs) predict wind speeds below derived thresholds near equinox, when available observations of lake surfaces have been acquired. Predicted increases in winds as Titan approaches summer solstice, however, will exceed expected thresholds and may provide constraints on lake composition and/or GCM accuracy through the presence or absence of waves during the Cassini Solstice Mission. A two-scale microwave backscatter model suggests that returns from wave-modified liquid hydrocarbon surfaces may be below the pixel-scale noise floor of Cassini radar images, but can be detectable using real-aperture scatterometry, pixel binning and/or observations obtained in a specular geometry.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39344, title ="The Force Balance of the Southern Ocean Meridional Overturning Circulation", author = "Mazloff, Matthew R. and Ferrari, Raffaele", journal = "Journal of Physical Oceanography", volume = "43", number = "6", pages = "1193-1208", month = "June", year = "2013", issn = "0022-3670", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130712-132448635", note = "© 2013 American Meteorological Society.\nManuscript received 9 April 2012, in final form 15 February 2013.\nWe acknowledge the National\nScience Foundation (NSF) for support of this research\nthrough Grants OCE-1233832, OCE-1234473, and OPP-\n0961218. SOSE was produced using the Extreme Science\nand Engineering Discovery Environment (XSEDE),\nwhich is supported by National Science Foundation Grant\nMCA06N007.\n", revision_no = "12", abstract = "The Southern Ocean (SO) limb of the meridional overturning circulation (MOC) is characterized by three vertically stacked cells, each with a transport of about 10 Sv (Sv ≡ 10^6 m^3 s^(−1)). The buoyancy transport in the SO is dominated by the upper and middle MOC cells, with the middle cell accounting for most of the buoyancy transport across the Antarctic Circumpolar Current. A Southern Ocean state estimate for the years 2005 and 2006 with 1/6° resolution is used to determine the forces balancing this MOC. Diagnosing the zonal momentum budget in density space allows an exact determination of the adiabatic and diapycnal components balancing the thickness-weighted (residual) meridional transport. It is found that, to lowest order, the transport consists of an eddy component, a directly wind-driven component, and a component in balance with mean pressure gradients. Nonvanishing time-mean pressure gradients arise because isopycnal layers intersect topography or the surface in a circumpolar integral, leading to a largely geostrophic MOC even in the latitude band of Drake Passage. It is the geostrophic water mass transport in the surface layer where isopycnals outcrop that accomplishes the poleward buoyancy transport.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38823, title ="Predictions of thermal and gravitational signals of Jupiter’s deep zonal winds", author = "Liu, Junjun and Schneider, Tapio", journal = "Icarus", volume = "224", number = "1", pages = "114-125", month = "May", year = "2013", doi = "10.1016/j.icarus.2013.01.025", issn = "0019-1035", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130606-083406918", note = "© 2013 Elsevier Inc.\n\nReceived 15 March 2012;\nRevised 27 January 2013;\nAccepted 28 January 2013;\nAvailable online 21 February 2013.\n\nThis work was supported by a David and Lucile Packard Fellowship and by the NASA Outer Planets Research Program (Grant NNX10AQ05G). We thank Peter Goldreich for helpful discussions.", revision_no = "13", abstract = "NASA’s Juno spacecraft will make microwave and gravity measurements of Jupiter. These can reveal information about the composition of Jupiter’s atmosphere and about the temperature and density structure below the visible clouds, which is in balance with the structure of the zonal winds. Here we show that there exist strong physical constraints on the structure of the off-equatorial deep zonal winds, and that these imply dynamical constraints on the thermal and gravitational signals Juno will measure. The constraints derive from the facts that Jupiter is rapidly rotating, has nearly inviscid flow, and has strong intrinsic heat fluxes emanating from the deep interior. Because of the strong intrinsic heat fluxes, Jupiter’s interior is convecting, but the rapid rotation and weak viscosity constrain the convective motions away from the equator to occur primarily along cylinders parallel to the planet’s spin axis. As a consequence, convection is expected to approximately homogenize entropy along the spin axis, thereby adjusting the interior to a convectively and inertially nearly neutral state. In this state, entropy gradients perpendicular to the spin axis are constant but generally not zero on cylinders concentric with the spin axis. Additionally, thermal wind balance relates entropy gradients perpendicular to the spin axis to the zonal wind shear between the observed cloud-level winds and winds in the deep interior (pressures of order 10^6 bar), which must be much weaker because otherwise the Ohmic energy dissipation produced by the interaction of the zonal winds with the planetary magnetic field would exceed the planetary luminosity. Combining these physical constraints with thermal and electrical properties of the atmosphere, we obtain that zonal winds away from the equator likely extend deeply into Jupiter (to a depth between about 0.84R_J and 0.94R_J with Jupiter radius R_J) but have strengths similar to cloud level winds only within the outer few percent of Jupiter’s radius. Meridional equator-to-pole temperature contrasts in thermal wind balance with the zonal winds increase with depth and reach ∼1–2 K at 50 bar; they would reach O(10 K) if the winds were shallowly confined, as has been proposed previously. Such temperature contrasts will be detectable by Juno’s microwave instrument and are expected to be much larger than those associated with variations in water vapor abundance. The associated gravitational signals of the zonal winds will also be detectable by Juno, but they will be more difficult to distinguish from those implied by other flow models with deep zonal flows. The combination of Juno’s gravity and microwave instruments should be able to distinguish deep flows (detectable gravitational signals) from shallow flows (detectable thermal signals), providing strong constraints on the penetration depth of substantial zonal winds.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38112, title ="The Tropical Precipitation Response to Orbital Precession", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of Climate", volume = "26", number = "6", pages = "2010-2021", month = "March", year = "2013", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130425-090941220", note = "© 2013 American Meteorological Society.\n\nManuscript received 1 April 2012, in final form 30 July 2012.\n\nWe appreciate comments by Andy\nThompson, Jess Adkins, and three anonymous reviewers.\nThis work was supported by a National Science\nFoundation Graduate Research Fellowship, a Princeton\nCenter for Theoretical Science Fellowship, and National\nScience Foundation Grant AGS-1049201. The program\ncode for the simulations, based on the Flexible Modeling\nSystem of the Geophysical Fluid Dynamics Laboratory,\nas well as the simulation results themselves, are available\nfrom the authors upon request.", revision_no = "11", abstract = "Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius–Clapeyron relationship. Here it is argued that a different nonlinearity related to moisture advection by the atmospheric circulation is more important. When perihelion changes from one hemisphere’s summer solstice to the other’s in an idealized aquaplanet atmospheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into “thermodynamic” and “dynamic” components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land–sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precession-induced seasonal humidity changes correlate with the seasonal cycle in low-level convergence. This interpretation is confirmed using simulations in which the Clausius–Clapeyron relationship is explicitly linearized. The thermodynamic mechanism also operates in simulations with an idealized representation of land, although in these simulations the dynamic component of the precipitation changes is also important, adding to the thermodynamic precipitation changes in some latitudes and offsetting it in others.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/37401, title ="Hadley Circulation Response to Orbital Precession. Part I: Aquaplanets", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of Climate", volume = "26", number = "3", pages = "740-753", month = "February", year = "2013", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130308-075759181", note = "© 2013 American Meteorological Society.\n\n\nManuscript received 6 December 2011, in final form 24 July 2012.\n\nA careful reading of Tim Merlis’s\nPh.D. thesis by Andy Thompson is greatly appreciated.\nWe thank Rob Korty and Damianos Mantsis for helpful\ncomments on the manuscript. This work was supported\nby a National Science Foundation Graduate Research\nFellowship, a Princeton Center for Theoretical Science\nFellowship, and National Science Foundation Grant\nAGS-1049201. The program codes for the simulations,\nbased on the Flexible Modeling System of the Geophysical\nFluid Dynamics Laboratory, as well as the\nsimulation results themselves, are available from the\nauthors upon request.", revision_no = "12", abstract = "The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with an aquaplanet slab-ocean lower boundary. Contrary to expectations, the simulated monsoonal Hadley circulation is weaker when perihelion occurs at the summer solstice than when aphelion occurs at the summer solstice. The angular momentum balance and energy balance are examined to understand the mechanisms that produce this result. That the summer with stronger insolation has a weaker circulation is the result of an increase in the atmosphere’s energetic stratification, the gross moist stability, which increases more than the amount required to balance the change in atmospheric energy flux divergence necessitated by the change in top-of-atmosphere net radiation. The solstice-season changes result in annual-mean Hadley circulation changes (e.g., changes in circulation strength).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39774, title ="Continental arc–island arc fluctuations, growth of crustal carbonates, and long-term climate change", author = "Lee, Cin-Ty A. and Shen, Bing", journal = "Geosphere", volume = "9", number = "1", pages = "21-36", month = "February", year = "2013", doi = "10.1130/GES00822.1", issn = "1553-040X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130806-093803561", note = "© 2012 Geological Society of America.\n\nReceived 21 May 2012; Revision received 3 October 2012; Accepted 22 October 2012; Published online 13 December 2012.\n\nWe thank the Packard Foundation, the Miller Institute\nat the University of California–Berkeley, the\nAtmosphere and Ocean Research Institute of the University\nof Tokyo, and the National Science Foundation\nfor indirect support. We thank D. Morton, A.W.\nBally, B. Dyer, D. Scholl, D. Schrag, W. Leeman,\nD. Thomas, and F. Marcantonio for liberating discussions,\nand A. Maloof, R. Stern, K. Putirka, L. Kump,\nI. Dalziel, and K. Burke for constructive and critical\nreviews. We also thank D. Scholl and A. Fildani for\nhelp and encouragement.", revision_no = "12", abstract = "The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO_2. Hypotheses for the elevated CO_2 concentrations invoke an increase in volcanic CO_2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO_2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO_2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO_2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO_2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (>50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/37373, title ="Hadley Circulation Response to Orbital Precession. Part II: Subtropical Continent", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of Climate", volume = "26", number = "3", pages = "754-771", month = "February", year = "2013", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130307-111836691", note = "© 2013 American Meteorological Society.\n\nReceived: March 14, 2012; Accepted: July 3, 2012.\n\nWe thank three anonymous reviewers for their comments. This work was supported by a National Science Foundation Graduate Research Fellowship, a Princeton Center for Theoretical Science Fellowship, and National Science Foundation Grant AGS-1049201. We thank Sonja Graves for providing modifications to the GCM code. The program codes for the simulations, based on the Flexible Modeling System\nof the Geophysical Fluid Dynamics Laboratory as well\nas the simulation results themselves, are available from\nthe authors upon request.", revision_no = "10", abstract = "The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with a simplified representation of land surface processes in subtropical latitudes. When perihelion occurs in the summer of a hemisphere with a subtropical continent, changes in the top-of-atmosphere energy balance, together with a poleward shift of the monsoonal circulation boundary, lead to a strengthening of the monsoonal circulation. Spatial variations in surface heat capacity determine whether radiative perturbations are balanced by energy storage or by atmospheric energy fluxes. Although orbital precession does not affect annual-mean insolation, the annual-mean Hadley circulation does respond to orbital precession because its sensitivity to radiative changes varies over the course of the year: the monsoonal circulation in summer is near the angular momentum-conserving limit and responds directly to radiative changes; whereas in winter, the circulation is affected by the momentum fluxes of extratropical eddies and is less sensitive to radiative changes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47293, title ="The imprint of surface fluxes and transport on variations in total column carbon dioxide", author = "Keppel-Aleks, G. and Wennberg, P. O.", journal = "Biogeosciences", volume = "9", number = "3", pages = "875-891", month = "March", year = "2012", doi = "10.5194/bg-9-875-2012 ", issn = "1726-4170", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140717-110834015", note = "© 2012 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. \n\nReceived: 30 June 2011; Published in Biogeosciences Discuss.: 27 July 2011; Revised: 2 January 2012; Accepted: 9 February 2012; Published: 1 March 2012. \n\nSupport for this work from NASA Carbon Cycle Program grant NNX08AI86G is gratefully acknowledged.\nGKA acknowledges fellowships from NSF and AAUW. The\nsimulations used in this study were performed on the Caltech\nDivision of Geological and Planetary Sciences Dell Cluster. Lauder\nTCCON measurements are funded by New Zealand Foundation\nof Research Science and Technology contracts C01X0204,\nC01X0703, and C01X0406. HIPPO is supported by the National\nScience Foundation and the National Ocean and Atmosphere\nAdministration. CarbonTracker 2009 results were provided by\nNOAA ESRL, Boulder, Colorado, USA from the website at\nhttp://carbontracker.noaa.gov. LEF flux tower observations were\nmade possible with assistance from A. Andrews (NOAA), J. Thom\n(UW), D. Baumann and M. Kubiske (USFS), and R. Strand and\nJ. Ayers of the Wisconsin Educational Communications Board, and\nsupported by Department of Energy (DOE) Office of Biological\nand Environmental Research (BER) National Institute for Climatic\nChange Research (NICCR) Midwestern Region Subagreement\n050516Z19 and the National Science Foundation (NSF) Biology\nDirectorate Grant DEB-0845166. RJA was sponsored by\nUS Department of Energy, Office of Science, Biological and\nEnvironmental Research (BER) programs and performed at Oak\nRidge National Laboratory (ORNL) under US Department of\nEnergy contract DE-AC05-00OR22725. We acknowledge financial support by the Senate of Bremen and the EU projects IMECC and\nGEOmon as well as maintainance and logistical work provided by\nAeroMeteo Service (Bialystok). ", revision_no = "21", abstract = "New observations of the vertically integrated CO_2 mixing ratio, ⟨CO_2⟩, from ground-based remote sensing show that variations in CO_2⟩ are primarily determined by large-scale flux patterns. They therefore provide fundamentally different information than observations made within the boundary layer, which reflect the combined influence of large-scale and local fluxes. Observations of both ⟨CO_2⟩ and CO_2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in ⟨CO_2⟩ in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO_2, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in ⟨CO_2⟩ from covariations in ⟨CO_2⟩ and potential temperature, θ, a dynamical tracer, on synoptic timescales to evaluate surface flux estimates commonly used in carbon cycle models. We find that simulations using Carnegie Ames Stanford Approach (CASA) biospheric fluxes underestimate both the ⟨CO_2⟩ seasonal cycle amplitude throughout the Northern Hemisphere midlatitudes and the meridional gradient during the growing season. Simulations using CASA net ecosystem exchange (NEE) with increased and phase-shifted boreal fluxes better fit the observations. Our simulations suggest that climatological mean CASA fluxes underestimate boreal growing season NEE (between 45–65° N) by ~40%. We describe the implications for this large seasonal exchange on inference of the net Northern Hemisphere terrestrial carbon sink.