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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 28 Nov 2023 18:15:21 +0000A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100114713
Authors: Callies, J.; Marotzke, J.
Year: 2012
DOI: 10.5194/os-8-49-2012
A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile.
This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models.
Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.https://authors.library.caltech.edu/records/mf6e3-af621Interpreting Energy and Tracer Spectra of Upper-Ocean Turbulence in the Submesoscale Range (1–200 km)
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100115052
Authors: Callies, Jörn; Ferrari, Raffaele
Year: 2013
DOI: 10.1175/JPO-D-13-063.1
Submesoscale (1–200 km) wavenumber spectra of kinetic and potential energy and tracer variance are obtained from in situ observations in the Gulf Stream region and in the eastern subtropical North Pacific. In the Gulf Stream region, steep kinetic energy spectra at scales between 200 and 20 km are consistent with predictions of interior quasigeostrophic–turbulence theory, both in the mixed layer and in the thermocline. At scales below 20 km, the spectra flatten out, consistent with a growing contribution of internal-wave energy at small scales. In the subtropical North Pacific, the energy spectra are flatter and inconsistent with predictions of interior quasigeostrophic–turbulence theory. The observed spectra and their dependence on depth are also inconsistent with predictions of surface quasigeostrophic–turbulence theory for the observed ocean stratification. It appears that unbalanced motions, most likely internal tides at large scales and the internal-wave continuum at small scales, dominate the energy spectrum throughout the submesoscale range. Spectra of temperature variance along density surfaces, which are not affected by internal tides, are also inconsistent with predictions of geostrophic-turbulence theories. Reasons for this inconsistency could be the injection of energy in the submesoscale range by small-scale baroclinic instabilities or modifications of the spectra by coupling between surface and interior dynamics or by ageostrophic frontal effects.https://authors.library.caltech.edu/records/ar67j-kmt66Asymmetries between Wavenumber Spectra of Along- and Across-Track Velocity from Tandem Mission Altimetry
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100115331
Authors: Wortham, Cimarron; Callies, Jörn; Scharffenberg, Martin G.
Year: 2014
DOI: 10.1175/JPO-D-13-0153.1
Satellite altimetry has proven to be one of the most useful oceanographic datasets, providing a continuous, near-global record of surface geostrophic currents, among other uses. One limitation of observations from a single satellite is the difficulty of estimating the full geostrophic velocity field. The 3-yr Jason-1–Ocean Topography Experiment (TOPEX)/Poseidon tandem mission, with two satellites flying parallel tracks, promised to overcome this limitation. However, the wide track separation severely limits the tandem mission's resolution and reduces the observed velocity variance. In this paper, the effective filter imposed by the track separation is discussed and two important consequences for any application of the tandem mission velocities are explained. First, while across-track velocity is simply low-pass filtered, along-track velocity is attenuated also at wavelengths much longer than the track separation. Second, velocity wavenumber spectral slopes are artificially steepened by a factor of k−2 at wavelengths smaller than the track separation. Knowledge of the effective filter has several applications, including reconstruction of the full velocity spectrum from the heavily filtered observations. Here, the hypothesis that the tandem mission flow field is horizontally nondivergent and isotropic is tested. The effective filter is also used to predict the fraction of the eddy kinetic energy (EKE) that is captured for a given track separation. The EKE captured falls off rapidly for track separations greater than about 20 km.https://authors.library.caltech.edu/records/4y7m5-2jn44Wave–vortex decomposition of one-dimensional ship-track data
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100115650
Authors: Bühler, Oliver; Callies, Jörn; Ferrari, Raffaele
Year: 2014
DOI: 10.1017/jfm.2014.488
