When atmospheric storms sweep across the sea surface they excite near-inertial waves (NIWs) whose frequency sits close to the local Coriolis frequency. By generating shear-driven mixing in the upper ocean, NIWs mediate the storm\u2013ocean coupling and, in turn, influence climate. Oceanographers have long known that the fate of wind-generated NIWs hinges on how they interact with the background circulation -- especially with mesoscale eddies -- but observations show that this interaction ranges from negligible to dominant depending on location. Those regional contrasts matter: eddy-modified NIWs can dramatically reshape the pattern and intensity of mixing. My thesis uses a mix of theory, observations, and numerical modelling to better understand the dynamics that governs NIW-mesoscale interactions.
\r\n\r\nIn trying to understand how NIWs behave in the presence of mesoscale eddies, the work of theoreticians culminated in the YBJ (Young and Ben-Jelloul) equation which describes the evolution of NIWs, including the physics of advection and refraction of NIWs by mesoscale eddies. I test whether this equation, subject to observations of the wind stress, stratification and mesoscale eddy field, can capture the observed dynamics of NIWs. Simulations of the YBJ equation can be compared to observations from a mooring array in the North Atlantic. The simulation reproduces the amplitude, phase, and across-array structure of the waves, and it reveals strong concentration of NIWs in anticyclones. In contrast, the traditional slab model -- lacking mesoscale interaction physics -- performs poorly. Potential energy budget diagnostics further show that, in this region, the net NIW\u2013eddy energy exchange is small compared to other terms in the mesoscale energy budget.
\r\n\r\nGiven the utility of the YBJ equation in understanding observations, I next try to understand theoretically what governs the impact that mesoscale eddies have on NIWs. This analysis heavily leverages the connection between the YBJ equation and the Schr\u00f6dinger equation of quantum mechanics. The key governing parameter in the YBJ equation is the wave dispersiveness which quantifies the ratio of wave dispersion to wave refraction. Analytical calculations of the eigenmodes of the YBJ equation show that strongly dispersive waves are marginally affected by the eddies. However, eddies strongly imprint onto weakly dispersive NIWs. In the weak dispersion limit, the ray-tracing equations emerge from the YBJ equation, resolving some controversies regarding the applicability of ray-tracing to NIWs.
\r\n\r\nFinally, I try to understand how these different regimes may be distributed throughout the ocean. Observations from the Global Drifter Program can be used to calculate NIW spectra. Separating these spectra by vorticity reveals the impact of NIW-mesoscale interactions. NIW frequency shifts correlate strongly with vorticity, signalling weakly dispersive dynamics. Only a patch of the North Pacific shows a muted impact of mesoscale eddies. In high energy regions, such as western boundary currents and the Antarctic Circumpolar Current, NIWs exhibit a net negative frequency shift -- a potential sign of strongly dispersive waves. The true NIW signal is composed of many wave modes, each with a different dispersiveness, and each contributing to the observations. Idealised simulations of the YBJ equation are able to replicate the observed spectra from drifters well. The eigenmode approach is also useful in understanding the underlying physics that results in the observed spectral characteristics. The drifters confirm that trapping in anticyclones is common, but not universal.
", "doi": "10.7907/z5n5-s931", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18681", "collection": "thesis", "collection_id": "18681", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292026-001739784", "primary_object_url": { "basename": "RuthMoormanPhDThesisFinal.pdf", "content": "final", "filesize": 87639640, "license": "other", "mime_type": "application/pdf", "url": "/18681/1/RuthMoormanPhDThesisFinal.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Sources and Sinks of Warm Circumpolar Deep Water on the Antarctic Continental Shelf", "author": [ { "family_name": "Moorman", "given_name": "Ruth", "orcid": "0000-0001-5054-1559", "clpid": "Moorman-Ruth" } ], "thesis_advisor": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" } ], "thesis_committee": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Stewart", "given_name": "Andrew L.", "orcid": "0000-0001-5861-4070", "clpid": "Stewart-A-L" }, { "family_name": "Minchew", "given_name": "Brent", "orcid": "0000-0002-5991-3926", "clpid": "Minchew-B-M" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Circumpolar Deep Water (CDW) is the heat source driving ice loss from the Antarctic Ice Sheet. These warm ocean waters can be found everywhere north of the continental shelf break around Antarctica. Along most of the Antarctic coastline, the Antarctic Ice Sheet is protected from this heat reservoir by a buffer of surface cooled continental shelf waters. But where this buffer fails, and warm CDW encroaches on the Antarctic coast without venting its heat to the atmosphere, these warm waters rapidly melt floating ice shelves from below and accelerate the flow of upstream glaciers into the ocean. For decades this has been observed along the Pacific coast of Antarctica, leading to dramatic ice loss from the West Antarctic Ice Sheet. This thesis aims to improve our understanding of the dynamics controlling the supply of warm CDW to the Antarctic continental shelf and determining the fate of warm CDW after it crosses the continental shelf break. An atypical focus on the latter suite of processes, i.e. the \"sinks\" of CDW on the continental shelf, yields novel insights into the Antarctic coastal environment and the drivers of ice shelf melt variability.