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/32974, title ="Recovery and characterization of Neptune’s near-polar stratospheric hot spot", author = "Orton, Glenn S. and Fletcher, Leigh N.", journal = "Planetary and Space Science", volume = "61", number = "1", pages = "161-167", month = "February", year = "2012", doi = "10.1016/j.pss.2011.06.013", issn = "0032-0633", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120807-101117478", note = "© 2011 Elsevier. Received 4 December 2010. Received in revised form 15 June 2011. Accepted 18 June 2011. Available online 30 June 2011. We thank Erich Karkoschka for helpful comments. Orton and Yanamandra-Fisher conducted a portion of this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. They thank Mark Hofstadter, the Principal Investigator of the relevant program, for this support. De Pater was supported by NSF Grant AST-0908575. Fletcher was supported by a Glasstone Science Fellowship at the University of Oxford. Schneider and Liu acknowledge support by a David and Lucile Packard Fellowship and by the NASA Outer Planets Research Program (Grant NNX10AQ05G).Fujiyoshi and Fuse were supported by the National Astronomical Observatory of Japan. Edwards and Geballe were supported by the Gemini Observatory, on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministrio da Cinica e Tecnologia (Brazil) and Ministerio de Cienica, Tecnologia e Innovacin Productiva (Argentina). The simulations on which Fig. 5 is based were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. The new results presented in this paper were based, in part, on observations made at the European Southern Observatory telescopes, in programs 081C-0496(A) and 083C-0163(A) and (B), obtained from the ESO/ST-ECF Science Archive Facility; on data collected at Subaru Telescope in program IDS08-032, which is operated by the National Astronomical Observatory of Japan; and on data obtained from the Gemini North Telescope in program GN-2007B-A-105 and the Gemini South Telescope in programs GS-2007B-Q-47, GS-2010B-A-42, which are operated by the Association of Universities for Research in Astronomy. ", revision_no = "35", abstract = "Images of Neptune obtained in 2006 at ESO's Very Large Telescope (Orton et al., 2007, Astronomy & Astrophysics 473, L5) revealed a near-polar hot spot near 70°S latitude that was detectable in filters sampling both stratospheric methane (7 μm) and ethane (∼12 μm) emission. Such a feature was not present in 2003 Keck and 2005 Gemini North observations, which showed only a general warming trend toward Neptune's pole that was longitudinally homogeneous. Because of the paucity of longitudinal sampling in the 2003, 2005 and 2006 images, it was not clear whether the failure to see this phenomenon in 2003 and 2005 was simply the result of insufficient longitudinal sampling or whether the phenomenon was truly variable in time. To unravel these two possibilities, we made follow-up observations on large telescopes that were capable of resolving Neptune at thermal-infrared wavelengths: Gemini South in 2007 and 2010 using the T-ReCS instrument, Subaru in 2008 using the COMICS instrument and VLT in 2008 and 2009 using the VISIR instrument. Two serendipitous T-ReCS images of Neptune were also obtained in 2007 using a broad N-band (8–14 μm) filter, whose radiance is dominated by stratospheric emission from both methane and ethane. The feature was recovered (i) in 2007 with T-ReCS in the broad N-band image and (ii) in 2008 with COMICS in a 12.5-μm image. However, T-ReCS observations in 2010 that covered up to 250° of longitude did not show evidence of an off-polar hot spot. Although we have not definitively ruled out the possibility that various observers have simply missed a semi-permanent feature, it seems statistically very unlikely to be the case. With only 3 sightings in 13 independent observing epochs, it is likely that the phenomenon is ephemeral in time. A possible origin for the phenomenon is a large planetary wave that is dynamically confined to the high-latitude regions characterized by prograde zonal winds. It may be episodically excited by dynamical activity deeper in the atmosphere. This must be coupled with mixing near the poles that destroys or at least substantially attenuates the hot spot over the south pole that leads to an appearance of the typical polar stratospheric hot spot being offset in latitude.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28790, title ="Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle", author = "Schneider, T. and Graves, S. D. B.", journal = "Nature", volume = "481", number = "7379", pages = "58-61", month = "January", year = "2012", doi = "10.1038/nature10666", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120113-130450759", note = "© 2012 Macmillan Publishers Limited. \n\nReceived 04 August 2011. Accepted 20 October 2011. Published online 04 January 2012. \n\nWe are grateful for support by a NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship. We thank I. Eisenman for code for the insolation calculations, and O. Aharonson, A. Hayes and A. Soto for comments on a draft. The simulations were done on the California Institute of Technology’s Division of Geological and Planetary Sciences Dell cluster. \n\nAuthor Contributions: T.S. and M.E.B. conceived the study; T.S., S.D.B.G. and E.L.S. developed the GCM; E.L.S. and M.E.B. provided data; and T.S. and S.D.B.G. wrote the paper, with contributions and comments from all authors. \n\nThe authors declare no competing financial interests.", revision_no = "21", abstract = "Titan has a methane cycle akin to Earth's water cycle. It has lakes in polar regions, preferentially in the north; dry low latitudes with fluvial features and occasional rainstorms; and tropospheric clouds mainly (so far) in southern middle latitudes and polar regions. Previous models have explained the low-latitude dryness as a result of atmospheric methane transport into middle and high latitudes. Hitherto, no model has explained why lakes are found only in polar regions and preferentially in the north; how low-latitude rainstorms arise; or why clouds cluster in southern middle and high latitudes. Here we report simulations with a three-dimensional atmospheric model coupled to a dynamic surface reservoir of methane. We find that methane is cold-trapped and accumulates in polar regions, preferentially in the north because the northern summer, at aphelion, is longer and has greater net precipitation than the southern summer. The net precipitation in polar regions is balanced in the annual mean by slow along-surface methane transport towards mid-latitudes, and subsequent evaporation. In low latitudes, rare but intense storms occur around the equinoxes, producing enough precipitation to carve surface features. Tropospheric clouds form primarily in middle and high latitudes of the summer hemisphere, which until recently has been the southern hemisphere. We predict that in the northern polar region, prominent clouds will form within about two (Earth) years and lake levels will rise over the next fifteen years.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28953, title ="The Relative Humidity in an Isentropic Advection–Condensation Model: Limited Poleward Influence and Properties of Subtropical Minima", author = "O’Gorman, Paul A. and Lamquin, Nicolas", journal = "Journal of the Atmospheric Sciences", volume = "68", number = "12", pages = "3079-3093", month = "December", year = "2011", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120125-085020111", note = "© 2011 American Meteorological Society.\n\nManuscript received 24 February 2011, in final form 13 June 2011.\nWe are grateful for support by the\nNational Science Foundation (Grants ATM-0450059 and\nAGS-1019211) and by a David and Lucile Packard\nFellowship. The two-dimensional turbulence simulations\nwere performed using the spectral quasigeostrophic\nmodel developed by Shafer Smith. Thanks to Tim Merlis\nfor helpful comments.", revision_no = "16", abstract = "An idealized model of advection and condensation of water vapor is considered as a representation of processes influencing the humidity distribution along isentropic surfaces in the free troposphere. Results are presented for how the mean relative humidity distribution varies in response to changes in the distribution of saturation specific humidity and in the amplitude of a tropical moisture source. Changes in the tropical moisture source are found to have little effect on the relative humidity poleward of the subtropical minima, suggesting a lack of poleward influence despite much greater water vapor concentrations at lower latitudes. The subtropical minima in relative humidity are found to be located just equatorward of the inflection points of the saturation specific humidity profile along the isentropic surface. The degree of mean subsaturation is found to vary with the magnitude of the meridional gradient of saturation specific humidity when other parameters are held fixed.\n\nThe atmospheric relevance of these results is investigated by comparison with the positions of the relative humidity minima in reanalysis data and by examining poleward influence of relative humidity in simulations with an idealized general circulation model. It is suggested that the limited poleward influence of relative humidity may constrain the propagation of errors in simulated humidity fields.\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28510, title ="Convective Generation of Equatorial Superrotation in Planetary Atmospheres\n", author = "Liu, Junjun and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "68", number = "11", pages = "2742-2756", month = "November", year = "2011", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111219-092158853", note = "© 2011 American Meteorological Society. Manuscript received 7 December 2010, in final form 6 June 2011. This work was supported by a David and Lucile Packard Fellowship and by the NASA Outer Planets Research Program (Grant NNX10AQ05G). The GCM is based on the Flexible Modeling System of the\nGeophysical Fluid Dynamics Laboratory. The simulations\nwere performed on Caltechs Division of Geological and\nPlanetary Sciences Dell cluster. ", revision_no = "13", abstract = "In rapidly rotating planetary atmospheres that are heated from below, equatorial superrotation can occur through convective generation of equatorial Rossby waves. If the heating from below is sufficiently strong that convection penetrates into the upper troposphere, then the convection generates equatorial Rossby waves, which can induce the equatorward angular momentum transport necessary for superrotation. This paper investigates the conditions under which the convective generation of equatorial Rossby waves and their angular momentum transport lead to superrotation. It also addresses how the strength and width of superrotating equatorial jets are controlled.\n\nIn simulations with an idealized general circulation model (GCM), the relative roles of baroclinicity, heating from below, and bottom drag are explored systematically. Equatorial superrotation generally occurs when the heating from below is sufficiently strong. However, the threshold heating at which the transition to superrotation occurs increases as the baroclinicity or the bottom drag increases. The greater the baroclinicity is, the stronger the angular momentum transport out of low latitudes by baroclinic eddies of extratropical origin. This competes with the angular momentum transport toward the equator by convectively generated Rossby waves and thus can inhibit a transition to superrotation. Equatorial bottom drag damps both the mean zonal flow and convectively generated Rossby waves, weakening the equatorward angular momentum transport as the drag increases; this can also inhibit a transition to superrotation. The strength of superrotating equatorial jets scales approximately with the square of their width. When they are sufficiently strong, their width, in turn, scales with the equatorial Rossby radius and thus depends on the thermal stratification of the equatorial atmosphere.\n\nThe results have broad implications for planetary atmospheres, particularly for how superrotation can be generated in giant planet atmospheres and in terrestrial atmospheres in warm climates.\n\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27888, title ="Consistent Changes in the Sea Ice Seasonal Cycle in Response to Global Warming", author = "Eisenman, Ian and Schneider, Tapio", journal = "Journal of Climate", volume = "24", number = "20", pages = "5325-5335", month = "October", year = "2011", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111121-101315922", note = "© 2011 American Meteorological Society.\n\nReceived: September 3, 2010; Accepted: March 16, 2011\n\nThis work was supported by a TPF Postdoctoral Fellowship through the Caltech Division of Geological and Planetary Sciences, a NOAA Climate and Global Change Postdoctoral Fellowship administered by the University Corporation for Atmospheric Research, a David and Lucile Packard Fellowship, and the Davidow Discovery Fund. We thank the modeling groups and the Program for Climate Model Diagnosis and Intercomparison for making available the CMIP3 multimodel dataset.", revision_no = "17", abstract = "The Northern Hemisphere sea ice cover has diminished rapidly in recent years and is projected to continue to diminish in the future. The year-to-year retreat of Northern Hemisphere sea ice extent is faster in summer than winter, which has been identified as one of the most striking features of satellite observations as well as of state-of-the-art climate model projections. This is typically understood to imply that the sea ice cover is most sensitive to climate forcing in summertime, and previous studies have explained this by calling on factors such as the surface albedo feedback. In the Southern Hemisphere, however, it is the wintertime sea ice extent that retreats fastest in climate model projections. Here, it is shown that the interhemispheric differences in the model projections can be attributed to differences in coastline geometry, which constrain where sea ice can occur. After accounting for coastline geometry, it is found that the sea ice changes simulated in both hemispheres in most climate models are consistent with sea ice retreat being fastest in winter in the absence of landmasses. These results demonstrate that, despite the widely differing rates of ice retreat among climate model projections, the seasonal structure of the sea ice retreat is robust among the models and is uniform in both hemispheres.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27776, title ="Downstream Self-Destruction of Storm Tracks", author = "Kaspi, Yohai and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "68", number = "10", pages = "2459-2464", month = "October", year = "2011", doi = "10.1175/JAS-D-10-05002.1", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111114-153036980", note = "© 2011 American Meteorological Society.\n\nManuscript received 18 November 2010, in final form 18 February 2011.\n\nWe thank Isaac Held, Xavier Levine,\nand Tim Merlis for useful discussions and comments. This\nresearch was supported by the NOAA Climate and Global\nChange Postdoctoral Fellowship administered by the University\nCorporation for Atmospheric Research, by a David\nand Lucile Packard Fellowship, and by NSF Grant\nAGS-1019211. The GCM is based on the Flexible Modeling\nSystem of the Geophysical Fluid Dynamics Laboratory;\nthe simulations were performed on Caltech’s Division\nof Geological and Planetary Sciences Dell cluster.", revision_no = "20", abstract = "The Northern Hemisphere storm tracks have maximum intensity over the Pacific and Atlantic basins; their intensity is reduced over the continents downstream. Here, simulations with an idealized aquaplanet general circulation model are used to demonstrate that even without continents, storm tracks have a self-determined longitudinal length scale. Their length is controlled primarily by the planetary rotation rate and is similar to that of Earth’s storm tracks for Earth’s rotation rate. Downstream, storm tracks self-destruct: the downstream eddy kinetic energy is lower than it would be without the zonal asymmetries that cause localized storm tracks. Likely involved in the downstream self-destruction of storm tracks are the energy fluxes associated with them. The zonal asymmetries that cause localized storm tracks enhance the energy transport through the generation of stationary eddies, and this leads to a reduced baroclinicity that persists far downstream of the eddy kinetic energy maxima.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27174, title ="Changes in Zonal Surface Temperature Gradients and Walker Circulations in a Wide Range of Climates", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of Climate", volume = "24", number = "17", pages = "4757-4768", month = "September", year = "2011", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111012-074304154", note = "© 2011 American Meteorological Society. Manuscript received 30 August 2010, in final form 9 March 2011. We thank Simona Bordoni, Isaac Held, Yohai Kaspi, Xavier Levine, and Paul O’Gorman for helpful discussions and technical assistance. The comments of two anonymous reviewers helped clarify\nthe presentation of our work. This work was supported\nby a National Defense Science and Engineering Graduate\nFellowship, a National Science Foundation Graduate\nResearch Fellowship, and a David and Lucile Packard Fellowship. The GCM simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. The program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, as well as the simulation results themselves are available from the authors upon request.", revision_no = "16", abstract = "Variations in zonal surface temperature gradients and zonally asymmetric tropical overturning circulations (Walker circulations) are examined over a wide range of climates simulated with an idealized atmospheric general circulation model (GCM). The asymmetry in the tropical climate is generated by an imposed ocean energy flux, which does not vary with climate. The range of climates is simulated by modifying the optical thickness of an idealized longwave absorber (representing greenhouse gases).