We present a simple two-step method by which one-dimensional spectra of horizontal velocity and buoyancy measured along a ship track can be decomposed into a wave component consisting of inertia–gravity waves and a vortex component consisting of a horizontal flow in geostrophic balance. The method requires certain assumptions for the data regarding stationarity, homogeneity, and horizontal isotropy. In the first step an exact Helmholtz decomposition of the horizontal velocity spectra into rotational and divergent components is performed and in the second step an energy equipartition property of hydrostatic inertia–gravity waves is exploited that allows a diagnosis of the wave energy spectrum solely from the observed horizontal velocities. The observed buoyancy spectrum can then be used to compute the residual vortex energy spectrum. Further wave–vortex decompositions of the observed fields are possible if additional information about the frequency content of the waves is available. We illustrate the method on two recent oceanic data sets from the North Pacific and the Gulf Stream. Notably, both steps in our new method might be of broader use in the theoretical and observational study of atmosphere and ocean fluid dynamics.https://authors.library.caltech.edu/records/wk31w-mrb23Transition from geostrophic turbulence to inertia–gravity waves in the atmospheric energy spectrum
https://resolver.caltech.edu/CaltechAUTHORS:20180112-085941209
Authors: Callies, Jörn; Ferrari, Raffaele; Bühler, Oliver
Year: 2014
DOI: 10.1073/pnas.1410772111
PMCID: PMC4260586
Midlatitude fluctuations of the atmospheric winds on scales of thousands of kilometers, the most energetic of such fluctuations, are strongly constrained by the Earth's rotation and the atmosphere's stratification. As a result of these constraints, the flow is quasi-2D and energy is trapped at large scales—nonlinear turbulent interactions transfer energy to larger scales, but not to smaller scales. Aircraft observations of wind and temperature near the tropopause indicate that fluctuations at horizontal scales smaller than about 500 km are more energetic than expected from these quasi-2D dynamics. We present an analysis of the observations that indicates that these smaller-scale motions are due to approximately linear inertia–gravity waves, contrary to recent claims that these scales are strongly turbulent. Specifically, the aircraft velocity and temperature measurements are separated into two components: one due to the quasi-2D dynamics and one due to linear inertia–gravity waves. Quasi-2D dynamics dominate at scales larger than 500 km; inertia–gravity waves dominate at scales smaller than 500 km.https://authors.library.caltech.edu/records/0e40h-ta690Seasonality in submesoscale turbulence
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100116002
Authors: Callies, Jörn; Ferrari, Raffaele; Klymak, Jody M.; Gula, Jonathan
Year: 2015
DOI: 10.1038/ncomms7862
PMCID: PMC4410631
Although the strongest ocean surface currents occur at horizontal scales of order 100 km, recent numerical simulations suggest that flows smaller than these mesoscale eddies can achieve important vertical transports in the upper ocean. These submesoscale flows, 1–100 km in horizontal extent, take heat and atmospheric gases down into the interior ocean, accelerating air–sea fluxes, and bring deep nutrients up into the sunlit surface layer, fueling primary production. Here we present observational evidence that submesoscale flows undergo a seasonal cycle in the surface mixed layer: they are much stronger in winter than in summer. Submesoscale flows are energized by baroclinic instabilities that develop around geostrophic eddies in the deep winter mixed layer at a horizontal scale of order 1–10 km. Flows larger than this instability scale are energized by turbulent scale interactions. Enhanced submesoscale activity in the winter mixed layer is expected to achieve efficient exchanges with the permanent thermocline below.https://authors.library.caltech.edu/records/dsx6y-cw150The LatMix Summer Campaign: Submesoscale Stirring in the Upper Ocean
https://resolver.caltech.edu/CaltechAUTHORS:20180112-130231208
Authors: Shcherbina, Andrey Y.; Callies, Jörn
Year: 2015
DOI: 10.1175/BAMS-D-14-00015.1
Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.https://authors.library.caltech.edu/records/28cxn-gs205The role of mixed-layer instabilities in submesoscale turbulence
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100116271
Authors: Callies, Jörn; Flierl, Glenn; Ferrari, Raffaele; Fox-Kemper, Baylor
Year: 2016
DOI: 10.1017/jfm.2015.700