\r\n\r\nThe depth of the thermocline partitioning warm CDW from colder overlying Winter Waters (WW), effectively the thickness of the CDW layer, correlates well with West Antarctic ice shelf melt rates. Using a simple conceptual model, we show that variations in coastal sea ice formation rates can generate large amplitude, decadal-scale thermocline depth variations, even when the supply of CDW from the shelf-break is steady. The modeled variability is sustained by feedbacks between ice shelf melt rates, vertical mixing of CDW across the thermocline, and thermocline stratification strength, here a function of WW density. In a later chapter, relationships between thermocline and WW properties are investigated throughout the West Antarctic continental shelf using observations. This assessment reveals consistent relationships between thermocline depth and thermocline stratification strength throughout the region, but suggests sea ice processes exert differing degrees of influence on CDW thickness in the Amundsen and Bellingshausen sectors. These studies frame the West Antarctic thermocline as a potential site of CDW modification, modulating the diversion of CDW heat away from ice shelves towards the atmosphere.
\r\n\r\nWhen warm CDW manages to access Antarctic ice shelves and melt glacial ice, it loses some of its heat to that process. Though this statement may appear banal, the majority of ocean and climate models fail to account for the heat exchanges needed to melt Antarctic glacial ice. Using simulations that represent ice shelf and iceberg melt, we present a detailed heat budget of Antarctica's coastal oceans, demonstrating that this omission neglects the largest ocean heat sink on the Antarctic continental shelf. Experiments where we suppress this heat sink to evaluate the impact of its omission reveal that the supply CDW heat to the continental shelf is dynamically coupled to the consumption of CDW heat on the continental shelf.
\r\n\r\nUltimately, this thesis serves as a reminder to consider the processes keeping Antarctica\u2019s coastal oceans cold when investigating the drivers ice shelf melt variability.
", "doi": "10.7907/kj0x-b924", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17680", "collection": "thesis", "collection_id": "17680", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09152025-210456988", "primary_object_url": { "basename": "PhD_Thesis_revision.pdf", "content": "final", "filesize": 88662785, "license": "other", "mime_type": "application/pdf", "url": "/17680/2/PhD_Thesis_revision.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Submesoscale Dynamics in the Upper Ocean: Air--Sea Interactions and Energy Transfers", "author": [ { "family_name": "Bai", "given_name": "Yue (Luna)", "orcid": "0009-0003-6431-8413", "clpid": "Bai-Yue-Luna" } ], "thesis_advisor": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" } ], "thesis_committee": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Villas B\u00f4as", "given_name": "Ana Beatriz", "orcid": "0000-0001-6767-6556", "clpid": "Villas B\u00f4as-A-B" }, { "family_name": "Klein", "given_name": "Patrice", "orcid": "0000-0002-3089-3896", "clpid": "Klein-P" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Submesoscale dynamics, with horizontal scales of O(1\u201310)~km, are ubiquitous in the surface ocean. Recent observations and simulations reveal enhanced sea surface temperature and velocity gradients at these scales, with sharpened buoyancy fronts attracting particular attention for their potential of energy transfer to link large-scale energy reservoirs to the small scales where dissipation occurs. This thesis addresses the following questions: How do the ocean and atmosphere interact at submesoscale? To what extent can balanced dynamics be applied to interpret submesoscale variability, and, given that, how well do balanced frameworks represent energy transfer at submesoscale fronts?
\r\n\r\nAir--sea interaction has been extensively studied at mesoscale, O(100)~km, but remains less well quantified at submesoscale. Mesoscale sea surface temperature and vorticity imprint their own spatial structure on wind stress gradients through thermal and current feedbacks (TFB and CFB). These feedbacks have typically been assessed separately, with limited attention to their collocation and potential joint impacts. Atmospheric wind stress modifications feed back on the ocean by altering surface vertical transports through Ekman dynamics. In submesoscale-resolving air--sea coupled simulations, examining TFB and CFB jointly reveals their combined influence on surface wind stress. The coupled effect produces anomalous wind stress curls an order of magnitude stronger than those at mesoscale. The resulting nonlinear Ekman velocities from wind stress curl changes reach magnitudes roughly ten times larger than at mesoscale, underscoring the potential importance of submesoscale air--sea coupling for vertical exchange in the surface ocean.
\r\n\r\nThe submesoscale range covers a transition from balanced to unbalanced motion as spatial scales decrease. This transition scale is critical for understanding energy transfers and tracer transport in the upper ocean, yet remains poorly constrained. Because balanced motions evolve on time scales much longer than Earth\u2019s rotation period, the temporal scale of submesoscale motions can indicate their dynamical regime. Spatio-temporal analyses in submesoscale-resolving simulations and surface mooring observations show that flow-following Lagrangian time scale of submesoscale flow is much longer than that registered by a fixed Eulerian observer, placing the transition to unbalanced dynamics at ~1 km, much shorter than suggested by Eulerian diagnostics. This implies that balanced theory remain applicable to submesoscale motions down to O(1)~km.
\r\n\r\nAlthough balanced quasi-geostrophic (QG) and semi-geostrophic (SG) theories reproduce submesoscale frontal development, the associated energy transfers under balanced conditions are less well understood. We derive and assess the full kinetic energy budget in spectral space and in physical space using coarse-graining for an isolated front in QG and SG frameworks. A geostrophic strain field by itself is sufficient to produce forward transfer of kinetic energy in QG frontogenesis. In SG, ageostrophic advection generates a dipole in energy transfer linked to the convergent--divergent surface flow structure, but this balanced ageostrophic circulation is too weak to overcome the dominant forward transfer from the strain field. These results demonstrate that balanced dynamics can generate surface convergence and drive downscale transfers, and that ageostrophic circulation is essential to produce the asymmetry of energy fluxes at submesoscale fronts.