\n\nThe zonal surface temperature gradient in low latitudes generally decreases as the climate warms in the idealized GCM simulations. A scaling relationship based on a two-term balance in the surface energy budget accounts for the changes in the zonally asymmetric component of the GCM-simulated surface temperature.\n\nThe Walker circulation weakens as the climate warms in the idealized simulations, as it does in comprehensive simulations of climate change. The wide range of climates allows a systematic test of energetic arguments that have been proposed to account for these changes in the tropical circulation. The analysis shows that a scaling estimate based on changes in the hydrological cycle (precipitation rate and saturation specific humidity) accounts for the simulated changes in the Walker circulation. However, it must be evaluated locally, with local precipitation rates. If global-mean quantities are used, the scaling estimate does not generally account for changes in the Walker circulation, and the extent to which it does is the result of compensating errors in changes in precipitation and saturation specific humidity that enter the scaling estimate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23743, title ="Sources of variations in total column carbon dioxide", author = "Keppel-Aleks, G. and Wennberg, P. O.", journal = "Atmospheric Chemistry and Physics", volume = "11", number = "8", pages = "3581-3593", month = "April", year = "2011", doi = "10.5194/acp-11-3581-2011", issn = "1680-7316", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110520-080425147", note = "© 2011 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. \n\nReceived: 11 November 2010. Published in Atmos. Chem. Phys. Discuss.: 16 December 2010. Revised: 15 March 2011. Accepted: 24 March 2011. Published: 18 April 2011. Article first published online: 21 Jun 2010. \n\nWe thank Peter Rayer and Dietrich Feist for helpful reviews of this manuscript. Support for this work from the NASA Carbon Cycle Program grant NNX08AI86G is gratefully acknowledged. GKA acknowledges support from a NSF graduate fellowship and an AAUW dissertation fellowship. The simulations shown were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. CarbonTracker 2009 data were provided by NOAA ESRL, Boulder, Colorado, USA from the\nwebsite at http://carbontracker.noaa.gov. \nEdited by: C. Gerbig.", revision_no = "26", abstract = "Observations of gradients in the total CO_2 column,\n(CO2), are expected to provide improved constraints\non surface fluxes of CO_2. Here we use a general circulation\nmodel with a variety of prescribed carbon fluxes to investigate how variations in (CO_2) arise. On diurnal scales, variations are small and are forced by both local fluxes and advection. On seasonal scales, gradients are set by the north-south flux distribution. On synoptic scales, variations arise due to large-scale eddy-driven disturbances of the meridional gradient. In this case, because variations in (CO_2) are tied to synoptic\nactivity, significant correlations exist between (CO_2)\nand dynamical tracers. We illustrate how such correlations\ncan be used to describe the north-south gradients of (CO_2)\nand the underlying fluxes on continental scales. These simulations suggest a novel analysis framework for using column observations in carbon cycle science.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23517, title ="Response of the Hadley Circulation to Climate Change in an Aquaplanet GCM Coupled to a Simple Representation of Ocean Heat Transport\n", author = "Levine, Xavier J. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "68", number = "4", pages = "769-783", month = "April", year = "2011", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110502-105810785", note = "© 2011 American Meteorological Society.\n\nManuscript received 18 May 2010, in final form 5 November 2010).\n\nWe thank Paul O’Gorman for\nperforming the simulations without ocean heat transport\nand for helpful comments and discussions. We are\ngrateful for support by the National Science Foundation\n(Grants ATM-0450059 and AGS-1019211), the Davidow\nDiscovery Fund, and a David and Lucile Packard Fellowship.\nThe simulations were performed on the Division\nof Geological and Planetary Sciences’s Dell\ncluster at the California Institute of Technology. The\nprogram code for the simulations described in this paper,\nand the simulation results themselves, are available\nfrom the authors upon request.", revision_no = "17", abstract = "It is unclear how the width and strength of the Hadley circulation are controlled and how they respond to climate changes. Simulations of global warming scenarios with comprehensive climate models suggest the Hadley circulation may widen and weaken as the climate warms. But these changes are not quantitatively consistent among models, and how they come about is not understood. Here, a wide range of climates is simulated with an idealized moist general circulation model (GCM) coupled to a simple representation of ocean heat transport, in order to place past and possible future changes in the Hadley circulation into a broader context and to investigate the mechanisms responsible for them.\n\nBy comparison of simulations with and without ocean heat transport, it is shown that it is essential to take low-latitude ocean heat transport and its coupling to wind stress into account to obtain Hadley circulations in a dynamical regime resembling Earth’s, particularly in climates resembling present-day Earth’s and colder. As the optical thickness of an idealized longwave absorber in the simulations is increased and the climate warms, the Hadley circulation strengthens in colder climates and weakens in warmer climates; it has maximum strength in a climate close to present-day Earth’s. In climates resembling present-day Earth’s and colder, the Hadley circulation strength is largely controlled by the divergence of angular momentum fluxes associated with eddies of midlatitude origin; the latter scale with the mean available potential energy in midlatitudes. The importance of these eddy momentum fluxes for the Hadley circulation strength gradually diminishes as the climate warms. The Hadley circulation generally widens as the climate warms, but at a modest rate that depends sensitively on how it is determined.\n\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23384, title ="Winter cold of eastern continental boundaries induced by warm ocean waters\n", author = "Kaspi, Yohai and Schneider, Tapio", journal = "Nature", volume = "471", number = "7340", pages = "621-624", month = "March", year = "2011", doi = "10.1038/nature09924 ", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110419-142949447", note = "© 2011 Macmillan Publishers Limited. Received 30 August 2010, accepted 10 February 2011, Published online 30 March 2011. We thank D. Abbot, I. Eisenman, X. Levine and T. Merlis for\ncomments. This research was supported by the National Oceanic and Atmospheric\nAdministration Climate and Global Change Postdoctoral Fellowship administered by\nthe University Corporation for Atmospheric Research (Y.K.), by a David and Lucile\nPackard Fellowship (T.S.), and by a grant from the National Science Foundation\n(AGS-1019211). The simulations were performed on the California Institute of\nTechnology’s Division of Geological and Planetary Sciences Dell cluster.\nAuthor Contributions: Y.K. and T.S. designed the study and wrote the paper; Y.K.\nperformed the numerical simulations and data analyses.", revision_no = "19", abstract = "In winter, northeastern North America and northeastern Asia are both colder than other regions at similar latitudes. This has been attributed to the effects of stationary weather systems set by elevated terrain (orography), and to a lack of maritime influences from the prevailing westerly winds. However, the differences in extent and orography between the two continents suggest that further mechanisms are involved. Here we show that this anomalous winter cold can result in part from westward radiation of large-scale atmospheric waves—nearly stationary Rossby waves—generated by heating of the atmosphere over warm ocean waters. We demonstrate this mechanism using simulations with an idealized general circulation model, with which we show that the extent of the cold region is controlled by properties of Rossby waves, such as their group velocity and its dependence on the planetary rotation rate. Our results show that warm ocean waters contribute to the contrast in mid-latitude winter temperatures between eastern and western continental boundaries not only by warming western boundaries, but also by cooling eastern boundaries.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/21452, title ="Mechanisms of Jet Formation on the Giant Planets\n", author = "Liu, Junjun and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "67", number = "11", pages = "3652-3672", month = "November", year = "2010", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20101220-141832153", note = "© 2010 American Meteorological Society.\n\nManuscript received 4 March 2010, in final form 4 June 2010.\nThis work was supported by a David\nand Lucile Packard Fellowship and by the NASA Outer\nPlanets Research Program (Grant NNX10AQ05G). The\nGCM is based on the Flexible Modeling System of the\nGeophysical Fluid Dynamics Laboratory; the simulations\nwere performed on Caltech’s Division of Geological and\nPlanetary Sciences Dell cluster. We thank Andy Ingersoll\nand Yohai Kaspi for helpful discussions and comments on\ndrafts of this paper.", revision_no = "13", abstract = "The giant planet atmospheres exhibit alternating prograde (eastward) and retrograde (westward) jets of different speeds and widths, with an equatorial jet that is prograde on Jupiter and Saturn and retrograde on Uranus and Neptune. The jets are variously thought to be driven by differential radiative heating of the upper atmosphere or by intrinsic heat fluxes emanating from the deep interior. However, existing models cannot account for the different flow configurations on the giant planets in an energetically consistent manner. Here a three-dimensional general circulation model is used to show that the different flow configurations can be reproduced by mechanisms universal across the giant planets if differences in their radiative heating and intrinsic heat fluxes are taken into account. Whether the equatorial jet is prograde or retrograde depends on whether the deep intrinsic heat fluxes are strong enough that convection penetrates into the upper troposphere and generates strong equatorial Rossby waves there. Prograde equatorial jets result if convective Rossby wave generation is strong and low-latitude angular momentum flux divergence owing to baroclinic eddies generated off the equator is sufficiently weak (Jupiter and Saturn). Retrograde equatorial jets result if either convective Rossby wave generation is weak or absent (Uranus) or low-latitude angular momentum flux divergence owing to baroclinic eddies is sufficiently strong (Neptune). The different speeds and widths of the off-equatorial jets depend, among other factors, on the differential radiative heating of the atmosphere and the altitude of the jets, which are vertically sheared. The simulations have closed energy and angular momentum balances that are consistent with observations of the giant planets. They exhibit temperature structures closely resembling those observed and make predictions about as yet unobserved aspects of flow and temperature structures.\n\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/19255, title ="Water vapor and the dynamics of climate changes", author = "Schneider, Tapio and O'Gorman, Paul A.", journal = "Reviews of Geophysics", volume = "48", pages = "Art. No. RG3001", month = "July", year = "2010", issn = "8755-1209", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100803-093823508", note = "© 2010 American Geophysical Union.\nReceived 29 June 2009; accepted 3 December 2009; published 2 July 2010.\nWe are grateful for support\nby the Davidow Discovery Fund, the National Science Foundation\n(grant ATM‐0450059), and a David and Lucile Packard Fellowship.\nThe simulations shown were performed on Caltech’s Division of\nGeological and Planetary Sciences Dell cluster. Portions of\nsection 2 were presented previously by Schneider and O’Gorman\n[2007] at the 15th ’Aha Huliko’a Hawaiian Winter Workshop.\nWe thank Ian Eisenman, Yohai Kaspi, Tim Merlis, Steven\nSherwood, and two reviewers for helpful comments on a draft of\nthis paper.\nThe Editor responsible for this paper was Gerald North. He\nthanks Robert Korty and an additional anonymous reviewer.", revision_no = "14", abstract = "Water vapor is not only Earth's dominant greenhouse gas. Through the release of latent heat when it condenses, it also plays an active role in dynamic processes that shape the global circulation of the atmosphere and thus climate. Here we present an overview of how latent heat release affects atmosphere dynamics in a broad range of climates, ranging from extremely cold to extremely warm. Contrary to widely held beliefs, atmospheric circulation statistics can change nonmonotonically with global-mean surface temperature, in part because of dynamic effects of water vapor. For example, the strengths of the tropical Hadley circulation and of zonally asymmetric tropical circulations, as well as the kinetic energy of extratropical baroclinic eddies, can be lower than they presently are both in much warmer climates and in much colder climates. We discuss how latent heat release is implicated in such circulation changes, particularly through its effect on the atmospheric static stability, and we illustrate the circulation changes through simulations with an idealized general circulation model. This allows us to explore a continuum of climates, to constrain macroscopic laws governing this climatic continuum, and to place past and possible future climate changes in a broader context. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/18841, title ="Regime Transitions of Steady and Time-Dependent Hadley Circulations:\nComparison of Axisymmetric and Eddy-Permitting Simulations", author = "Bordoni, Simona and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "67", number = "5", pages = "1643-1654", month = "May", year = "2010", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100629-085126837", note = "© 2010 American Meteorological Society. Received: August 24, 2009; Accepted: November 25, 2009. SB was supported by NCAR’s Advanced Study Program. We are grateful for support by a David and Lucile Packard Fellowship. The simulations were carried out on Caltech’s Geological and Planetary Sciences Dell cluster. We thank Rich Rotunno, Mitch Moncrieff, Rodrigo Caballero, and an anonymous reviewer for thoughtful comments on the manuscript.", revision_no = "17", abstract = "Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which\nthermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state. The latitude\nϕ_0 of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time\nto study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are\ncompared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations\nfor all ϕ_0 approach the nearly inviscid, angular-momentum-conserving limit, despite the presence of\nfinite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting\nsimulations, the Hadley circulations undergo a regime transition as ϕ_0 is increased, from an equinox regime\n(small ϕ_0) in which eddy momentumfluxes strongly influence both Hadley cells to a solstice regime (large ϕ_0)\nin which the cross-equatorial winter Hadley cell more closely approaches the angular-momentum-conserving\nlimit. In axisymmetric time-dependent simulations, the Hadley cells undergo transitions between a linear\nequinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddypermitting\nsimulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in\nthe axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddypermitting\nsimulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum\nbudget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the\neddy-permitting simulations, a mean-flow feedback—involving the upper-level zonal winds, the lower-level\ntemperature gradient, and the poleward boundary of the cross-equatorial Hadley cell—makes it possible for\nthe circulation fields to change at the transition more rapidly than can be explained by the steady-state response\nto the thermal forcing. However, the regime transitions in the axisymmetric simulations are less sharp\nthan those in the eddy-permitting simulations because eddy–mean flow feedbacks in the eddy-permitting\nsimulations additionally sharpen the transitions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70733, title ="Atmospheric Dynamics of Earth-Like Tidally Locked Aquaplanets", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of Advances in Modeling Earth Systems", volume = "2", number = "4", pages = "Art. No. 13", month = "April", year = "2010", doi = "10.3894/JAMES.2010.2.13", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-155139882", note = "© 2010 American Geophysical Union. This work is licensed under a Creative Commons Attribution 3.0 License. \n\nManuscript submitted 26 January 2010; in final form 14 June 2010. \n\nTimothy Merlis was supported by a National Defense Science and Engineering Graduate fellowship and a National Science Foundation Graduate Research fellowship. We thank Dorian Abbot and Sonja Graves for providing modifications to the GCM code and Simona Bordoni, Ian Eisenman, Andy Ingersoll, Yohai