Upper-ocean turbulence at scales smaller than the mesoscale is believed to exchange surface and thermocline waters, which plays an important role in both physical and biogeochemical budgets. But what energizes this submesoscale turbulence remains a topic of debate. Two mechanisms have been proposed: mesoscale-driven surface frontogenesis and baroclinic mixed-layer instabilities. The goal here is to understand the differences between the dynamics of these two mechanisms, using a simple quasi-geostrophic model. The essence of mesoscale-driven surface frontogenesis is captured by the well-known surface quasi-geostrophic model, which describes the sharpening of surface buoyancy gradients and the subsequent breakup in secondary roll-up instabilities. We formulate a similarly archetypical Eady-like model of submesoscale turbulence induced by mixed-layer instabilities. The model captures the scale and structure of this baroclinic instability in the mixed layer. A wide range of scales are energized through a turbulent inverse cascade of kinetic energy that is fuelled by the submesoscale mixed-layer instability. Major differences to mesoscale-driven surface frontogenesis are that mixed-layer instabilities energize the entire depth of the mixed layer and produce larger vertical velocities. The distribution of energy across scales and in the vertical produced by our simple model of mixed-layer instabilities compares favourably to observations of energetic wintertime submesoscale flows, suggesting that it captures the leading-order balanced dynamics of these flows. The dynamics described here in an oceanographic context have potential applications to other geophysical fluids with layers of different stratifications.https://authors.library.caltech.edu/records/9t2e3-9jw02The Dynamics of Mesoscale Winds in the Upper Troposphere and Lower Stratosphere
https://resolver.caltech.edu/CaltechAUTHORS:20180112-101230946
Authors: Callies, Jörn; Bühler, Oliver; Ferrari, Raffaele
Year: 2016
DOI: 10.1175/JAS-D-16-0108.1
Spectral analysis is applied to infer the dynamics of mesoscale winds from aircraft observations in the upper troposphere and lower stratosphere. Two datasets are analyzed: one collected aboard commercial aircraft and one collected using a dedicated research aircraft. A recently developed wave–vortex decomposition is used to test the observations' consistency with linear inertia–gravity wave dynamics. The decomposition method is shown to be robust in the vicinity of the tropopause if flight tracks vary sufficiently in altitude. For the lower stratosphere, the decompositions of both datasets confirm a recent result that mesoscale winds are consistent with the polarization and dispersion relations of inertia–gravity waves. For the upper troposphere, however, the two datasets disagree: only the research aircraft data indicate consistency with linear wave dynamics at mesoscales. The source of the inconsistency is a difference in mesoscale variance of the measured along-track wind component. To further test the observed flow's consistency with linear wave dynamics, the ratio between tropospheric and stratospheric mesoscale energy levels is compared to a simple model of upward-propagating waves that are partially reflected at the tropopause. For both datasets, the observed energy ratio is roughly consistent with the simple wave model, but wave frequencies diagnosed from the data draw into question the applicability of the monochromatic theory at wavelengths smaller than 10 km.https://authors.library.caltech.edu/records/6ywvj-48f11Baroclinic Instability in the Presence of Convection
https://resolver.caltech.edu/CaltechAUTHORS:20180112-100116533
Authors: Callies, Jörn; Ferrari, Raffaele
Year: 2018
DOI: 10.1175/JPO-D-17-0028.1
Baroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.https://authors.library.caltech.edu/records/ers83-tnt97Dynamics of an Abyssal Circulation Driven by Bottom-Intensified Mixing on Slopes
https://resolver.caltech.edu/CaltechAUTHORS:20180718-150324617
Authors: Callies, Jörn; Ferrari, Raffaele
Year: 2018
DOI: 10.1175/JPO-D-17-0125.1
The large-scale circulation of the abyssal ocean is enabled by small-scale diapycnal mixing, which observations suggest is strongly enhanced toward the ocean bottom, where the breaking of internal tides and lee waves is most vigorous. As discussed recently, bottom-intensified mixing induces a pattern of near-bottom up- and downwelling that is quite different from the traditionally assumed widespread upwelling. Here the consequences of bottom-intensified mixing for the horizontal circulation of the abyssal ocean are explored by considering planetary geostrophic dynamics in an idealized "bathtub geometry." Up- and downwelling layers develop on bottom slopes as expected, and these layers are well described by boundary layer theory. The basin-scale circulation is driven by flows in and out of these boundary layers at the base of the sloping topography, which creates primarily zonal currents in the interior and a net meridional exchange along western boundaries. The rate of the net overturning is controlled by the up- and downslope transports in boundary layers on slopes and can be predicted with boundary layer theory.https://authors.library.caltech.edu/records/d4kj5-1dd03Note on the Rate of Restratification in the Baroclinic Spindown of Fronts