", "doi": "10.7907/x7ve-tx51", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17386", "collection": "thesis", "collection_id": "17386", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032025-000538732", "primary_object_url": { "basename": "PhD_Thesis-1.pdf", "content": "final", "filesize": 6919777, "license": "other", "mime_type": "application/pdf", "url": "/17386/1/PhD_Thesis-1.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Mixing-Driven Abyssal Ocean Circulation over Sloping Topography", "author": [ { "family_name": "Peterson", "given_name": "Henry Grant", "orcid": "0000-0003-3491-7688", "clpid": "Peterson-Henry-Grant" } ], "thesis_advisor": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Adkins", "given_name": "Jess F.", "orcid": "0000-0002-3174-5190", "clpid": "Adkins-J-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Purkey", "given_name": "Sarah", "orcid": "0000-0002-1893-6224", "clpid": "Purkey-Sarah" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The planetary-scale overturning circulation of the ocean is maintained by small-scale diapycnal mixing in the abyss. Recent theory and observations suggest that this turbulence is bottom-enhanced, confining the upwelling needed to close this circulation to thin bottom boundary layers (BLs) over sloping topography. Developing an understanding of how this mixing shapes the abyssal circulation, both locally and at the basin scale, is the unifying goal of this thesis.
\r\n\r\nThe local response of a water column to mixing has previously been understood using a one-dimensional model of a rotating, stratified fluid over a sloping seafloor. Canonically, this model assumes no cross- or along-slope variations of the flow, pressure, and buoyancy anomalies. At steady state, it predicts a peculiar form of the net cross-slope transport, however, failing to consider its coupling to the global circulation. For symmetric bathymetry without along-slope variations, for instance, this large-scale context implies that all cross-slope BL transport must be exactly returned in the interior. This interior downwelling is then turned by the Coriolis acceleration, rapidly spinning up along-slope flow in balance with a cross-slope barotropic pressure gradient. With these added physics, the one-dimensional model better captures the local response to mixing over an idealized ridge, for example. Using BL theory, we explicitly describe how the BL and interior communicate in this model. The up-slope transport of dense water in the bottom BL contributes a net downward flux of buoyancy, creating an effective bottom boundary condition on the interior. The coupling goes both ways, with the interior stratification at the top of the BL setting the strength of the BL transport. Variations across the slope then allow for BL--interior exchange.
\r\n\r\nUltimately, the net transport of the local response must conserve potential vorticity at the basin scale. To better understand this coupling for arbitrary topography, we develop a novel finite element model of the planetary geostrophic equations. Using a combination of simulations and BL theory, we then study the mixing-driven abyssal circulation in an idealized bowl-shaped basin. In the absence of wind forcing and the joint effect of baroclinicity and relief, the leading-order barotropic transport flows along f/H contours, where f is the Coriolis frequency and H is the depth. The local response to mixing is coupled to this barotropic circulation, simultaneously constrained by the barotropic circulation and forcing it via a bottom stress curl. For closed f/H contours, a strong along-contour barotropic circulation spins up, reminiscent of the local response described above. On the other hand, if these contours intersect the boundary, a case more typical in the real ocean, the barotropic transport is suppressed. This decouples the leading-order local response from the large-scale circulation and intensifies bottom BL upwelling. This work therefore suggests that the local abyssal stratification in the presence of bottom-enhanced mixing strongly depends on the large-scale context.
", "doi": "10.7907/94gy-cy80", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16463", "collection": "thesis", "collection_id": "16463", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012024-234504692", "type": "thesis", "title": "Seismic Thermometry of the North Pacific and Equatorial Indian Oceans", "author": [ { "family_name": "Peng", "given_name": "Shirui", "orcid": "0000-0002-4616-4604", "clpid": "Peng-Shirui" } ], "thesis_advisor": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" }, { "family_name": "McPhaden", "given_name": "Michael J", "orcid": "0000-0002-8423-5805", "clpid": "McPhaden-Michael-J" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The ocean absorbs the majority of excess heat in the climate system. Ocean mixing is also critical in setting Earth's thermal inertia. Over the course of the past few decades, conventional observations like Argo floats have drastically improved the coverage of the global ocean. However, their temporal and spatial resolutions are still limited. Resolving trends and patterns of temperature variations in the ocean under climate change remains a challenging sampling problem. This dissertation seeks to reduce such sampling errors by developing seismic thermometry. It is an acoustic method that measures large-scale ocean temperature changes using sound waves generated by repeating earthquakes. The chapters in this thesis attempt to combine physical understanding with statistical analysis to improve and implement seismic thermometry in several ways. First, acoustic waves generated by earthquakes along the Japan Trench and received at Wake Island are used to constrain temperature variation in the Kuroshio Extension region. An inversion that combines these measurements for the time and azimuth dependence of the range-averaged deep temperatures reveals lateral and temporal variations due to Kuroshio Extension meanders, mesoscale eddies, and decadal water mass rearrangements. Second, a comprehensive covariance structure is proposed to represent variabilities due to stochastic mesoscale, regional trend, and large-scale seasonality. It demonstrates statistical consistency between conventional float data and seismic measurements, and shows quantitatively that seismic thermometry reduces basin-scale temperature uncertainty when combined with conventional measurements. Finally, seismic data are compared with ocean models in the equatorial Indian Ocean to study the vertical structure of biweekly Yanai waves. The comparison indicates qualitative agreements in biweekly variations, and regression analysis confirms their origin as west-propagating Yanai waves. Yet quantitative differences in the biweekly variance magnitude demand further calibrations in both models and the seismic inversion.", "doi": "10.7907/a814-nf75", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15158", "collection": "thesis", "collection_id": "15158", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05092023-230615398", "type": "thesis", "title": "The Role of Small-Scale Cloud, Aerosol, and Radiation Processes for Earth's Climate", "author": [ { "family_name": "Singer", "given_name": "Clare Emilie Elmendorf", "orcid": "0000-0002-1708-0997", "clpid": "Singer-Clare-Emilie-Elmendorf" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Wennberg", "given_name": "Paul O.", "orcid": "0000-0002-6126-3854", "clpid": "Wennberg-P-O" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Feingold", "given_name": "Graham", "orcid": "0000-0002-0774-2926", "clpid": "Feingold-Graham" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "What makes clouds ethereal and beautiful also makes them complex and challenging to understand and to model. The important (thermo)dynamical processes of clouds occur at scales from microns (cloud-aerosol interactions), to meters (turbulence), to thousands of kilometers (synoptic weather patterns), and every scale in between. In this thesis, I explore several facets of how clouds interact with, respond to, and shape Earth's climate. I focus on small-scale processes, using high-resolution models and theory, to understand phenomena that can have large-scale impacts.