Kaspi, and two anonymous reviewers for comments on the manuscript. The simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. The program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, and the simulation results themselves are available from the authors upon request.", revision_no = "11", abstract = "\nWe present simulations of atmospheres of Earth-like aquaplanets that are tidally locked to their star, that is, planets whose orbital period is equal to the rotation period about their spin axis, so that one side always faces the star and the other side is always dark. Such simulations are of interest in the study of tidally locked terrestrial exoplanets and as illustrations of how planetary rotation and the insolation distribution shape climate. As extreme cases illustrating the effects of slow and rapid rotation, we consider planets with rotation periods equal to one current Earth year and one current Earth day. The dynamics responsible for the surface climate (e.g., winds, temperature, precipitation) and the general circulation of the atmosphere are discussed in light of existing theories of atmospheric circulations. For example, as expected from the increasing importance of Coriolis accelerations relative to inertial accelerations as the rotation rate increases, the winds are approximately isotropic and divergent at leading order in the slowly rotating atmosphere but are predominantly zonal and rotational in the rapidly rotating atmosphere. Free-atmospheric horizontal temperature variations in the slowly rotating atmosphere are generally weaker than in the rapidly rotating atmosphere. Interestingly, the surface temperature on the night side of the planets does not fall below ∼240 K in either the rapidly or slowly rotating atmosphere; that is, heat transport from the day side to the night side of the planets efficiently reduces temperature contrasts in either case. Rotational waves and eddies shape the distribution of winds, temperature, and precipitation in the rapidly rotating atmosphere; in the slowly rotating atmosphere, these distributions are controlled by simpler divergent circulations. Both the slowly and rapidly rotating atmospheres exhibit equatorial superrotation. Systematic variation of the planetary rotation rate shows that the equatorial superrotation varies non-monotonically with rotation rate, whereas the surface temperature contrast between the day side and the night side does not vary strongly with changes in rotation rate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/17506, title ="The Maintenance of the Relative Humidity of the Subtropical Free Troposphere", author = "Couhert, Alexandre and Schneider, Tapio", journal = "Journal of Climate", volume = "23", number = "2", pages = "390-403", month = "January", year = "2010", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100217-112336766", note = "© 2010 American Meteorological Society. \n\nManuscript received 1 December 2008, in final form 3 August 2009.\nA. Couhert and T. Schneider gratefully\nacknowledge support by the National Science Foundation\n(Grant ATM-0450059) and a David and Lucile\nPackard Fellowship. J. Li and D. Waliser were supported\nby the Jet Propulsion Laboratory, California Institute of\nTechnology, under a contract with the National Aeronautics\nand Space Administration. We thank Anthony\nDel Genio and Adam Sobel for helpful comments and\ndiscussions.", revision_no = "11", abstract = "The relative importance of different processes in the water vapor balance of the troposphere is assessed, using high-resolution hindcast data from the ECMWF Integrated Forecast System (IFS) for December–February 1998/99 interpolated to isentropic coordinates. The focus is on elucidating the processes that maintain the relative humidity of the subtropical free troposphere. The dominant drying process in the subtropical free troposphere is cross-isentropic subsidence driven by radiative cooling. In some subtropical regions [e.g., over continents in the Southern (summer) Hemisphere and over western portions of ocean basins in the Northern (winter) Hemisphere], drying by radiative subsidence is partially offset or overcompensated by moistening by cross-isentropic dynamic transport of water vapor from the surface upward (e.g., in convection). Any resultant net drying or moistening of the subtropical free troposphere by cross-isentropic motions is regionally primarily balanced by isentropic mean and eddy transport of water vapor from moister into drier regions. Isentropic transport redistributes water vapor within the subtropics and moderates relative humidity contrasts; however, it does not consistently lead to a substantial net import or export of water vapor into or out of the subtropics.\n\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/16791, title ="Scaling of Precipitation Extremes over a Wide Range of Climates Simulated with an Idealized GCM", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Journal of Climate", volume = "22", number = "21", pages = "5676-5685", month = "November", year = "2009", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20091124-113114811", note = "© 2009 American Meteorological Society.\n\nManuscript received 30 June 2008; in final form 8 April 2009.\n\nWe are grateful for support by the\nNational Science Foundation (GrantATM-0450059) and\na David and Lucile Packard Fellowship. The simulations\nwere performed on Caltech’s Division of Geological and\nPlanetary Sciences Dell cluster. Some preliminary results\nof this work were reported in Schneider and O’Gorman\n(2007).\n\n", revision_no = "13", abstract = "Extremes of precipitation are examined in a wide range of climates simulated with an idealized aquaplanet GCM. The high percentiles of daily precipitation increase as the climate warms. Their fractional rate of increase with global-mean surface temperature is generally similar to or greater than that of mean precipitation, but it is less than that of atmospheric (column) water vapor content. A simple scaling is introduced for precipitation extremes that accounts for their behavior by including the effects of changes in the moist-adiabatic lapse rate, the circulation strength, and the temperature when the extreme events occur. The effects of changes in the moist-adiabatic lapse rate and circulation strength on precipitation extremes are important globally, whereas the difference in the mean temperature and the temperature at which precipitation extremes occur is important only at middle to high latitudes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/15789, title ="The physical basis for increases in precipitation extremes in simulations of 21st-century climate change", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Proceedings of the National Academy of Sciences of the United States of America", volume = "106", number = "35", pages = "14773-14777", month = "September", year = "2009", doi = "10.1073/pnas.0907610106", issn = "0027-8424", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090911-153558204", note = "Copyright ©2009 by the National Academy of Sciences. \n\nCommunicated by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, July 14, 2009 (received for review March 24, 2009). Published online before print August 19, 2009, doi: 10.1073/pnas.0907610106 \n\nWe thank the modeling groups, the Program for Climate Model Diagnosis and Intercomparison and the World Climate Research Programme's Working Group on Coupled Modelling for their roles in making available the World Climate Research Program Coupled Model Intercomparison Project phase 3 multimodel dataset. Support of this dataset was provided by the Office of Science, U.S. Department of Energy. This work was supported by David and Lucile Packard Fellowship and by the National Science Foundation Grant ATM-0450059. The Global Precipitation Climatology Project 1-degree daily precipitation dataset was downloaded from http://www1.ncdc.noaa.gov. National Centers for Environmental Prediction-Department of Energy Reanalysis 2 data were provided by the National Oceanic and Atmospheric Administration/Office of Oceanic and Atmospheric Research/Earth System Research Laboratory Physical Sciences Division at www.cdc.noaa.gov. \n\nAuthor contributions: P.A.O. and T.S. designed research; P.A.O. and T.S. performed research; P.A.O. analyzed data; and P.A.O. and T.S. wrote the paper. \n\nThe authors declare no conflict of interest. \n\nThis article contains supporting information online at www.pnas.org/cgi/content/full/0907610106/DCSupplemental.", revision_no = "21", abstract = "Global warming is expected to lead to a large increase in atmospheric water vapor content and to changes in the hydrological cycle, which include an intensification of precipitation extremes. The intensity of precipitation extremes is widely held to increase proportionately to the increase in atmospheric water vapor content. Here, we show that this is not the case in 21st-century climate change scenarios simulated with climate models. In the tropics, precipitation extremes are not simulated reliably and do not change consistently among climate models; in the extratropics, they consistently increase more slowly than atmospheric water vapor content. We give a physical basis for how precipitation extremes change with climate and show that their changes depend on changes in the moist-adiabatic temperature lapse rate, in the upward velocity, and in the temperature when precipitation extremes occur. For the tropics, the theory suggests that improving the simulation of upward velocities in climate models is essential for improving predictions of precipitation extremes; for the extratropics, agreement with theory and the consistency among climate models increase confidence in the robustness of predictions of precipitation extremes under climate change. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/15442, title ="Storms in the tropics of Titan", author = "Schaller, E. L. and Roe, H. G.", journal = "Nature", volume = "460", number = "7257", pages = "873-875", month = "August", year = "2009", doi = "10.1038/nature08193", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090828-231032364", note = "© 2009 Nature Publishing Group. \n\nReceived 5 March; accepted 2 June 2009. \n\nE.L.S. is supported by a Hubble Postdoctoral Fellowship. H.G.R is supported by the NASA Planetary Astronomy Program. M.E.B. is supported by an NSF Planetary Astronomy grant. We thank IRTF telescope operators, D. Griep, W. Golisch, P. Sears and E. Volquardsen. The IRTF is operated by the University of Hawaii under a cooperative agreement with the Planetary Astronomy Program of the NASA Science Mission Directorate. Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the International Gemini partnership. \n\nAuthor Contributions: E.L.S. analysed and interpreted the IRTF and Gemini observations and wrote the paper. H.G.R. was responsible for the Gemini observations, data reduction, and analysis. T.S. helped interpret the observations. T.S. and E.L.S. wrote the Supplementary Information. M.E.B. supervised the project. All authors discussed the results and implications and commented on the manuscript.", revision_no = "17", abstract = "Methane clouds, lakes and most fluvial features on Saturn's moon Titan have been observed in the moist high latitudes while the tropics have been nearly devoid of convective clouds and have shown an abundance of wind-carved surface features like dunes. The presence of small-scale channels and dry riverbeds near the equator observed by the Huygens probe at latitudes thought incapable of supporting convection (and thus strong rain) has been suggested to be due to geological seepage or other mechanisms not related to precipitation. Here we report the presence of bright, transient, tropospheric clouds in tropical latitudes. We find that the initial pulse of cloud activity generated planetary waves that instigated cloud activity at other latitudes across Titan that had been cloud-free for at least several years. These observations show that convective pulses at one latitude can trigger short-term convection at other latitudes, even those not generally considered capable of supporting convection, and may also explain the presence of methane-carved rivers and channels near the Huygens landing site.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/15252, title ="Scales of Linear Baroclinic Instability and Macroturbulence in Dry Atmospheres", author = "Merlis, Timothy M. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "66", number = "6", pages = "1821-1833", month = "June", year = "2009", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090821-151334152", note = "© 2009 American Meteorological Society. \n\n(Manuscript received 11 July 2008, in final form 20 November 2008) \n\nWe thank Chris Walker for performing most of the nonlinear simulations and Simona Bordoni and Paul O’Gorman for helpful comments and discussions. T. M. Merlis is supported by a National Defense Science and Engineering Graduate Fellowship. The linear stability analyses were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. The program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, and the simulation results themselves are available from the authors upon request.", revision_no = "10", abstract = "Linear stability analyses are performed on a wide range of mean flows simulated with a dry idealized general circulation model. The zonal length scale of the linearly most unstable waves is similar to the Rossby radius. It is also similar to the energy-containing zonal length scale in statistically steady states of corresponding nonlinear simulations. The meridional length scale of the linearly most unstable waves is generally smaller than the energy-containing meridional length scale in the corresponding nonlinear simulations. The growth rate of the most unstable waves increases with increasing Eady growth rate, but the scaling relationship is not linear in general. The available potential energy and barotropic and baroclinic kinetic energies of the linearly most unstable waves scale linearly with each other, with similar partitionings among the energy forms as in the corresponding nonlinear simulations. These results show that the mean flows in the nonlinear simulations are baroclinically unstable, yet there is no substantial inverse cascade of barotropic eddy kinetic energy from the baroclinic generation scale to larger scales, even in strongly unstable flows. Some aspects of the nonlinear simulations, such as partitionings among eddy energies, can be understood on the basis of linear stability analyses; for other aspects, such as the structure of heat and momentum fluxes, nonlinear modifications of the waves are important.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/14535, title ="Formation of jets and equatorial superrotation on Jupiter", author = "Schneider, Tapio and Liu, Junjun", journal = "Journal of the Atmospheric Sciences", volume = "66", number = "3", pages = "579-601", month = "March", year = "2009", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090709-092656156", note = "© 2009 American Meteorological Society.\n\nManuscript received 31 March 2008, in final form 24 September 2008.\n\nThis work was supported by a\nDavid and Lucile Packard Fellowship. The GCM is\nbased on the Flexible Modeling System of the Geophysical\nFluid Dynamics Laboratory; the simulations\nwere performed on Caltech’s Division of Geological\nand Planetary Sciences Dell cluster. We thank Isaac\nHeld, Andrew Ingersoll, Yohai Kaspi, Paul O’Gorman,\nAdam Sobel, David Stevenson, and Paul Wennberg for\ncomments on drafts of this paper and Paul O’Gorman\nfor providing code for the calculation of spectral energy\nfluxes.", revision_no = "16", abstract = "The zonal flow in Jupiter’s upper troposphere is organized into alternating retrograde and prograde jets, with\na prograde (superrotating) jet at the equator. Existing models posit as the driver of the flow either differential\nradiative heating of the atmosphere or intrinsic heat fluxes emanating from the deep interior; however, they do\nnot reproduce all large-scale features of Jupiter’s jets and thermal structure. Here it is shown that the difficulties\nin accounting for Jupiter’s jets and thermal structure resolve if the effects of differential radiative\nheating and intrinsic heat fluxes are considered together, and if upper-tropospheric dynamics are linked to a\nmagnetohydrodynamic(MHD)drag that acts deep in the atmosphere and affects the zonal flow away from but\nnot near the equator. Baroclinic eddies generated by differential radiative heating can account for the off-equatorial\njets; meridionally propagating equatorial Rossby waves generated by intrinsic convective heat\nfluxes can account for the equatorial superrotation. The zonal flow extends deeply into the atmosphere, with its\nspeed changing with depth, away from the equator up to depths at which the MHD drag acts. The theory is\nsupported by simulations with an energetically consistent general circulation model of Jupiter’s outer atmosphere.\nA simulation that incorporates differential radiative heating and intrinsic heat fluxes reproduces\nJupiter’s observed jets and thermal structure and makes testable predictions about as yet unobserved aspects\nthereof. A control simulation that incorporates only differential radiative heating but not intrinsic heat fluxes\nproduces off-equatorial jets but no equatorial superrotation; another control simulation that incorporates only\nintrinsic heat fluxes but not differential radiative heating produces equatorial superrotation but no off-equatorial\njets. The proposed mechanisms for the formation of jets and equatorial superrotation likely act\nin the atmospheres of all giant planets.