https://resolver.caltech.edu/CaltechAUTHORS:20180726-133913501
Authors: Callies, Jörn; Ferrari, Raffaele
Year: 2018
DOI: 10.1175/JPO-D-17-0175.1
This paper revisits how the restratifying buoyancy flux [overbar](w'b') generated by baroclinic mixed layer instabilities depends on environmental conditions. The frontal spindown is shown to produce buoyancy fluxes that increase significantly beyond the previously proposed and widely used scaling [overbar](w'b') ~ fΛ^2H^2 (f is the Coriolis parameter, Λ is the geostrophic shear, and H is the mixed layer depth), irrespective of whether the initial front is broad or narrow. This increase occurs after the initial phase of the nonlinear evolution, when the baroclinic eddies grow in size and develop velocities significantly in excess of the scaling assumption V ~ ΛH. Implications for parameterizing the restratification caused by baroclinic mixed layer instabilities in coarse-resolution models are discussed.https://authors.library.caltech.edu/records/j4bxj-gd949Restratification of Abyssal Mixing Layers by Submesoscale Baroclinic Eddies
https://resolver.caltech.edu/CaltechAUTHORS:20180906-135924988
Authors: Callies, Jörn
Year: 2018
DOI: 10.1175/JPO-D-18-0082.1
For small-scale turbulence to achieve water mass transformation and thus affect the large-scale overturning circulation, it must occur in stratified water. Observations show that abyssal turbulence is strongly enhanced in the bottom few hundred meters in regions with rough topography, and it is thought that these abyssal mixing layers are crucial for closing and shaping the overturning circulation. If it were left unopposed, however, bottom-intensified turbulence would mix away the observed mixing-layer stratification over the course of a few years. It is proposed here that the homogenizing tendency of mixing may be balanced by baroclinic restratification. It is shown that bottom-intensified mixing, if it occurs on a large-scale topographic slope such as a midocean ridge flank, not only erodes stratification but also tilts isopycnals in the bottom few hundred meters. This tilting of isopycnals generates a reservoir of potential energy that can be tapped into by submesoscale baroclinic eddies. The eddies slide dense water under light water and thus restratify the mixing layer, similar to what happens in the surface mixed layer. This restratification is shown to be effective enough to balance the homogenizing tendency of mixing and to maintain the observed mixing-layer stratification. This suggests that submesoscale baroclinic eddies may play a crucial role in providing the stratification mixing can act on, thus allowing sustained water mass transformation. Through their restratification of abyssal mixing layers, submesoscale eddies may therefore directly affect the strength and structure of the abyssal overturning circulation.https://authors.library.caltech.edu/records/bz5ec-x1p39Submesoscale Baroclinic Instability in the Bottom Boundary Layer
https://resolver.caltech.edu/CaltechAUTHORS:20180618-140655630
Authors: Wenegrat, Jacob O.; Callies, Jörn; Thomas, Leif N.
Year: 2018
DOI: 10.1175/JPO-D-17-0264.1
Weakly stratified layers over sloping topography can support a submesoscale baroclinic instability mode, a bottom boundary layer counterpart to surface mixed layer instabilities. The instability results from the release of available potential energy, which can be generated because of the observed bottom intensification of turbulent mixing in the deep ocean, or the Ekman adjustment of a current on a slope. Linear stability analysis suggests that the growth rates of bottom boundary layer baroclinic instabilities can be comparable to those of the surface mixed layer mode and are relatively insensitive to topographic slope angle, implying the instability is robust and potentially active in many areas of the global oceans. The solutions of two separate one-dimensional theories of the bottom boundary layer are both demonstrated to be linearly unstable to baroclinic instability, and results from an example nonlinear simulation are shown. Implications of these findings for understanding bottom boundary layer dynamics and processes are discussed.https://authors.library.caltech.edu/records/f5nyf-kzh15Some Expectations for Submesoscale Sea Surface Height Variance Spectra
https://resolver.caltech.edu/CaltechAUTHORS:20190911-150907009
Authors: Callies, Jörn; Wu, Weiguang
Year: 2019
DOI: 10.1175/JPO-D-18-0272.1