\r\n \r\nIn the first three chapters of this thesis, I explore the idea of stratocumulus-cumulus transitions. Chapters 1 and 2 develop and demonstrate a conceptual model of a cloud-topped atmospheric boundary layer, which is rooted in mixed-layer theory. This model is able to concisely explain both the spatial stratocumulus-cumulus transition observed in the historical period, as well as a transition that has only been hypothesized by models, which may occur in the future as the direct effect of extreme concentrations of atmospheric CO2, or which may have occurred in the past. I use this conceptual model to show the importance of sea surface temperature variations for driving the climatological transition, and on sea surface warming as a positive feedback for the CO2-induced transition. Chapter 3 extends this work to understand the global response to CO2-induced stratocumulus-cumulus transitions and the role for spatial teleconnections by embedding this conceptual model of the boundary layer into a global climate model (GCM). In the GCM we see both a fast adjustment in low cloud cover to CO2, as well as a slower surface temperature-mediated feedback. Under CO2 quadrupling, the stratocumulus cloud regions shrink in extent as the cloud-top longwave cooling is inhibited by CO2 and surface temperatures also increase.
\r\n\r\nThe final two chapters diverge from the previous theme to present two studies using very high-resolution models to explore how clouds interact with i) aerosols and ii) radiation. In Chapter 4, using a particle-based cloud microphysics model, I find that aerosol hygroscopicity, determined by the chemical composition of the particles, can alter stratocumulus cloud macrophysical properties, like liquid water path by up to 25% (in the regime of small aerosol sizes). I compare these results to a more standard moment-based microphysics model and find that this model is overly sensitive to aerosol hygroscopicity in the regime of small aerosol sizes, but realistically represents the negative sensitivity for large aerosol sizes. Finally, in Chapter 5, I use a Monte Carlo 3D radiative transfer solver to estimate the global albedo bias introduced in models which make the standard assumption that photon fluxes in the horizontal are zero (the so-called Independent Column Approximation). I extrapolate globally from a set of resolved tropical cloud fields, using a learned empirical relation between top-of-atmosphere flux bias and cloud water path. I conclude that in a global model that resolves clouds at small-enough spatial scales, the tropical-mean, annual-mean bias may be on the order of 3 W m-2.
", "doi": "10.7907/bd4s-w586", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15063", "collection": "thesis", "collection_id": "15063", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11152022-215747755", "primary_object_url": { "basename": "PhD_thesis__Ignacio_Lopez_Gomez_-revised.pdf", "content": "final", "filesize": 28614789, "license": "other", "mime_type": "application/pdf", "url": "/15063/1/PhD_thesis__Ignacio_Lopez_Gomez_-revised.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "A Unified Data-Informed Model of Turbulence and Convection for Climate Prediction", "author": [ { "family_name": "L\u00f3pez G\u00f3mez", "given_name": "Ignacio", "orcid": "0000-0002-7255-5895", "clpid": "L\u00f3pez-G\u00f3mez-Ignacio" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" }, { "family_name": "Teixeira", "given_name": "Joao", "clpid": "Teixeira-Joao" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "div_gps" } ], "abstract": "Resolving atmospheric turbulent and convective processes in global climate simulations is, and will remain for decades, an intractable computational problem. The strong influence of these processes on cloud formation and maintenance makes the task of modeling turbulence and convection one of the grand challenges in climate modeling, due to the outsized effect of clouds on climate. Current operational climate models fail to represent atmospheric turbulence and convection accurately and consistently across dynamical regimes and vertical levels; errors in the representation of these processes explain about half of the spread in climate projections. This dissertation seeks to reduce such representation errors by improving a recently proposed unified framework for modeling turbulence and convection, known as the extended eddy-diffusivity mass-flux scheme, in several ways. First, the framework is rederived by systematically coarse-graining the governing fluid equations, highlighting the assumptions about atmospheric motion that are necessary to yield the scheme. New terms related to turbulent entrainment processes are shown to arise from the derivation. Second, a generalized formulation of turbulent diffusion consistent with the framework is presented. This novel formulation is shown to accurately represent turbulent processes under statically stable and unstable conditions, including regimes with sharp lapse rate inversions such as the stratocumulus-topped boundary layer. Finally, a methodology to calibrate free parameters within the model from indirect data is proposed. The methodology, based on Kalman filtering, is shown to be efficient at calibrating imperfect black-box models from noisy data, and in its regularized unscented version approximately quantifies parametric uncertainty. The resulting unified data-informed model of turbulence and convection is shown to accurately represent a range of low-cloud regimes that are associated with the largest biases in current operational climate models. The response of the model to realistic climate perturbations is also shown to be consistent with the resolved climate response, although structural errors in the amount of condensate are still important at realistic vertical resolutions.", "doi": "10.7907/042m-9686", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15095", "collection": "thesis", "collection_id": "15095", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01302023-185542422", "primary_object_url": { "basename": "Thesis_20230130_finalized_submitted.pdf", "content": "final", "filesize": 79139952, "license": "other", "mime_type": "application/pdf", "url": "/15095/1/Thesis_20230130_finalized_submitted.