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70814, title ="Single-layer axisymmetric model for a Hadley circulation with parameterized eddy momentum forcing", author = "Sobel, Adam H. and Schneider, Tapio", journal = "Journal of Advances in Modeling Earth Systems", volume = "1", number = "3", pages = "Art. No. 10", month = "March", year = "2009", doi = "10.3894/JAMES.2009.1.10", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20161004-105301466", note = "© 2009 American Geophysical Union. \n\nManuscript submitted 26 May 2009; in final form 11 August 2009. First published: March 2009. \n\nWe are grateful for support by the National Science Foundation via grant nos. ATM-0450059 (TS) and ATM-0542736 (AHS) and a David and Lucile Packard Fellowship (TS). \n\nMuch of the work was done during the academic year 2007-2008 while the first author was a sabbatical visitor at the Centre for Australian Weather and Climate Research, Bureau of Meteorology, in Melbourne, Australia, and he thanks the scientists and administrative staff of that institution for their support and hospitality. We thank Paul O'Gorman for helpful discussion about and suggestions for the eddy momentum flux parameterization we used. \n\nWe thank Paul O’Gorman for helpful discussion about and suggestions for the eddy momentum flux parameterization we used.", revision_no = "17", abstract = "An axisymmetric single-layer model is used to study interactions of the Hadley circulation with extratropical eddies. Eddy momentum fluxes are parameterized using a simple closure motivated by calculations with an idealized dry general circulation model (GCM). Calculations are performed in which the heating is parameterized as Newtonian relaxation of temperatures toward a prescribed radiative-convective equilibrium (RCE) state. The latitude at which the maximum RCE temperature occurs is varied to represent seasonal variations. In the axisymmetric model, as in the GCM, qualitative changes in the zonal momentum budget occur as the RCE temperature maximum moves away from the equator past a threshold latitude. For RCE temperature maxima closer to the equator, eddy momentum fluxes play a dominant role in the zonal momentum budget, nonlinearity is weak, and the meridional circulation is a weak function of the degree of asymmetry about the equator. For RCE temperature maxima sufficiently far from the equator, the zonal momentum budget becomes more nonlinear, angular momentum is more nearly conserved, and the circulation is a stronger function of the degree of asymmetry about the equator. Since the axisymmetric model can capture this behavior while being much simpler than the GCM, it may be a useful step towards a more comprehensive theory of the zonal-mean general circulation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/12864, title ="Extent of Hadley circulations in dry atmospheres", author = "Korty, Robert L. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "35", number = "23", pages = "L23803", month = "December", year = "2008", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:KORgrl08", note = "Copyright 2008 by the American Geophysical Union. \n\nReceived 28 August 2008; revised 21 October 2008; accepted 27 October 2008; published 2 December 2008. \n\nThis work was supported by a David and Lucile Packard Fellowship and by the National Science Foundation (grant ATM-0450059). Three anonymous reviewers provided very helpful comments.", revision_no = "11", abstract = "The subtropical terminus of the Hadley circulation is interpreted as the latitude poleward of which vertical wave activity fluxes (meridional eddy entropy fluxes) become sufficiently deep to reach the upper troposphere. This leads to a sign change of the upper-tropospheric divergence of meridional wave activity fluxes (convergence of meridional eddy angular momentum fluxes) and marks the transition from the tropical Hadley cell to the extratropical Ferrel cell. A quantitative formulation for determining the depth of vertical wave activity fluxes and thus the terminus of the Hadley circulation is proposed based on the supercriticality, a measure of the slope of isentropes. The supercriticality assumes an approximately constant value at the terminus of the Hadley circulation in a series of simulations with an idealized dry general circulation model. However, it is unclear how to generalize this supercriticality-based formulation to moist atmospheres.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/12772, title ="Energy of Midlatitude Transient Eddies in Idealized Simulations of Changed Climates", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Journal of Climate", volume = "21", number = "22", pages = "5797-5806", month = "November", year = "2008", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:OGOjc08b", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 14 June 2007, in final form 2 April 2008) \n\nWe are grateful for support by the National Science Foundation (Grant ATM-450059), the Davidow Discovery Fund, and a David and Lucile Packard Fellowship. The simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. We thank Dargan Frierson for providing code for the convection and radiation schemes and Chris Walker for development of the postprocessing code.", revision_no = "10", abstract = "As the climate changes, changes in static stability, meridional temperature gradients, and availability of moisture for latent heat release may exert competing effects on the energy of midlatitude transient eddies. This paper examines how the eddy kinetic energy in midlatitude baroclinic zones responds to changes in radiative forcing in simulations with an idealized moist general circulation model. In a series of simulations in which the optical thickness of the longwave absorber is varied over a wide range, the eddy kinetic energy has a maximum for a climate with mean temperature similar to that of present-day earth, with significantly smaller values both for warmer and for colder climates. In a series of simulations in which the meridional insolation gradient is varied, the eddy kinetic energy increases monotonically with insolation gradient. In both series of simulations, the eddy kinetic energy scales approximately linearly with the dry mean available potential energy averaged over the baroclinic zones. Changes in eddy kinetic energy can therefore be related to the changes in the atmospheric thermal structure that affect the mean available potential energy.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/12540, title ="Moist Convection and the Thermal Stratification of the Extratropical Troposphere", author = "Schneider, Tapio and O'Gorman, Paul A.", journal = "Journal of the Atmospheric Sciences", volume = "65", number = "11", pages = "3571-3583", month = "November", year = "2008", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas08c", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 16 October 2007, in final form 11 April 2008) \n\nWe are grateful for support by the National Science Foundation (Grant ATM-0450059) and by a David and Lucile Packard Fellowship. The simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. Both the program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, and the simulation results themselves are available from the authors upon request.", revision_no = "12", abstract = "Simulations with an aquaplanet general circulation model show that sensible and latent heat transport by large-scale eddies influences the extratropical thermal stratification over a wide range of climates, even in relatively warm climates with small meridional surface temperature gradients. Variations of the lapse rate toward which the parameterized moist convection in the model relaxes atmospheric temperature profiles demonstrate that the convective lapse rate only marginally affects the extratropical thermal stratification in Earth-like and colder climates. In warmer climates, the convective lapse rate does affect the extratropical thermal stratification, but the effect is still smaller than would be expected if moist convection alone controlled the thermal stratification. A theory for how large-scale eddies modify the thermal stratification of dry atmospheres is consistent with the simulation results for colder climates. For warmer and moister climates, however, theories and heuristics that have been proposed to account for the extratropical thermal stratification are not consistent with the simulation results. Theories for the extratropical thermal stratification will generally have to take transport of sensible and latent heat by large-scale eddies into account, but moist convection may only need to be taken into account regionally and in sufficiently warm climates.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/11507, title ="The Hydrological Cycle over a Wide Range of Climates Simulated with an Idealized GCM", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Journal of Climate", volume = "21", number = "15", pages = "3815-3832", month = "August", year = "2008", doi = "10.1175/2007JCLI2065.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:OGOjc08a", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 29 May 2007, in final form 10 December 2007) \n\nWe are grateful for support by the National Science Foundation (Grant ATM-0450059), the Davidow Discovery Fund, and a David and Lucile Packard Fellowship. The simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster. We thank Dargan Frierson for providing code for the convection and radiation schemes, Chris Walker for development of the post-processing code, and Raymond Pierrehumbert and Duane Waliser for helpful discussions.", revision_no = "15", abstract = "A wide range of hydrological cycles and general circulations was simulated with an idealized general circulation model (GCM) by varying the optical thickness of the longwave absorber. While the idealized GCM does not capture the full complexity of the hydrological cycle, the wide range of climates simulated allows the systematic development and testing of theories of how precipitation and moisture transport change as the climate changes. The simulations show that the character of the response of the hydrological cycle to variations in longwave optical thickness differs in different climate regimes. \n\nThe global-mean precipitation increases linearly with surface temperature for colder climates, but it asymptotically approaches a maximum at higher surface temperatures. The basic features of the precipitation–temperature relation, including the rate of increase in the linear regime, are reproduced in radiative–convective equilibrium simulations. Energy constraints partially account for the precipitation–temperature relation but are not quantitatively accurate. \n\nLarge-scale condensation is most important in the midlatitude storm tracks, and its behavior is accounted for using a stochastic model of moisture advection and condensation. The precipitation associated with large-scale condensation does not scale with mean specific humidity, partly because the condensation region moves upward and meridionally as the climate warms, and partly because the mean condensation rate depends on isentropic specific humidity gradients, which do not scale with the specific humidity itself. \n\nThe local water vapor budget relates local precipitation to evaporation and meridional moisture fluxes, whose scaling in the subtropics and extratropics is examined. A delicate balance between opposing changes in evaporation and moisture flux divergence holds in the subtropical dry zones. The extratropical precipitation maximum follows the storm track in warm climates but lies equatorward of the storm track in cold climates.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/11753, title ="Monsoons as eddy-mediated regime transitions of the tropical overturning circulation", author = "Bordoni, Simona and Schneider, Tapio", journal = "Nature Geoscience", volume = "1", number = "8", pages = "515-519", month = "August", year = "2008", issn = "1752-0894", url = "https://resolver.caltech.edu/CaltechAUTHORS:BORng08", note = "© 2008 Nature Publishing Group. \n\nReceived 6 February 2008; accepted 4 June 2008; published 6 July 2008. \n\nThis work was supported by the Davidow Discovery Fund, a David and Lucile Packard Fellowship, a Moore Postdoctoral Fellowship and the National Science Foundation (grant no. ATM-0450059). The simulations were carried out on Caltech’s Geological and Planetary Science Dell Cluster, and the reanalysis data were provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation). Part of the research was carried while S.B. was with the Department of Atmospheric and Oceanic Sciences at UCLA (supported by a UCLA Dissertation Year Fellowship). We thank B. Stevens for comments on drafts of this paper.", revision_no = "15", abstract = "Monsoons are generally viewed as planetary-scale sea-breeze circulations, caused by contrasts in the thermal properties between oceans and land surfaces that lead to thermal contrasts upon radiative heating1, 2. But the radiative heating evolves gradually with the seasons, whereas the onset of monsoon precipitation, and the associated circulation changes such as reversal of surface winds, occur rapidly3, 4. Here we use reanalysis data to show that the onset of the Asian monsoon marks a transition between two circulation regimes that are distinct in the degree to which eddy momentum fluxes control the strength of the tropical overturning circulation. Rapid transitions of the circulation between the two regimes can occur as a result of feedbacks between large-scale extratropical eddies and the tropical circulation5. Using simulations with an aquaplanet general circulation model, we demonstrate that rapid, eddy-mediated monsoon transitions occur even in the absence of surface inhomogeneities, provided the planet surface has sufficiently low thermal inertia. On the basis of these results, we propose a view of monsoons in which feedbacks between large-scale extratropical eddies and the tropical circulation are essential for the development of monsoons, whereas surface inhomogeneities such as land-sea contrasts are not.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/11369, title ="Scaling Laws and Regime Transitions of Macroturbulence in Dry Atmospheres", author = "Schneider, Tapio and Walker, Christopher C.", journal = "Journal of the Atmospheric Sciences", volume = "65", number = "7", pages = "2153-5173", month = "July", year = "2008", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas08b", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 7 September 2007, in final form 5 December 2007) \n\nWe are grateful for support by the Davidow Discovery Fund, by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059); for computing resources provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation); and for very helpful comments on drafts of this paper by Tim Merlis and Paul O’Gorman. Both the program code for the simulations, based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory, and the simulation results themselves are available from the authors upon request.", revision_no = "13", abstract = "In simulations of a wide range of circulations with an idealized general circulation model, clear scaling laws of dry atmospheric macroturbulence emerge that are consistent with nonlinear eddy–eddy interactions being weak. The simulations span several decades of eddy energies and include Earth-like circulations and circulations with multiple jets and belts of surface westerlies in each hemisphere. In the simulations, the eddy available potential energy and the barotropic and baroclinic eddy kinetic energy scale linearly with each other, with the ratio of the baroclinic eddy kinetic energy to the barotropic eddy kinetic energy and eddy available potential energy decreasing with increasing planetary radius and rotation rate. Mean values of the meridional eddy flux of surface potential temperature and of the vertically integrated convergence of the meridional eddy flux of zonal momentum generally scale with functions of the eddy energies and the energy-containing eddy length scale, with a few exceptions in simulations with statically near-neutral or neutral extratropical thermal stratifications. Eddy energies scale with the mean available potential energy and with a function of the supercriticality, a measure of the near-surface slope of isentropes. Strongly baroclinic circulations form an extended regime in which eddy energies scale linearly with the mean available potential energy. Mean values of the eddy flux of surface potential temperature and of the vertically integrated eddy momentum flux convergence scale similarly with the mean available potential energy and other mean fields. \n\nThe scaling laws for the dependence of eddy fields on mean fields exhibit a regime transition between a regime in which the extratropical thermal stratification and tropopause height are controlled by radiation and convection and a regime in which baroclinic entropy fluxes modify the extratropical thermal stratification and tropopause height. At the regime transition, for example, the dependence of the eddy flux of surface potential temperature and the dependence of the vertically integrated eddy momentum flux convergence on mean fields changes -— a result with implications for climate stability and for the general circulation of an atmosphere, including its tropical Hadley circulation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/11156, title ="Statistics of an Unstable Barotropic Jet from a Cumulant Expansion", author = "Marston, J. B. and Conover, E.", journal = "Journal of the Atmospheric Sciences", volume = "65", number = "6", pages = "1955-1966", month = "June", year = "2008", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:MARjas08", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 3 May 2007, in final form 17 October 2007) \n\nWe thank Greg Holloway, Paul Kushner, Ookie Ma, and Peter Weichman for helpful discussions. This work was supported in part by the National Science Foundation under Grants DMR-0213818 and DMR-0605619. It was initiated during the summer 2005 Aspen Center for Physics workshop “Novel Approaches to Climate,” and JBM and TS thank the Center for its support.", revision_no = "12", abstract = "Low-order equal-time statistics of a barotropic flow on a rotating sphere are investigated. The flow is driven by linear relaxation toward an unstable zonal jet. For relatively short relaxation times, the flow is dominated by critical-layer waves. For sufficiently long relaxation times, the flow is turbulent. Statistics obtained from a second-order cumulant expansion are compared to those accumulated in direct numerical simulations, revealing the strengths and limitations of the expansion for different relaxation times.