In anticipation of the Surface Water and Ocean Topography (SWOT) wide-swath altimetry mission, this study reviews expectations for sea surface height (SSH) variance spectra at wavelengths of 10–100 km. Kinetic energy spectra from in situ observations and numerical simulations indicate that SSH variance spectra associated with balanced flow drop off steeply with wavenumber, with at least the negative fourth power of the wavenumber. Such a steep drop-off implies that even drastic reductions in altimetry noise yield only a modest improvement in the resolution of balanced flow. This general expectation is made concrete by extrapolating SSH variance spectra from existing altimetry to submesoscales, the results of which suggest that in the extratropics (poleward of 20° latitude) SWOT will improve the resolution from currently about 100 km to a median of 51 or 74 km, depending on whether or not submesoscale balanced flows are energetic. Internal waves, in contrast to balanced flow, give rise to SSH variance spectra that drop off relatively gently with wavenumber, so SSH variance should become strongly dominated by internal waves in the submesoscale range. In situ observations of the internal-wave field suggest that the internal-wave signal accessible by SWOT will be largely dominated by internal tides. The internal-wave continuum is estimated to have a spectral level close to but somewhat lower than SWOT's expected noise level.https://authors.library.caltech.edu/records/xx4vs-dec48SIDEBAR. Submesoscale Dynamics Inferred from Oleander Data
https://resolver.caltech.edu/CaltechAUTHORS:20190926-101952892
Authors: Callies, Jörn
Year: 2019
DOI: 10.5670/oceanog.2019.320
CMV Oleander III data played a key role in advancing our understanding of submesoscale turbulence, which is suspected to control the exchange of heat, carbon, and nutrients between the surface and the interior ocean (e.g., Ferrari, 2011; Mahadevan, 2016). Insight largely came from wavenumber spectra of kinetic energy, which decompose the flow observed with the shipboard ADCP into contributions from different spatial scales that cover a range of about 10–500 km in the case of Oleander.https://authors.library.caltech.edu/records/czz4m-58t67Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons
https://resolver.caltech.edu/CaltechAUTHORS:20200130-154916027
Authors: Ruan, Xiaozhou; Callies, Jörn
Year: 2020
DOI: 10.1175/JPO-D-19-0174.1
To close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing.https://authors.library.caltech.edu/records/tsjsv-18n59Time Scales of Submesoscale Flow Inferred from a Mooring Array
https://resolver.caltech.edu/CaltechAUTHORS:20201022-112713482
Authors: Callies, Jörn; Barkan, Roy; Naveira Garabato, Alberto
Year: 2020
DOI: 10.1175/jpo-d-19-0254.1
While the distribution of kinetic energy across spatial scales in the submesoscale range (1–100 km) has been estimated from observations, the associated time scales are largely unconstrained. These time scales can provide important insight into the dynamics of submesoscale turbulence because they help quantify to what degree the flow is subinertial and thus constrained by Earth's rotation. Here a mooring array is used to estimate these time scales in the northeast Atlantic. Frequency-resolved structure functions indicate that energetic wintertime submesoscale turbulence at spatial scales around 10 km evolves on time scales of about 1 day. While these time scales are comparable to the inertial period, the observed flow also displays characteristics of subinertial flow that is geostrophically balanced to leading order. An approximate Helmholtz decomposition shows the order 10-km flow to be dominated by its rotational component, and the root-mean-square Rossby number at these scales is estimated to be 0.3. This rotational dominance and Rossby numbers below one persist down to 2.6 km, the smallest spatial scale accessible by the mooring array, despite substantially superinertial Eulerian evolution. This indicates that the Lagrangian evolution of submesoscale turbulence is slower than the Eulerian time scale estimated from the moorings. The observations therefore suggest that, on average, submesoscale turbulence largely follows subinertial dynamics in the 1–100-km range, even if Doppler shifting produces superinertial Eulerian evolution. Ageostrophic motions become increasingly important for the evolution of submesoscale turbulence as the scale is reduced—the root-mean-square Rossby number reaches 0.5 at a spatial scale of 2.6 km.https://authors.library.caltech.edu/records/9qpzy-k1m55Abyssal Circulation Driven By Near-Boundary Mixing: Water Mass Transformations and Interior Stratification
https://resolver.caltech.edu/CaltechAUTHORS:20200716-085229115
Authors: Drake, Henri F.; Ferrari, Raffaele; Callies, Jörn
Year: 2020
DOI: 10.1175/JPO-D-19-0313.1