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Tracer Transport in Three Dimensions: Dispersion of Methane on Mars, Coupled Chemistry and Dynamics on Exoplanets, and Submesoscale Mixing in the Ocean", "author": [ { "family_name": "Luo", "given_name": "Yangcheng", "orcid": "0000-0003-0983-3650", "clpid": "Luo-Yangcheng" } ], "thesis_advisor": [ { "family_name": "Yung", "given_name": "Yuk L.", "orcid": "0000-0002-4263-2562", "clpid": "Yung-Y-L" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "thesis_committee": [ { "family_name": "Ehlmann", "given_name": "Bethany L.", "orcid": "0000-0002-2745-3240", "clpid": "Ehlmann-B-L" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Yung", "given_name": "Yuk L.", "orcid": "0000-0002-4263-2562", "clpid": "Yung-Y-L" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "One-dimensional (1D) modeling from a horizontally averaged perspective can oftentimes greatly simplify problems in atmospheric and oceanic sciences and thus capture leading-order physics. Meanwhile, 1D numerical models have great advantages such as numerical stability and time efficiency, hence they are widely used to gain insights into complex problems. However, oversimplification by 1D models may cause failures in finding solutions, revealing novel phenomena, and discovering scaling laws in the three-dimensional (3D) real world, and those are when 3D thinking proves its value. Also, the rise in computational power has allowed investigations using 3D numerical models. This thesis discusses three examples of how 3D modeling transcends the limitations of 1D modeling and reveals new solutions, phenomena, and scalings in planetary atmospheres and Earth\u2019s ocean.
\r\n\r\nChapter 2 is focused on the dispersion of methane plumes on Mars and how it can reconcile the discrepancy between observations. In the face of ostensibly inconsistent observational results of methane on Mars, we adopt a novel approach\u2014inverse Lagrangian modeling in 3D space\u2014to find the scenarios in which the inconsistency in the observations can be reconciled and locate the methane source. We find that the inconsistency between the results of the near-surface in situ methane measurements and the satellite remote sensing measurements can be reconciled if and only if an active methane emission hot spot is located in the immediate vicinity of the Curiosity rover in northwestern Gale crater, or unknown physical or chemical processes are rapidly removing methane.
\r\n\r\nChapter 3 presents a novel phenomenon that could exist on exoplanets\u2014self-sustained photochemical oscillations, which is only produced by 3D atmospheric models. We use a 3D, fully coupled, chemistry-radiation-dynamics model to simulate the ozone-NOx-HOx photochemistry in the atmosphere of a tidally locked Earth-like exoplanet in the circumstellar habitable zone, and calculate the transmission spectra during transits. We find that under certain conditions, biological nitrogen fixation like the one on the Earth can drive large-magnitude, self-sustained photochemical oscillations in the atmospheres of terrestrial exoplanets. The resulting large temporal variability in ozone abundance on exoplanets, if observed, may suggest a strong surface NOx emission source, which could signal extrasolar life participating in the nitrogen cycle on exoplanets. Fully coupled, three-dimensional atmospheric chemistry-radiation-dynamics models can reveal new phenomena that may not exist in one-dimensional models, and hence they are powerful tools for future planetary atmospheric research.
\r\n\r\nChapter 4 uses a 3D fluid dynamics model to study the vertical exchange in the upper part of Earth\u2019s ocean that potentially has great implications for the marine ecosystem. We develop scaling laws for the exchange rate between the surface ocean and the ocean interior which is critical to the rate of nutrient supply to phytoplankton near the ocean surface. These scaling laws could substitute the crude 1D parameterizations that are currently widely used in ocean models. We find that submesoscale turbulence energized by baroclinic instability in the ocean mixed layer can induce tracer exchange between the surface ocean and the ocean interior. Various environmental physical parameters affect the exchange rate. The exchange is stronger where the ocean mixed layer is thicker, the Richardson number (defined as the ratio of the squared buoyancy frequency to the squared vertical shear of the horizontal flow) of the thermocline is smaller, and the Richardson number of ocean mixed layer is larger. The associated nutrient supply from the ocean interior to the surface ocean is also expected to be stronger under these conditions.