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/10102, title ="Eddy-Mediated Regime Transitions in the Seasonal Cycle of a Hadley Circulation and Implications for Monsoon Dynamics", author = "Schneider, Tapio and Bordoni, Simona", journal = "Journal of the Atmospheric Sciences", volume = "65", number = "3", pages = "915-934", month = "March", year = "2008", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas08a", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 2 February 2007, in final form 12 June 2007) \n\nWe are grateful for support by the National Science Foundation (Grant ATM-0450059), by the National Aeronautics and Space Administration (Grant NAG512559), by a David and Lucile Packard Fellowship, and by a UCLA Dissertation Year Fellowship. We thank Chris Walker for assistance with the GCM and related software and Paul O’Gorman, Bjorn Stevens, and Duane Waliser for helpful comments on drafts of this paper. The simulations reported in this paper were carried out on Caltech’s Geological and Planetary Sciences Dell cluster.", revision_no = "6", abstract = "In a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions, the Hadley cells undergo transitions between two regimes distinguishable according to whether large-scale eddy momentum fluxes strongly or weakly influence the strength of a cell. The center of the summer and equinox Hadley cell lies in a latitude zone of upper-level westerlies and significant eddy momentum flux divergence; the influence of eddy momentum fluxes on the strength of the cell is strong. The center of the cross-equatorial winter Hadley cell lies in a latitude zone of upper-level easterlies and is shielded from the energy-containing midlatitude eddies; the influence of eddy momentum fluxes on the strength of the cell is weak. Mediated by feedbacks between eddy fluxes, mean zonal winds at upper levels, and the mean meridional circulation, the dominant balance in the zonal momentum equation at the center of a Hadley cell shifts at the transitions between the regimes, from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell. At the transitions, a feedback involving changes in the strength of the lower-level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross-equatorial winter cell. The transitions resemble the onset and end of monsoons, for example, in the shift in the dominant zonal momentum balance, rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels, rapid changes in strength of the upward mass flux, and changes in direction and strength of the zonal wind at upper and lower levels. In the monsoonal regime, the maximum upward mass flux occurs in an off-equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic. Similar dynamic mechanisms as at the regime transitions in the simulation—mechanisms that can act irrespective of land–sea contrasts and other inhomogeneities in lower boundary conditions—may be implicated in large-scale monsoon dynamics in Earth’s atmosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/9783, title ="Weather-Layer Dynamics of Baroclinic Eddies and Multiple Jets in an Idealized General Circulation Model", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "65", number = "2", pages = "524-535", month = "February", year = "2008", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:OGOjas08", note = "© 2008 American Meteorological Society. \n\n(Manuscript received 24 August 2006, in final form 30 April 2007) \n\nWe are grateful for support by an Alfred P. Sloan Research Fellowship and by the National Science Foundation (Grant ATM-0450059). The numerical simulations were performed on Caltech’s Division of Geological and Planetary Sciences Dell cluster.", revision_no = "7", abstract = "The general circulation and the behavior of multiple jets and baroclinic eddies are described for an atmosphere in which meridional potential temperature gradients and eddies are confined to a weather layer. The weather layer is separated from the frictional lower boundary by a statically stable barotropic layer with significant mass. Closure of the zonal momentum budget in the resulting circulation is achieved through ageostrophic meridional cells that extend to the lower boundary, at which momentum is dissipated. In a series of simulations with a multilevel primitive equation model, dynamic changes in the static stability of the weather layer are found to be critical in determining the scaling of the baroclinic eddies, an effect not captured in quasigeostrophic models. For simulations with a single jet in each hemisphere, the static stability of the weather layer adjusts so that a significant inverse energy cascade to scales larger than the Rossby deformation radius does not occur. The eddy length is found to scale with both the Rossby deformation radius and the Rhines scale. Simulations with larger planetary radii and low pole-to-equator temperature gradients exhibit multiple jets in each hemisphere. Eddy lengths and energies for the jet nearest the equator in each hemisphere have the same scaling as those in the single-jet simulations. Similar scalings are found for jets farther poleward but with different constants of proportionality that are consistent with more supercritical eddies. The local eddy length is found to have only a weak variation with latitude, and the local meridional jet spacing is found to scale with the local eddy length in all cases. Insights from the weather-layer simulations may be relevant to circulations in gas giant planets and the ocean.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/9550, title ="A Climatology of the Tropospheric Thermal Stratification Using Saturation Potential Vorticity", author = "Korty, Robert L. and Schneider, Tapio", journal = "Journal of Climate", volume = "20", number = "24", pages = "5977-5991", month = "December", year = "2007", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:KORjc07", note = "© 2007 American Meteorological Society. \n\n(Manuscript received 27 November 2006, in final form 10 April 2007) \n\nWe thank Kerry Emanuel for sharing numerous insights and beneficial conversations about this topic with us. Anthony Del Genio and two anonymous reviewers provided helpful comments on an earlier draft of this manuscript. We are grateful for support from the Davidow Discovery Fund and the National Science Foundation (Grant ATM-0450059).", revision_no = "6", abstract = "The condition of convective neutrality is assessed in the troposphere by calculating the saturation potential vorticity P* from reanalysis data. Regions of the atmosphere in which saturation entropy is constant along isosurfaces of absolute angular momentum, a state indicative of slantwise-convective neutrality, have values of P* equal to zero. In a global reanalysis dataset spanning the years 1970–2004, tropospheric regions are identified in which P* is near zero, implying that vertical convection or slantwise convection may be important in determining the local thermal stratification. Convectively neutral air masses are common not only in the Tropics but also in higher latitudes, for example, over midlatitude continents in summer and in storm tracks over oceans in winter. Large-scale eddies appear to stabilize parts of the lower troposphere, particularly in winter.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70732, title ="Recovery of atmospheric flow statistics in a general circulation model without nonlinear eddy-eddy interactions", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "34", number = "22", pages = "Art. No. L22801", month = "November", year = "2007", doi = "10.1029/2007GL031779", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-154729045", note = "© 2007 American Geophysical Union. \n\nReceived 22 August 2007; revised 3 October 2007; accepted 12 October 2007; published 16 November 2007. \n\nThis work was supported by the National Science Foundation (Grant ATM-0450059), the Davidow Discovery Fund, and a David and Lucile Packard Fellowship. The numerical simulations were performed on Caltech's Division of Geological and Planetary Sciences Dell cluster.", revision_no = "8", abstract = "The closure problem of turbulence arises because nonlinear interactions among turbulent fluctuations (eddies) lead to an infinite hierarchy of moment equations for flow statistics. Here we demonstrate with an idealized general circulation model (GCM) that many atmospheric flow statistics can already be recovered if the hierarchy of moment equations is truncated at second order, corresponding to the elimination of nonlinear eddy-eddy interactions. Some, but not all, features of the general circulation remain the same. The atmospheric eddy kinetic energy spectrum retains a −3 power-law range even though this is usually explained in terms of an enstrophy cascade mediated by nonlinear eddy-eddy interactions. Our results suggest that it may be possible to construct fast general circulation models that solve for atmospheric flow statistics directly rather than via simulation of individual eddies and their interactions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55825, title ="Uncertainty in climate-sensitivity estimates", author = "Schneider, Tapio", journal = "Nature", volume = "446", number = "7131", pages = "E1", month = "March", year = "2007", doi = "10.1038/nature05707", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150317-075915988", note = "© 2007 Nature Publishing Group.\n\nReceived 19 May 2006; accepted 13 December 2006. Published online 28 February 2007.\n\nCompeting financial interests: declared none.", revision_no = "13", abstract = "Based on reconstructions of past temperatures from proxy data, Hegerl et al. estimate a confidence interval for climate sensitivity that suggests a substantially reduced probability of very high climate sensitivity compared with previous empirical estimates. Here I show that the inference procedure used by Hegerl et al. neglects uncertainties in temperature reconstructions and in the estimated climate sensitivity and can even be used to infer that the climate sensitivity is zero with vanishing uncertainty. Similar procedures based on temperature reconstructions from proxy data generally underestimate uncertainties in climate sensitivity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/7323, title ="Comment on \"Spatio-temporal filling of missing points in geophysical data sets\" by D. Kondrashov and M. Ghil, Nonlin. Processes Geophys., 13, 151–159, 2006", author = "Schneider, T.", journal = "Nonlinear Processes in Geophysics", volume = "14", number = "1", pages = "1-2", month = "January", year = "2007", doi = "10.5194/npg-14-1-2007", issn = "1023-5809", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNnpg07", note = "© Author(s) 2007. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License. Published by Copernicus GmbH on behalf of the European Geosciences Union and the American Geophysical Union. \n\nReceived: 10 July 2006 – Revised: 1 October 2006 – Accepted: 22 December 2006 – Published: 15 January 2007. \n\nSee also:\n\noriginal article -- \"Spatio-temporal filling of missing points in geophysical data sets\" by D. Kondrashov and M. Ghil, Nonlin. Processes Geophys., 13, 151–159, 2006. \n\nauthors' reply to comment: Reply to T. Schneider's comment on \"Spatio-temporal filling of missing points in geophysical data sets\", Nonlin. Processes Geophys., 14, 3-4, 2007.", revision_no = "7", abstract = "Kondrashov and Ghil (2006) (KG hereafter) describe a method for imputing missing values in incomplete datasets that can exploit both spatial and temporal covariability to estimate missing values from available values. Temporal covariability has not been exploited as widely as spatial covariability in imputing missing values in geophysical datasets, but, as KG show, doing so can improve estimates of missing values. However, there are several inaccuracies in KG’s paper. Since similar inaccuracies have surfaced in other recent papers, for example, in the literature on paleo-climate reconstructions, I would like to point them out here.", } @book_section {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70668, title ="The thermal stratification of the extratropical troposphere", author = "Schneider, Tapio", pages = "47-77", month = "January", year = "2007", isbn = "9780691121819", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160929-131829928", note = "© 2007 Princeton University Press.", revision_no = "7", abstract = "This chapter discusses the dynamical mechanisms responsible for the maintenance and variability of the extratropical thermal stratification and tropopause in the zonal mean. Figure 3.1 shows the zonal-mean temperature lapse rate of Earth’s atmosphere for boreal winter and summer. The zonal-mean lapse rate in the free troposphere is relatively uniform (about 6.5 K km^(−1) ) and varies only weakly with season — observations that motivated the assumption of a fixed thermal stratification in quasigeostrophic theory. Regions of smaller lapse rate (statically more stable stratification) are seen near the surface in the subtropics and in high latitudes, particularly in winter. At the tropopause, the lapse rate decreases, in many regions to zero or less, marking the transition from the troposphere to the more stably stratified stratosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/6974, title ="Eddy Influences on Hadley Circulations: Simulations with an Idealized GCM", author = "Walker, Christopher C. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "63", number = "12", pages = "3333-3360", month = "December", year = "2006", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:WALjas06", note = "© 2006 American Meteorological Society \n\nManuscript received 10 November 2005, in final form 19 April 2006 \n\nWe thank Dorian Abbot, Richard Lindzen, Edwin Schneider, Adam Sobel, and an anonymous reviewer for very helpful comments on an earlier version of this paper. We are grateful for financial support by the Davidow Discovery Fund, by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059) and for computing resources provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation). The program code for the simulations described in this paper, as well as the simulation results themselves, are available from the authors upon request.", revision_no = "112", abstract = "An idealized GCM is used to investigate how the strength and meridional extent of the Hadley circulation depend on the planet radius, rotation rate, and thermal driving. Over wide parameter ranges, the strength and meridional extent of the Hadley circulation display clear scaling relations with regime transitions, which are not predicted by existing theories of axisymmetric Hadley circulations. For example, the scaling of the strength as a function of the radiative-equilibrium equator-to-pole temperature contrast exhibits a regime transition corresponding to a regime transition in scaling laws of baroclinic eddy fluxes. The scaling of the strength of the cross-equatorial Hadley cell as a function of the latitude of maximum radiative-equilibrium temperature exhibits a regime transition from a regime in which eddy momentum fluxes strongly influence the strength to a regime in which the influence of eddy momentum fluxes is weak. \n\nOver a wide range of flow parameters, albeit not always, the Hadley circulation strength is directly related to the eddy momentum flux divergence at the latitude of the streamfunction extremum. Simulations with hemispherically symmetric thermal driving span circulations with local Rossby numbers in the horizontal upper branch of the Hadley circulation between 0.1 and 0.8, indicating that neither nonlinear nearly inviscid theories, valid for Ro → 1, nor linear theories, valid for Ro → 0, of axisymmetric Hadley circulations can be expected to be generally adequate. Nonlinear theories of axisymmetric Hadley circulations may account for aspects of the circulation when the maximum radiative-equilibrium temperature is displaced sufficiently far away from the equator, which results in cross-equatorial Hadley cells with nearly angular momentum-conserving upper branches. \n\nThe dependence of the Hadley circulation on eddy fluxes, which are themselves dependent on extratropical circulation characteristics such as meridional temperature gradients, suggests that tropical circulations depend on the extratropical climate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/6855, title ="A Climatology of Tropospheric Zonal-Mean Water Vapor Fields and Fluxes in Isentropic Coordinates", author = "Schneider, Tapio and Smith, Karen L.", journal = "Journal of Climate", volume = "19", number = "22", pages = "5918-5933", month = "November", year = "2006", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjc06", note = "© 2006 American Meteorological Society \n\nManuscript received 5 January 2006, in final form 27 February 2006 \n\nWe are grateful for financial support by the Davidow Discovery Fund, by a Davidow Graduate Fellowship (K. Smith), by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059), and