The emerging view of the abyssal circulation is that it is associated with bottom-enhanced mixing, which results in downwelling in the stratified ocean interior and upwelling in a bottom boundary layer along the insulating and sloping seafloor. In the limit of slowly varying vertical stratification and topography, however, boundary layer theory predicts that these upslope and downslope flows largely compensate, such that net water mass transformations along the slope are vanishingly small. Using a planetary geostrophic circulation model that resolves both the boundary layer dynamics and the large-scale overturning in an idealized basin with bottom-enhanced mixing along a midocean ridge, we show that vertical variations in stratification become sufficiently large at equilibrium to reduce the degree of compensation along the midocean ridge flanks. The resulting large net transformations are similar to estimates for the abyssal ocean and span the vertical extent of the ridge. These results suggest that boundary flows generated by mixing play a crucial role in setting the global ocean stratification and overturning circulation, requiring a revision of abyssal ocean theories.https://authors.library.caltech.edu/records/0be7b-21b78Seismic ocean thermometry
https://resolver.caltech.edu/CaltechAUTHORS:20200918-085145090
Authors: Wu, Wenbo; Zhan, Zhongwen; Peng, Shirui; Ni, Sidao; Callies, Jörn
Year: 2020
DOI: 10.1126/science.abb9519
More than 90% of the energy trapped on Earth by increasingly abundant greenhouse gases is absorbed by the ocean. Monitoring the resulting ocean warming remains a challenging sampling problem. To complement existing point measurements, we introduce a method that infers basin-scale deep-ocean temperature changes from the travel times of sound waves that are generated by repeating earthquakes. A first implementation of this seismic ocean thermometry constrains temperature anomalies averaged across a 3000-kilometer-long section in the equatorial East Indian Ocean with a standard error of 0.0060 kelvin. Between 2005 and 2016, we find temperature fluctuations on time scales of 12 months, 6 months, and ~10 days, and we infer a decadal warming trend that substantially exceeds previous estimates.https://authors.library.caltech.edu/records/xyz2a-9v642Kinetic Energy Transfers between Mesoscale and Submesoscale Motions in the Open Ocean's Upper Layers
https://resolver.caltech.edu/CaltechAUTHORS:20220721-7971000
Authors: Naveira Garabato, Alberto C.; Yu, Xiaolong; Callies, Jörn; Barkan, Roy; Polzin, Kurt L.; Frajka-Williams, Eleanor E.; Buckingham, Christian E.; Griffies, Stephen M.
Year: 2022
DOI: 10.1175/jpo-d-21-0099.1
Mesoscale eddies contain the bulk of the ocean's kinetic energy (KE), but fundamental questions remain on the cross-scale KE transfers linking eddy generation and dissipation. The role of submesoscale flows represents the key point of discussion, with contrasting views of submesoscales as either a source or a sink of mesoscale KE. Here, the first observational assessment of the annual cycle of the KE transfer between mesoscale and submesoscale motions is performed in the upper layers of a typical open-ocean region. Although these diagnostics have marginal statistical significance and should be regarded cautiously, they are physically plausible and can provide a valuable benchmark for model evaluation. The cross-scale KE transfer exhibits two distinct stages, whereby submesoscales energize mesoscales in winter and drain mesoscales in spring. Despite this seasonal reversal, an inverse KE cascade operates throughout the year across much of the mesoscale range. Our results are not incompatible with recent modeling investigations that place the headwaters of the inverse KE cascade at the submesoscale, and that rationalize the seasonality of mesoscale KE as an inverse cascade-mediated response to the generation of submesoscales in winter. However, our findings may challenge those investigations by suggesting that, in spring, a downscale KE transfer could dampen the inverse KE cascade. An exploratory appraisal of the dynamics governing mesoscale–submesoscale KE exchanges suggests that the upscale KE transfer in winter is underpinned by mixed layer baroclinic instabilities, and that the downscale KE transfer in spring is associated with frontogenesis. Current submesoscale-permitting ocean models may substantially understate this downscale KE transfer, due to the models' muted representation of frontogenesis.https://authors.library.caltech.edu/records/bfje1-sw878Rapid Spinup and Spindown of Flow along Slopes
https://resolver.caltech.edu/CaltechAUTHORS:20220517-214255122
Authors: Peterson, Henry G.; Callies, Jörn
Year: 2022
DOI: 10.1175/JPO-D-21-0173.1