", "doi": "10.7907/91p7-gg59", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15065", "collection": "thesis", "collection_id": "15065", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11172022-023850059", "primary_object_url": { "basename": "Williams_Ethan_2022_final_thesis.pdf", "content": "final", "filesize": 40985102, "license": "other", "mime_type": "application/pdf", "url": "/15065/2/Williams_Ethan_2022_final_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Probing Solid-Earth, Ocean, and Structural Dynamics with Distributed Fiber-Optic Sensing", "author": [ { "family_name": "Williams", "given_name": "Ethan Francis", "orcid": "0000-0002-6471-4497", "clpid": "Williams-Ethan-Francis" } ], "thesis_advisor": [ { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" } ], "thesis_committee": [ { "family_name": "Clayton", "given_name": "Robert W.", "orcid": "0000-0003-3323-3508", "clpid": "Clayton-R-W" }, { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" }, { "family_name": "Heaton", "given_name": "Thomas H.", "orcid": "0000-0003-3363-2197", "clpid": "Heaton-T-H" }, { "family_name": "Kohler", "given_name": "Monica D.", "orcid": "0000-0002-4703-190X", "clpid": "Kohler-M-D" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "Observational geophysics conventionally relies on point sensors to document and monitor Earth\u2019s dynamic processes, from locating earthquakes and imaging subsurface structure with seismometers to forecasting coastal wave heights and detecting tsunamis with buoys. Distributed acoustic sensing (DAS) offers a fundamentally different paradigm: distributed instead of point sensing. DAS converts fiber-optic cables into dense arrays of broadband, linear strainmeters, with spatial resolution as fine as one meter and temporal resolution up to several thousand samples per second. The first four chapters of this thesis concern ocean-bottom DAS, repurposing pre-existing telecommunications and power cables as distributed seafloor sensing networks for seismology and physical oceanography. In Chapter 2, we analyze one of the first ocean-bottom DAS datasets, demonstrating that seismic and ocean waves observed on the same array are related by a classic theory of double-frequency microseism generation. We also extract the principal body-wave phases of a M8.2 deep earthquake, demonstrating the earthquake detection capabilities of DAS even in a shallow water environment. In Chapter 3, we apply ambient noise interferometry to a one-hour of ocean-bottom DAS data and derive a shallow shear-wave velocity model. We also isolate spurious arrivals in noise cross-correlations associated with nearby offshore wind turbines, suggesting potential for remote monitoring. In Chapter 4, we adapt ambient noise interferometry to the ocean surface gravity wavefield, and estimate the tidal current velocity along a short cable segment in the Strait of Gibraltar with a waveform stretching method. In Chapter 5, we explore the application of DAS as a temperature sensor at long periods, documenting temperature signals up to 4 K associated with internal wave and boundary layer dynamics. We demonstrate that while ocean-bottom DAS exhibits sufficient strain sensitivity to record seafloor geodetic processes, oceanic temperature transients may overprint such signals. The last part of this thesis concerns a different frontier in geophysical instrumentation: long time-series. With a 20-year continuous record of ambient vibrations from a single accelerometer located on the ninth floor of a concrete building, we document long-term, passive changes in the building\u2019s natural frequencies as well as complex, time-dependent nonlinear elasticity during earthquakes.", "doi": "10.7907/vehm-dd85", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14613", "collection": "thesis", "collection_id": "14613", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252022-055331911", "primary_object_url": { "basename": "idini_benjamin_2022_thesis_v5.pdf", "content": "final", "filesize": 32598471, "license": "other", "mime_type": "application/pdf", "url": "/14613/6/idini_benjamin_2022_thesis_v5.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Earthquakes and the New Paradigm of Diluted Cores in Gas Giant Planets", "author": [ { "family_name": "Idini Zabala", "given_name": "Benjam\u00edn Rodo", "orcid": "0000-0002-2697-3893", "clpid": "Idini-Zabala-Benjam\u00edn-Rodo" } ], "thesis_advisor": [ { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" } ], "thesis_committee": [ { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "Simons", "given_name": "Mark", "orcid": "0000-0003-1412-6395", "clpid": "Simons-M" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "In this thesis, I present results on two distinct topics within geophysics: earthquake mechanics and the core of gas giant planets. A common element connecting this work is the similar research approach that I use to address each topic. Each chapter in this thesis attempts to provide a simple physical understanding on the fundamental aspects relevant to the system in question. Further, I use numerical models to expand my arguments in some cases, while in others I build up my case with mathematical modeling only.
\r\n\r\nChapters II-IV focus on the gravitational field of Jupiter and connect radio science observations from NASA's Juno mission to the structure of Jupiter's dilute core. In Chapter II, I use dynamical tides to interpret a nonhydrostatic component in Jupiter's degree-2 tidal response -- represented by the Love number k\u2082 -- observed by Juno at the mid-mission perijove (PJ) 17. The results presented here show how the Coriolis acceleration contributes with a dynamical effect to Jupiter's tidal response, providing a satisfactory fit to Juno's observed k\u2082. From these results, I conclude that Juno obtained the first unambiguous detection of the gravitational effect of dynamical tides in a gas giant planet.
\r\n\r\nIn Chapter III, I build a perturbation theory to show that the high-degree tidal gravitational field of Jupiter is dominated by spherical harmonic coupling promoted by Jupiter's oblate figure as forced by the centrifugal effect. Based on this novel understanding of Jupiter's high-degree tidal gravitational field, I establish that Juno observed a 7\u03c3 nonhydrostatic component in k\u2084\u2082 at mid-mission.
\r\n\r\nIn Chapter IV, I invoke a core-orbital resonance between internal gravity waves trapped in Jupiter's dilute core and the orbital motion of Io to explain the 7\u03c3 nonhydrostatic component in the high-degree tidal response of Jupiter as observed by Juno at mid-mission -- namely the Love number k\u2084\u2082. These results suggest that an extended dilute core in Jupiter (r \u2273 0.7RJup) reconciles the k\u2084\u2082 nonhydrostatic component. This explanation of Juno's observation requires two ingredients: a dilute core in Jupiter that becomes smoother or shrinks over geological time, alongside with a high amount of dissipation provided by resonantly excited internal gravity waves.