for computing resources and reanalysis data provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation). T. Schneider gratefully acknowledges discussions with Isaac Held in 2001, in which the idea to analyze water vapor fluxes in isentropic coordinates originated, and with Adam Sobel, which helped to clarify the relation between our results and those of Galewsky et al. (2005). We thank Kerry Emanuel for helpful comments on the manuscript.", revision_no = "83", abstract = "Based on reanalysis data for the years 1980–2001 from the European Centre for Medium-Range Weather Forecasts (ERA-40 data), a climatology of tropospheric zonal-mean water vapor fields and fluxes in isentropic coordinates is presented. In the extratropical free troposphere, eddy fluxes dominate the meridional flux of specific humidity along isentropes. At all levels, isentropic eddy fluxes transport water vapor from the deep Tropics through the subtropics into the extratropics. Isentropic eddy fluxes of specific humidity diverge near the surface and in the tropical and subtropical free troposphere; they converge in the extratropical free troposphere. Isentropic mean advective fluxes of specific humidity play a secondary role in the meridional water vapor transport in the free troposphere; however, they dominate the meridional flux of specific humidity near the surface, where they transport water vapor equatorward and, in the solstice seasons, across the equator. Cross-isentropic mean advective fluxes of specific humidity are especially important in the Hadley circulation, in whose ascending branches they moisten and in whose descending branches they dry the free troposphere. \n\nNear the minima of zonal-mean relative humidity in the subtropical free troposphere, the divergence of the cross-isentropic mean advective flux of specific humidity in the descending branches of the Hadley circulation is the dominant divergence in the mean specific humidity balance; it is primarily balanced by convergence of cross-isentropic turbulent fluxes that transport water vapor from the surface upward. Although there are significant isentropic eddy fluxes of specific humidity through the region of the subtropical relative humidity minima, their divergence near the minima is generally small compared with the divergence of cross-isentropic mean advective fluxes, implying that moistening by eddy transport from the Tropics into the region of the minima approximately balances drying by eddy transport into the extratropics. That drying by cross-isentropic mean subsidence near the subtropical relative humidity minima is primarily balanced by moistening by upward turbulent fluxes of specific humidity, likely in convective clouds, suggests cloud dynamics may play a central role in controlling the relative humidity of the subtropical free troposphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/6809, title ="Stochastic Models for the Kinematics of Moisture Transport and Condensation in Homogeneous Turbulent Flows", author = "O'Gorman, Paul A. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "63", number = "11", pages = "2992-3005", month = "November", year = "2006", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:OGOjas06", note = "© 2006 American Meteorological Society \n\nManuscript received 17 November 2005, in final form 3 March 2006 \n\nWe are grateful for support by the Davidow Discovery Fund, by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059). The pseudospectral code used in section 4 is a modified version of the spectral quasigeostropic model developed by Shafer Smith. We gratefully acknowledge helpful suggestions from James Gleeson, Raymond Pierrehumbert, and an anonymous referee.", revision_no = "41", abstract = "The transport of a condensing passive scalar is studied as a prototype model for the kinematics of moisture transport on isentropic surfaces. Condensation occurs whenever the scalar concentration exceeds a specified local saturation value. Since condensation rates are strongly nonlinear functions of moisture content, the mean moisture flux is generally not diffusive. To relate the mean moisture content, mean condensation rate, and mean moisture flux to statistics of the advecting velocity field, a one-dimensional stochastic model is developed in which the Lagrangian velocities of air parcels are independent Ornstein–Uhlenbeck (Gaussian colored noise) processes. The mean moisture evolution equation for the stochastic model is derived in the Brownian and ballistic limits of small and large Lagrangian velocity correlation time. The evolution equation involves expressions for the mean moisture flux and mean condensation rate that are nonlocal but remarkably simple. In a series of simulations of homogeneous two-dimensional turbulence, the dependence of mean moisture flux and mean condensation rate on mean saturation deficit is shown to be reproducible by the one-dimensional stochastic model, provided eddy length and time scales are taken as given. For nonzero Lagrangian velocity correlation times, condensation reduces the mean moisture flux for a given mean moisture gradient compared with the mean flux of a noncondensing scalar.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/4137, title ="Global Circulation of the Atmosphere (2004)", author = "Schneider, Tapio and Sobel, Adam", journal = "Bulletin of the American Meteorological Society", volume = "87", number = "6", pages = "807-809", month = "June", year = "2006", issn = "0003-0007", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNbams06b", note = "© Copyright by American Meteorological Society 2006 \n\nIn final form 13 December 2005 \n\nThe conference was made possible by a grant from the Davidow Research Fund. The National Science Foundation (ATM-0437392) provided travel support for students and postdocs.", revision_no = "6", abstract = "GLOBAL CIRCULATION OF THE ATMOSPHERE\nWHAT: Experts assembled to assess understanding of the global circulation with an eye toward identifying outstanding questions and improving the framework for synthesizing observations and simulations.\nWHEN: 4–6 November 2004\nWHERE: Pasadena, California", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/4481, title ="Self-Organization of Atmospheric Macroturbulence into Critical States of Weak Nonlinear Eddy–Eddy Interactions", author = "Schneider, Tapio and Walker, Christopher C.", journal = "Journal of the Atmospheric Sciences", volume = "63", number = "6", pages = "1569-1586", month = "June", year = "2006", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas06", note = "© Copyright by American Meteorological Society 2006 \n\n(Manuscript received 7 June 2005, in final form 1 November 2005) \n\nWe are grateful for support by the Davidow Discovery Fund, by an Alfred P. Sloan Research Fellowship, and by the National Science Foundation (Grant ATM-0450059); for computing resources provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation); and for comments on drafts of this paper by Paul O'Gorman and Tim DelSole. The program code for the simulations described in this paper, as well as the simulation results themselves, are available from the authors upon request.", revision_no = "6", abstract = "It is generally held that atmospheric macroturbulence can be strongly nonlinear. Yet weakly nonlinear models successfully account for scales and structures of baroclinic eddies in Earth's atmosphere. Here a theory and simulations with an idealized GCM are presented that suggest weakly nonlinear models are so successful because atmospheric macroturbulence organizes itself into critical states of weak nonlinear eddy–eddy interactions. By modifying the thermal structure of the extratropical atmosphere such that its supercriticality remains limited, macroturbulence inhibits nonlinear eddy–eddy interactions and the concomitant inverse energy cascade from the length scales of baroclinic instability to larger scales. For small meridional surface temperature gradients, the extratropical thermal stratification and tropopause height are set by radiation and convection, and the supercriticality is less than one; for sufficiently large meridional surface temperature gradients, the extratropical thermal stratification and tropopause height are modified by baroclinic eddies such that the supercriticality does not significantly exceed one. In either case, the scale of the energy-containing eddies is similar to the scale of the linearly most unstable baroclinic waves, and eddy kinetic and available potential energies are equipartitioned. The theory and simulations point to fundamental constraints on the thermal structures and global circulations of the atmospheres of Earth and other planets, for example, by providing limits on the tropopause height and estimates for eddy scales, eddy energies, and jet separation scales.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/5016, title ="The general circulation of the atmosphere", author = "Schneider, Tapio", journal = "Annual Review of Earth and Planetary Sciences", volume = "34", pages = "655-688", month = "May", year = "2006", issn = "0084-6597", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNareps06", note = "\"Reprinted, with permission, from the Annual Review of Earth and Planetary Science, Volume 34 copyright 2006 by Annual Reviews, www.annualreviews.org\" \n\nCollaborations and conversations with Isaac Held shaped my view of the general circulation and are reflected in this review. Chris Walker and Paul O’Gorman kindly provided simulation results and reanalysis data in a format that helped to produce the figures. I thank Paul O’Gorman, Olivier Pauluis, Adam Sobel, and Geoffrey Vallis for reviewing the manuscript and for offering helpful suggestions. I gratefully acknowledge financial support from the Davidow Discovery Fund, the Alfred P. Sloan Foundation, the National Science Foundation (grant no. 0450059), and the Aspen Center for Physics, where I wrote part of this review. The National Center for Atmospheric Research (which is sponsored by the National Science Foundation) provided computational support and the reanalysis data.", revision_no = "8", abstract = "Theories of how Earth's surface climate may change in the future, of how it may have been in the past, and of how it is related to climates of other planets must build upon a theory of the general circulation of the atmosphere. The view of the atmospheric general circulation presented here focuses not on Earth's general circulation as such but on a continuum of idealized circulations with axisymmetric flow statistics. Analyses of observational data for Earth's atmosphere, simulations with idealized general circulation models, and theoretical considerations suggest how characteristics of the tropical Hadley circulation, of the extratropical circulation, and of atmospheric macroturbulence may depend on parameters such as the planet radius and rotation rate and the strength of the differential heating at the surface.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/7123, title ="Zonal momentum balance, potential vorticity dynamics, and mass fluxes on near-surface isentropes", author = "Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "62", number = "6", pages = "1884-1900", month = "June", year = "2005", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas05", note = "© 2005 American Meteorological Society. \n\nManuscript received 2 December 2003, in final form 13 October 2004. \n\nI am grateful to Chris Walker for performing the simulations with the idealized GCM and for providing analyses of observational data with which I verified the scaling estimates of section 3e; to Isaac Held for discussions that helped to clarify commonalities and differences between dynamics in isentropic coordinates and in quasigeostrophic theory; to Tieh-Yong Koh and Peter Haynes for comments on drafts of this paper; and to the Davidow Research Fund for financial support. The balance condition (12) was originally included in a section of a paper jointly authored with Isaac Held and Stephen Garner (Schneider et al. 2003), a section that was later removed to reduce the length of that paper. I thank Isaac Held and Stephen Garner for helpful discussions during the writing of that section.", revision_no = "6", abstract = "While it has been recognized for some time that isentropic coordinates provide a convenient framework for theories of the global circulation of the atmosphere, the role of boundary effects in the zonal momentum balance and in potential vorticity dynamics on isentropes that intersect the surface has remained unclear. Here, a balance equation is derived that describes the temporal and zonal mean balance of zonal momentum and of potential vorticity on isentropes, including the near-surface isentropes that sometimes intersect the surface. Integrated vertically, the mean zonal momentum or potential vorticity balance leads to a balance condition that relates the mean meridional mass flux along isentropes to eddy fluxes of potential vorticity and surface potential temperature. The isentropic-coordinate balance condition formally resembles balance conditions well known in quasigeostrophic theory, but on near-surface isentropes it generally differs from the quasigeostrophic balance conditions. Not taking the intersection of isentropes with the surface into account, quasigeostrophic theory does not adequately represent the potential vorticity dynamics and mass fluxes on near-surface isentropes—a shortcoming that calls into question the relevance of quasigeostrophic theories for the macroturbulence and global circulation of the atmosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70690, title ="Response of idealized Hadley circulations to seasonally varying heating", author = "Walker, Chris C. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "32", number = "6", pages = "Art. No. L06813", month = "March", year = "2005", doi = "10.1029/2004GL022304", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-110409542", note = "© 2005 American Geophysical Union. \n\nReceived 23 December 2004; revised 17 February 2005; accepted 1 March 2005; published 30 March 2005. \n\n[28] We thank Paul O'Gorman for helpful discussions; the Davidow Research Fund for financial support; and the National Center for Atmospheric Research, which is sponsored by the National Science Foundation, for providing computing time used in this research.", revision_no = "7", abstract = "[1] The response of Hadley circulations to displacements of the latitude of maximum heating is investigated in idealized axisymmetric and eddy-permitting models. Consistent with an earlier study and with theory for the nearly inviscid limit (Lindzen and Hou, 1988), the strength of the Hadley circulation is sensitive to displacements of heating: the winter cell strengthens and summer cell weakens when the maximum heating is displaced off the equator. However, in conflict with the nearly inviscid limit but consistent with observations of Earth's atmosphere, the strength of an annually averaged Hadley circulation is comparable to the Hadley circulation driven by an annually averaged heating. The disagreement between these results and the nearly inviscid limit is ascribed to vertical diffusion of momentum and dry static energy in the axisymmetric model and to baroclinic eddy fluxes in the eddy-permitting model. Nonlinear amplification of the annually averaged Hadley circulation is only seen near the upper boundary in simulations with a rigid lid near the tropopause, suggesting that the amplification is an artifact of the upper boundary condition.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70689, title ="Using generalized cross-validation to select parameters in inversions for regional carbon fluxes", author = "Krakauer, Nir Y. and Schneider, Tapio", journal = "Geophysical Research Letters", volume = "31", number = "19", pages = "Art. No. L19108", month = "October", year = "2004", doi = "10.1029/2004GL020323", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-104900408", note = "© 2004 American Geophysical Union. \n\nReceived 21 April 2004; revised 6 July 2004; accepted 26 August 2004; version of record online 14 October 2004. \n\n[22] We thank K. R. Gurney for help with the TransCom 3 protocol, Z. H. Yang for reviewing drafts of this paper, and P. J. Rayner and an anonymous referee for helpful comments. NYK was supported by a graduate fellowship from the Betty and Gordon Moore Foundation. JTR gratefully acknowledges support from NASA (NAG5-11200) and NOAA (NA03OAR4310059).", revision_no = "11", abstract = "[1] Estimating CO_2 fluxes from the pattern of atmospheric CO_2 concentrations with atmospheric transport models is an ill-posed inverse problem, whose solution is stabilized using prior information. Weights assigned to prior information and to CO_2 concentrations at different locations are quantified by parameters that are not well known, and differences in the choice of these parameters contribute to differences among published estimates of the regional partitioning of CO_2 fluxes. Following the TransCom 3 protocol to estimate CO_2 fluxes for 1992–1996, we find that the partitioning of the CO_2 sink between land and oceans and between North America and Eurasia depends on parameters that quantify the relative weight given to prior flux estimates and the extent to which CO_2 concentrations at different stations are differentially weighted. Parameter values that minimize an estimated prediction error can be chosen by generalized cross-validation (GCV). The GCV parameter values yield fluxes in northern regions similar to those obtained with the TransCom parameter values, but the GCV fluxes are smaller in the poorly constrained equatorial and southern regions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/7146, title ="The tropopause and the thermal stratification in the extratropics of a dry atmosphere", author = "Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "61", number = "12", pages = "1317-1340", month = "June", year = "2004", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas04", note = "© 2004 American Meteorological Society. \n\nManuscript received 8 October 2002, in final form 5 December 2003. \n\nMy thanks go to Isaac Held for advice on the research on which this paper is based and on constructing the idealized GCM, and for many discussions over several years that helped to clarify, among other things, the commonalities and differences between the theoretical developments of this paper and quasigeostrophic theory. The simulations described in section 4c were prompted by discussions with Kerry Emanuel, Richard Lindzen, and Alan Plumb. I also thank Vladimir Gryanik, Peter Haynes, Paul Kushner, Shafer Smith, Ka-Kit Tung, and a reviewer for helpful discussions and comments on the paper, and Heidi Swanson for editing the manuscript. Parts of the research on which this paper is based were carried out while I was with the Courant Institute of Mathematical Sciences at New York University (supported partially by NSF Grant DMS-9972865 and ONR Grant N00014-96-1-0043) and with the Atmospheric and Oceanic Sciences Program at Princeton University (supported by a NASA Earth System Science Fellowship).", revision_no = "6", abstract = "A dynamical constraint on the extratropical tropopause height and thermal stratification is derived by considerations of entropy fluxes, or isentropic mass fluxes, and their different magnitudes in the troposphere and stratosphere. The dynamical constraint is based on a relation between isentropic mass fluxes and eddy fluxes of potential vorticity and surface potential temperature and on diffusive eddy flux closures. It takes baroclinic eddy fluxes as central for determining the extratropical tropopause height and thermal stratification and relates the tropopause potential temperature approximately linearly to the surface potential temperature and its gradient.