The near-bottom mixing that allows abyssal waters to upwell tilts isopycnals and spins up flow over the flanks of midocean ridges. Meso- and large-scale currents along sloping topography are subjected to a delicate balance of Ekman arrest and spindown. These two seemingly disparate oceanographic phenomena share a common theory, which is based on a one-dimensional model of rotating, stratified flow over a sloping, insulated boundary. This commonly used model, however, lacks rapid adjustment of interior flows, limiting its ability to capture the full physics of spinup and spindown of along-slope flow. Motivated by two-dimensional dynamics, the present work extends the one-dimensional model by constraining the vertically integrated cross-slope transport and allowing for a barotropic cross-slope pressure gradient. This produces a closed secondary circulation by forcing Ekman transport in the bottom boundary layer to return in the interior. The extended model can thus capture Ekman spinup and spindown physics: the interior return flow is turned by the Coriolis acceleration, leading to rapid rather than slow diffusive adjustment of the along-slope flow. This transport-constrained one-dimensional model accurately describes two-dimensional mixing-generated spinup over an idealized ridge and provides a unified framework for understanding the relative importance of Ekman arrest and spindown of flow along a slope.https://authors.library.caltech.edu/records/kbyxn-ng922Surface Gravity Wave Interferometry and Ocean Current Monitoring With Ocean‐Bottom DAS
https://resolver.caltech.edu/CaltechAUTHORS:20220609-879699300
Authors: Williams, Ethan F.; Zhan, Zhongwen; Martins, Hugo F.; Fernández-Ruiz, María R.; Martín-López, Sonia; González-Herráez, Miguel; Callies, Jörn
Year: 2022
DOI: 10.1029/2021jc018375
The cross-correlation of a diffuse or random wavefield at two points has been demonstrated to recover an empirical estimate of the Green's function under a wide variety of source conditions. Over the past two decades, the practical development of this principle, termed ambient noise interferometry, has revolutionized the fields of seismology and acoustics. Yet, because of the spatial sparsity of conventional water column and seafloor instrumentation, such array-based processing approaches have not been widely utilized in oceanography. Ocean-bottom distributed acoustic sensing (OBDAS) repurposes pre-existing optical fibers laid in seafloor cables as dense arrays of broadband strain sensors, which observe both seismic waves and ocean waves. The thousands of sensors in an OBDAS array make ambient noise interferometry of ocean waves straightforward for the first time. Here, we demonstrate the application of ambient noise interferometry to surface gravity waves observed on an OBDAS array near the Strait of Gibraltar. We focus particularly on a 3-km segment of the array on the continental shelf, containing 300 channels at 10-m spacing. By cross-correlating the raw strain records, we compute empirical ocean surface gravity wave Green's functions for each pair of stations. We first apply beamforming to measure the time-averaged dispersion relation along the cable. Then, we exploit the non-reciprocity of waves propagating in a flow to recover the depth-averaged current velocity as a function of time using a waveform stretching method. The result is a spatially continuous matrix of current velocity measurements with resolution <100 m and <1 hr.https://authors.library.caltech.edu/records/h0e90-htt06Coupling Between Abyssal Boundary Layers and the Interior Ocean in the Absence of Along-Slope Variations
https://resolver.caltech.edu/CaltechAUTHORS:20220517-214258487
Authors: Peterson, Henry G.; Callies, Jörn
Year: 2022
DOI: 10.48550/arXiv.2204.05946
To close the overturning circulation, dense bottom water must upwell via turbulent mixing. Recent studies have identified thin bottom boundary layers (BLs) as locations of intense upwelling, yet it remains unclear how they interact with and shape the large-scale circulation of the abyssal ocean. The current understanding of this BL--interior coupling is shaped by 1D theory, suggesting that variations in locally produced BL transport generate exchange with the interior and thus a global circulation. Until now, however, this picture has been based on a 1D theory that fails to capture the local evolution in even highly idealized 2D geometries. The present work applies BL theory to revised 1D dynamics, which more naturally generalizes to two and three dimensions. The BL is assumed to be in quasi-equilibrium between the upwelling of dense water and the convergence of downward buoyancy fluxes. The BL transport, for which explicit formulae are presented, exerts an influence on the interior by modifying the bottom boundary condition. In 1D, this BL transport is independent of the interior evolution, but in 2D the BL and interior are fully coupled. Once interior variables and the bottom slope are allowed to vary in the horizontal, the resulting convergences and divergences in the BL transport exchange mass with the interior. This framework allows for the analysis of previously inaccessible problems such as the BL--interior coupling in the presence of an exponential interior stratification, laying the foundation for developing a full theory for the abyssal circulation.https://authors.library.caltech.edu/records/xxg9g-ca734Slantwise convection in the Irminger Sea