\r\n\r\nIn Chapter V, I connect observations of earthquake modes of propagation to the damaged rock often found around tectonic fault zones. Previous work showed that pulse-like rupture -- a propagation mode where slip propagates as a narrow pulse -- can be induced by the dynamic effect of seismic waves reflected at the boundary of a cavity formed by the damaged material in fault zones. My main result shows that pulses are easier to produce than previously thought; pulses can appear in a highly damaged fault zone even in the absence of reflected seismic waves. In addition, these results provide a new explanation for back-propagating rupture fronts recently observed during large earthquakes and the rapid-tremor-reversal slip patterns observed in Cascadia and Japan.
\r\n\r\nIn summary, the results contained in these four chapters advance our knowledge in fundamental problems related to geophysics. In relation to gas giant planets, my results include the development of a novel technique to reveal the structure of Jupiter's core using spacecraft observations of the tidal gravitational field. In relation to earthquakes, my results connect earthquake ruptures to observable fault zone properties.
", "doi": "10.7907/hqtw-ka38", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:11179", "collection": "thesis", "collection_id": "11179", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09082018-212920204", "primary_object_url": { "basename": "Sridhar_Akshay_Thesis_2019.pdf", "content": "final", "filesize": 6023838, "license": "other", "mime_type": "application/pdf", "url": "/11179/1/Sridhar_Akshay_Thesis_2019.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Large-Eddy Simulation of Turbulent Boundary Layers with Spatially Varying Roughness", "author": [ { "family_name": "Sridhar", "given_name": "Akshay", "orcid": "0000-0002-2642-8246", "clpid": "Sridhar-Akshay" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "orcid": "0009-0007-5991-2863", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Pullin", "given_name": "Dale Ian", "orcid": "0009-0007-5991-2863", "clpid": "Pullin-D-I" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "This dissertation addresses high Reynolds number turbulent boundary layers flows with different inhomogeneous surface roughness distributions using large eddy simulations. The stretched vortex subgrid scale model for the outer flow LES is coupled with a virtual-wall model for the friction velocity with a correction accounting for local roughness effects.
\r\n\r\nA semi-empirical model that describes a fully developed rough-walled turbulent boundary layer with sand-grain roughness length-scale ks = \u03b1x that varies linearly with streamwise distance is first developed, with \u03b1 a dimensionless constant. For large Rex and a free-stream velocity U\u221e ~ xm, a simple log-wake model of the local turbulent mean-velocity profile is used that contains a standard mean-velocity correction for the asymptotic, fully rough regime. A two parameter (\u03b1; m) family of solutions is obtained for which U\u221e+ (or equivalently Cf) and boundary-layer measures can be calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable z/ks where z is the wall-normal co-ordinate. Results over a range of \u03b1 are discussed for cases including the zero-pressure gradient (m = 0) and sink-flow (m = -1) boundary layers. Model trends are supported by high Re wall-modeled LES. Linear streamwise growth of boundary layer measures is confirmed, while for each \u03b1, mean-velocity profiles and streamwise turbulent stresses are shown to collapse against z/(\u03b1x). Inner scaled velocity defects are shown to collapse against z/\u0394, where \u0394 is the Rotta-Clauser parameter. The present results suggest that these flows may be interpreted as the fully-rough limit for boundary layers in the presence of small-scale, linear roughness.
\r\n\r\nNext, an LES study of a flat-plate turbulent boundary layer at high Re under nonequilibrium flow conditions due to the presence of abrupt changes in surface roughness is presented. Two specific cases, smooth-rough (SR) and rough-smooth (RS) transition are examined in detail. Streamwise developing velocity and turbulent stress profiles are considered and sharp departures from equilibrium flow properties with subsequent relaxation are shown downstream. Relaxation trends are studied using integral parameters and higher-order mean flow statistics with emphasis on Re\u03c4 and ks+ dependence. Results are compared with RS experiments at matched Re\u03c4, and show good agreement in terms of recovery rates.
\r\n\r\nFinally, the case of static, impulsive wall-roughness in flows at high Re is addressed using the same LES framework. The initial perturbation from smooth-to-rough appears to dominate the flow behaviour with the length of the impulsive patch showing little effect on recovery rates at matched Re\u03c4 and ks+. The resulting trends show good agreement with low Re experiments and support the wall-modeled LES framework as a suitable method for analysing high Re flows in practical applications.
", "doi": "10.7907/8YWS-B862", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11568", "collection": "thesis", "collection_id": "11568", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302019-121137858", "primary_object_url": { "basename": "Ruan_thesis_final.pdf", "content": "final", "filesize": 9455377, "license": "other", "mime_type": "application/pdf", "url": "/11568/1/Ruan_thesis_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Oceanic Bottom Boundary Layers and Abyssal Overturning Circulation", "author": [ { "family_name": "Ruan", "given_name": "Xiaozhou", "orcid": "0000-0003-1240-1584", "clpid": "Ruan-Xiaozhou" } ], "thesis_advisor": [ { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" } ], "thesis_committee": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "McKeon", "given_name": "Beverley J.", "clpid": "McKeon-B-J" }, { "family_name": "Stewart", "given_name": "Andrew L.", "clpid": "Stewart-A-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The vast amount of carbon and heat exchange between the abyssal and upper ocean and subsequently the atmosphere is paced by the abyssal overturning circulation. A key component of the abyssal overturning circulation is the formation and consumption of the densest water mass on Earth, Antarctic Bottom Water (AABW), namely the conversion of North Atlantic Deep Water (NADW) to AABW and the consumption of AABW via small-scale diapycnal mixing. Yet, this pathway of AABW spanning thousands of kilometers has not been successfully reproduced in large-scale general circulation models (GCM). What is missing is essentially the understanding and resolution of small-scale physics involved in converting deep and bottom waters from one density class to another, the water mass transformation (WMT). In this thesis, we focus on small-scale (both in the horizontal and vertical directions) dynamics near the BBL, where enhanced shear, mixing and turbulence exist to facilitate effective WMT above the seafloor.