\n\nSimulations with an idealized GCM point to the possibility of an extratropical climate in which baroclinic eddy fluxes maintain a statically stable thermal stratification and, in interaction with large-scale diabatic processes, lead to the formation of a sharp tropopause. The simulations show that the extratropical tropopause height and thermal stratification are set locally by extratropical processes and do not depend on tropical processes and that, across a wide range of atmospheric circulations, the dynamical constraint describes the relation between tropopause and surface potential temperatures well. An analysis of observational data shows that the dynamical constraint, derived for an idealized dry atmosphere, can account for interannual variations of the tropopause height and thermal stratification in the extratropics of the earth's atmosphere.\n\nThe dynamical constraint implies that if baroclinic eddies determine the tropopause height and thermal stratification, an atmosphere organizes itself into a state in which nonlinear interactions among eddies are inhibited. The inhibition of nonlinear eddy–eddy interactions offers an explanation for the historic successes of linear and weakly nonlinear models of large-scale extratropical dynamics.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/7127, title ="Boundary effects in potential vorticity dynamics", author = "Schneider, Tapio and Held, Isaac M.", journal = "Journal of the Atmospheric Sciences", volume = "60", number = "8", pages = "1024-1040", month = "April", year = "2003", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjas03", note = "©2003 American Meteorological Society. \n\nManuscript received December 19, 2001, in final form August 30, 2002. \n\nWe thank Richard Rotunno, Vanda Grubiši, and Piotr Smolarkiewicz for their willingness to share their simulation data; Piotr Smolarkiewicz for making the data available in a format convenient for us; Robert Hallberg, Peter Haynes, David Marshall, Olivier Pauluis, and Richard Rotunno for helpful comments on drafts of this paper; and Heidi Swanson for editing the manuscript. Parts of this paper where drafted while T. Schneider was with the Atmospheric and Oceanic Sciences Program at Princeton University, where he was supported by a NASA Earth System Science Fellowship.", revision_no = "6", abstract = "Many aspects of geophysical flows can be described compactly in terms of potential vorticity dynamics. Since potential temperature can fluctuate at boundaries, however, the boundary conditions for potential vorticity dynamics are inhomogeneous, which complicates considerations of potential vorticity dynamics when boundary effects are dynamically significant. \n\nA formulation of potential vorticity dynamics is presented that encompasses boundary effects. It is shown that, for arbitrary flows, the generalization of the potential vorticity concept to a sum of the conventional interior potential vorticity and a singular surface potential vorticity allows one to replace the inhomogeneous boundary conditions for potential vorticity dynamics by simpler homogeneous boundary conditions (of constant potential temperature). Functional forms of the surface potential vorticity are derived from field equations in which the potential vorticity and a potential vorticity flux appear as sources of flow quantities in the same way in which an electric charge and an electric current appear as sources of fields in electrodynamics. For the generalized potential vorticity of flows that need be neither balanced nor hydrostatic and that can be influenced by diabatic processes and friction, a conservation law holds that is similar to the conservation law for the conventional interior potential vorticity. The conservation law for generalized potential vorticity contains, in the quasigeostrophic limit, the well-known dual relationship between fluctuations of potential temperature at boundaries and fluctuations of potential vorticity in the interior of quasigeostrophic flows. A nongeostrophic effect described by the conservation law is the induction of generalized potential vorticity by baroclinicity at boundaries, an effect that plays a role, for example, in mesoscale flows past topographic obstacles. Based on the generalized potential vorticity concept, a theory is outlined of how a wake with lee vortices can form in weakly dissipative flows past a mountain. Theoretical considerations and an analysis of a simulation show that a wake with lee vortices can form by separation of a generalized potential vorticity sheet from the mountain surface, similar to the separation of a friction-induced vorticity sheet from an obstacle, except that the generalized potential vorticity sheet can be induced by baroclinicity at the surface.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/3973, title ="Analysis of Incomplete Climate Data: Estimation of Mean Values and Covariance Matrices and Imputation of Missing Values", author = "Schneider, Tapio", journal = "Journal of Climate", volume = "14", number = "5", pages = "853-871", month = "March", year = "2001", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjc01b", note = "© Copyright by American Meteorological Society 2001 \n\n(Manuscript received December 3, 1999, in final form March 27, 2000) \n\nThis research project was supported by a NASA Earth System Science Fellowship. I thank Keith Dixon for providing the ensemble of climate simulations; Isaac Held, John Lanzante, Michael Mann, and Arnold Neumaier for comments on drafts of this paper; and Heidi Swanson for editing the manuscript.", revision_no = "6", abstract = "Estimating the mean and the covariance matrix of an incomplete dataset and filling in missing values with imputed values is generally a nonlinear problem, which must be solved iteratively. The expectation maximization (EM) algorithm for Gaussian data, an iterative method both for the estimation of mean values and covariance matrices from incomplete datasets and for the imputation of missing values, is taken as the point of departure for the development of a regularized EM algorithm. In contrast to the conventional EM algorithm, the regularized EM algorithm is applicable to sets of climate data, in which the number of variables typically exceeds the sample size. The regularized EM algorithm is based on iterated analyses of linear regressions of variables with missing values on variables with available values, with regression coefficients estimated by ridge regression, a regularized regression method in which a continuous regularization parameter controls the filtering of the noise in the data. The regularization parameter is determined by generalized cross-validation, such as to minimize, approximately, the expected mean-squared error of the imputed values. The regularized EM algorithm can estimate, and exploit for the imputation of missing values, both synchronic and diachronic covariance matrices, which may contain information on spatial covariability, stationary temporal covariability, or cyclostationary temporal covariability. A test of the regularized EM algorithm with simulated surface temperature data demonstrates that the algorithm is applicable to typical sets of climate data and that it leads to more accurate estimates of the missing values than a conventional noniterative imputation technique.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70730, title ="Algorithm 808: ARfit—a Matlab package for the estimation of parameters and eigenmodes of multivariate autoregressive models", author = "Schneider, Tapio and Neumaier, Arnold", journal = "ACM Transactions on Mathematical Software (TOMS)", volume = "27", number = "1", pages = "58-65", month = "March", year = "2001", doi = "10.1145/382043.382316", issn = "0098-3500", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160930-153541748", note = "© 2001 ACM. \n\nReceived: September 1997; revised: January 2000; accepted: October 2000.", revision_no = "9", abstract = "ARfit is a collection of Matlab modules for modeling and analyzing multivariate time series with autoregressive (AR) models. ARfit contains modules to given time series data, for analyzing eigen modes of a fitted model, and for simulating AR processes. ARfit estimates the parameters of AR models from given time series data with a stepwise least squares algorithm that is computationally efficient, in particular when the data are high-dimensional. ARfit modules construct approximate confidence intervals for the estimated parameters and compute statistics with which the adequacy of a fitted model can be assessed. Dynamical characteristics of the modeled time series can be examined by means of a decomposition of a fitted AR model into eigenmodes and associated oscillation periods, damping times, and excitations. The ARfit module that performs the eigendecomposition of a fitted model also constructs approximate confidence intervals for the eigenmodes and their oscillation periods and damping times.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/3966, title ="Discriminants of Twentieth-Century Changes in Earth Surface Temperatures", author = "Schneider, Tapio and Held, Isaac M.", journal = "Journal of Climate", volume = "14", number = "3", pages = "249-254", month = "February", year = "2001", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHN01a", note = "© Copyright by American Meteorological Society 2001\n\n(Manuscript received July 25, 2000, in final form August 18, 2000) \n\nT. Schneider was supported by a NASA Earth System Science Fellowship. We thank Keith Dixon for providing an ensemble of climate simulations with which we tested the analysis methods; Tom Delworth, Stephen Griffies, Tom Knutson, Paul Kushner, Thomas Müller, and Francis Zwiers for comments on drafts of this paper; and Heidi Swanson for editing the manuscript.", revision_no = "6", abstract = "An approach to identifying climate changes is presented that does not hinge on simulations of natural climate variations or anthropogenic changes. Observed interdecadal climate variations are decomposed into several discriminants, mutually uncorrelated spatiotemporal components with a maximal ratio of interdecadal-to-intradecadal variance. The dominant discriminants of twentieth-century variations in surface temperature exhibit large-scale warming in which, particularly in the Northern Hemisphere summer months, localized cooling is embedded. The structure of the large-scale warming is consistent with expected effects of increases in greenhouse gas concentrations. The localized cooling, with maxima on scales of 1000–2000 km over East Asia, eastern Europe, and North America, is suggestive of radiative effects of anthropogenic sulfate aerosols.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/3972, title ="The Surface Branch of the Zonally Averaged Mass Transport Circulation in the Troposphere", author = "Held, Isaac M. and Schneider, Tapio", journal = "Journal of the Atmospheric Sciences", volume = "56", number = "11", pages = "1688-1697", month = "November", year = "1999", issn = "0022-4928", url = "https://resolver.caltech.edu/CaltechAUTHORS:HELjas99", note = "© Copyright by American Meteorological Society 1999\n\n(Manuscript received February 10, 1998, in final form July 23, 1998) \n\nWe wish to express our thanks to Robert Hallberg, Paul Kushner, and an anonymous reviewer, whose comments on an earlier draft of this paper led to significant improvements in the final version.", revision_no = "6", abstract = "The near-surface branch of the overturning mass transport circulation in the troposphere, containing the equatorward flow, is examined in isentropic and geometric coordinates. A discussion of the zonal momentum balance within isentropic layers shows that the equatorward flow at a given latitude is confined to isentropic layers that typically intersect the surface at that latitude. As a consequence of mass transport within the surface mixed layer, much of the equatorward flow occurs in layers with potential temperatures below the mean surface potential temperature. \n\nIn the conventional transformed Eulerian mean formulation for geometric coordinates, the surface branch of the overturning circulation is represented in an unrealistic manner: streamlines of the residual circulation do not close above the surface. A modified residual circulation is introduced that is free from this defect and has the additional advantage that its computation, unlike that of the conventional residual circulation, does not require division by the static stability, which may approach zero in the planetary boundary layer. It is then argued that cold air advection by the residual circulation is responsible for the formation of surface inversions at all latitudes in idealized GCMs with weak thermal damping. Also included is a discussion of how a general circulation theory for the troposphere must be built upon a theory for the near-surface meridional mass fluxes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/3971, title ="A Conceptual Framework for Predictability Studies", author = "Schneider, Tapio and Griffies, Stephen M.", journal = "Journal of Climate", volume = "12", number = "10", pages = "3133-3155", month = "October", year = "1999", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:SCHNjc99", note = "© Copyright by American Meteorological Society 1999 \n\n(Manuscript received September 24, 1998, in final form January 19, 1999) \n\nWe wish to express our thanks to Jeff Anderson and Arnold Neumaier, who drew our attention to some of the referenced literature on climatic predictability and discriminant analysis, respectively. Jeff Anderson, Ruth Michaels, Thomas Müller, Arnold Neumaier, Heidi Swanson, and Jens Timmer carefully read drafts of this paper. We gratefully acknowledge their comments and criticism, which led to substantial improvements in the final version.", revision_no = "6", abstract = "A conceptual framework is presented for a unified treatment of issues arising in a variety of predictability studies. The predictive power (PP), a predictability measure based on information–theoretical principles, lies at the center of this framework. The PP is invariant under linear coordinate transformations and applies to multivariate predictions irrespective of assumptions about the probability distribution of prediction errors. For univariate Gaussian predictions, the PP reduces to conventional predictability measures that are based upon the ratio of the rms error of a model prediction over the rms error of the climatological mean prediction. \n\nSince climatic variability on intraseasonal to interdecadal timescales follows an approximately Gaussian distribution, the emphasis of this paper is on multivariate Gaussian random variables. Predictable and unpredictable components of multivariate Gaussian systems can be distinguished by predictable component analysis, a procedure derived from discriminant analysis: seeking components with large PP leads to an eigenvalue problem, whose solution yields uncorrelated components that are ordered by PP from largest to smallest. \n\nIn a discussion of the application of the PP and the predictable component analysis in different types of predictability studies, studies are considered that use either ensemble integrations of numerical models or autoregressive models fitted to observed or simulated data. \n\nAn investigation of simulated multidecadal variability of the North Atlantic illustrates the proposed methodology. Reanalyzing an ensemble of integrations of the Geophysical Fluid Dynamics Laboratory coupled general circulation model confirms and refines earlier findings. With an autoregressive model fitted to a single integration of the same model, it is demonstrated that similar conclusions can be reached without resorting to computationally costly ensemble integrations.", }