\r\n\r\nFrom high-resolution ocean glider observations around Antarctica, we find that a portion of Lower Circumpolar Deep Water, a branch of NADW, becomes lighter via mixing with light shelf water over the continental slope and shelf, instead of being converted into dense AABW under sea ice. This mixing is likely induced by submesoscale symmetric instability coming from a strong boundary current interacting with the sloping topography in the BBL. We then consider how to sustain the consumption of AABW in the global mid-ocean ridge system. Using numerical models, we show that submesoscale baroclinic eddies are crucial to maintaining strong stratification over the flanks of the mid-ocean ridges and thus permitting effective WMT. Lastly, we consider the interaction between external mean flows and stratified BBL over sloping topography. With the large-scale turbulence resolved in a large-eddy simulation model, we propose a new theoretical framework to describe the evolution of the BBL and the Eulerian advection of its associated stratification when external barotropic flows are present. This new framework can be used to parameterize bottom friction, important for closing the kinetic energy budget of the global ocean. We further extend this interaction to a horizontally-sheared and temporally-oscillating external mean flow and explore the response of the BBL and the BBL-interior mass exchange with simple turbulent parameterizations.
\r\n\r\nUsing a combination of different approaches, we confirm that the long-overlooked oceanic BBL is the key location for closing the abyssal overturning circulation. More importantly, without appropriate techniques to tackle the currently unresolved small-scale processes, they will likely remain a narrow bottleneck in understanding the abyssal overturning circulation.
", "doi": "10.7907/6EZA-R251", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11568", "collection": "thesis", "collection_id": "11568", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302019-121137858", "primary_object_url": { "basename": "Ruan_thesis_final.pdf", "content": "final", "filesize": 9455377, "license": "other", "mime_type": "application/pdf", "url": "/11568/1/Ruan_thesis_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Oceanic Bottom Boundary Layers and Abyssal Overturning Circulation", "author": [ { "family_name": "Ruan", "given_name": "Xiaozhou", "orcid": "0000-0003-1240-1584", "clpid": "Ruan-Xiaozhou" } ], "thesis_advisor": [ { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" } ], "thesis_committee": [ { "family_name": "Schneider", "given_name": "Tapio", "clpid": "Schneider-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "clpid": "Thompson-A-F" }, { "family_name": "Callies", "given_name": "Joern", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J" }, { "family_name": "McKeon", "given_name": "Beverley J.", "clpid": "McKeon-B-J" }, { "family_name": "Stewart", "given_name": "Andrew L.", "clpid": "Stewart-A-L" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The vast amount of carbon and heat exchange between the abyssal and upper ocean and subsequently the atmosphere is paced by the abyssal overturning circulation. A key component of the abyssal overturning circulation is the formation and consumption of the densest water mass on Earth, Antarctic Bottom Water (AABW), namely the conversion of North Atlantic Deep Water (NADW) to AABW and the consumption of AABW via small-scale diapycnal mixing. Yet, this pathway of AABW spanning thousands of kilometers has not been successfully reproduced in large-scale general circulation models (GCM). What is missing is essentially the understanding and resolution of small-scale physics involved in converting deep and bottom waters from one density class to another, the water mass transformation (WMT). In this thesis, we focus on small-scale (both in the horizontal and vertical directions) dynamics near the BBL, where enhanced shear, mixing and turbulence exist to facilitate effective WMT above the seafloor.
\r\n\r\nFrom high-resolution ocean glider observations around Antarctica, we find that a portion of Lower Circumpolar Deep Water, a branch of NADW, becomes lighter via mixing with light shelf water over the continental slope and shelf, instead of being converted into dense AABW under sea ice. This mixing is likely induced by submesoscale symmetric instability coming from a strong boundary current interacting with the sloping topography in the BBL. We then consider how to sustain the consumption of AABW in the global mid-ocean ridge system. Using numerical models, we show that submesoscale baroclinic eddies are crucial to maintaining strong stratification over the flanks of the mid-ocean ridges and thus permitting effective WMT. Lastly, we consider the interaction between external mean flows and stratified BBL over sloping topography. With the large-scale turbulence resolved in a large-eddy simulation model, we propose a new theoretical framework to describe the evolution of the BBL and the Eulerian advection of its associated stratification when external barotropic flows are present. This new framework can be used to parameterize bottom friction, important for closing the kinetic energy budget of the global ocean. We further extend this interaction to a horizontally-sheared and temporally-oscillating external mean flow and explore the response of the BBL and the BBL-interior mass exchange with simple turbulent parameterizations.
\r\n\r\nUsing a combination of different approaches, we confirm that the long-overlooked oceanic BBL is the key location for closing the abyssal overturning circulation. More importantly, without appropriate techniques to tackle the currently unresolved small-scale processes, they will likely remain a narrow bottleneck in understanding the abyssal overturning circulation.
", "doi": "10.7907/6EZA-R251", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" } ]