https://resolver.caltech.edu/CaltechAUTHORS:20220725-414674000
Authors: Le Bras, Isabela Alexander-Astiz; Callies, Jörn; Straneo, Fiamma; Carrilho Biló, Tiago; Holte, James; Johnson, Helen Louise
Year: 2022
DOI: 10.1002/essoar.10511966.1
The subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large-scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water and triggers slantwise convection. We estimate that this down-front wind forcing is four times stronger than air--sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large-scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.https://authors.library.caltech.edu/records/8wh22-ygp32Seasonality and Spatial Dependence of Mesoscale and Submesoscale Ocean Currents from Along-Track Satellite Altimetry
https://resolver.caltech.edu/CaltechAUTHORS:20230103-817548100.10
Authors: Lawrence, Albion; Callies, Jörn
Year: 2022
DOI: 10.1175/jpo-d-22-0007.1
Along-track wavenumber spectral densities of sea surface height (SSH) are estimated from Jason-2 altimetry data as a function of spatial location and calendar month to understand the seasonality of meso- and submesoscale balanced dynamics across the global ocean. Regions with significant mode-1 and mode-2 baroclinic tides are rejected, restricting the analysis to the extratropics. Where balanced motion dominates, the SSH spectral density is averaged over all pass segments in a region for each calendar month and is fit to a four-parameter model consisting of a flat plateau at low wavenumbers, a transition at wavenumber k₀ to a red power law spectrum k⁻⁵, and a white spectrum at high wavenumbers that models the altimeter noise. The monthly time series of the model parameters are compared to the evolution of the mixed layer. The annual mode of the spectral slope s reaches a minimum after the mixed layer deepens, and the annual mode of the bandpassed kinetic energy in the ranges [2k₀, 4k₀] and [k₀, 2k₀] peak ~2 and ~4 months, respectively, after the maximum of the annual mode of the mixed layer depth. This analysis is consistent with an energization of the submesoscale by a winter mixed layer instability followed by an inverse cascade of kinetic energy to the mesoscale, in agreement with prior modeling studies and in situ measurements. These results are compared to prior modeling, in situ, and satellite investigations of specific regions and are broadly consistent with them within measurement uncertainties.https://authors.library.caltech.edu/records/5rg7f-4xn22Slantwise Convection in the Irminger Sea
https://resolver.caltech.edu/CaltechAUTHORS:20221017-12657600.22
Authors: Le Bras, I. A.-A.; Callies, J.; Straneo, F.; Biló, T. C.; Holte, J.; Johnson, H. L.
Year: 2022
DOI: 10.1029/2022jc019071
The subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large-scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing in this region. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water, potentially triggering slantwise convection. We estimate that this down-front wind forcing is four times stronger than air–sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large-scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.https://authors.library.caltech.edu/records/yc755-17m04Low-Reynolds-number oscillating boundary layers on adiabatic slopes
https://resolver.caltech.edu/CaltechAUTHORS:20221024-123751900.9
Authors: Kaiser, Bryan E.; Pratt, Lawrence J.; Callies, Jörn
Year: 2022
DOI: 10.1017/jfm.2022.794
We investigate the instabilities and transition mechanisms of Boussinesq stratified boundary layers on sloping boundaries when subjected to oscillatory body forcing parallel to the slope. We examine idealized forms of boundary layers on hydraulically smooth abyssal slopes in tranquil mid- to low-latitude regions, where low-wavenumber internal tides gently heave isopycnals up and down adiabatic slopes in the absence of mean flows, high-wavenumber internal tides, shelf breaks, resonant tide–bathymetry interactions (critical slopes) and other phenomena associated with turbulence 'hot spots'. In non-rotating low-Reynolds-number flow, increased stratification on the downslope phase has a relaminarizing effect, while on the upslope phase we find transition-to-turbulence pathways arise from shear production triggered by gravitational instabilities. When rotation is significant (low slope Burger numbers) we find that boundary layer turbulence is sustained throughout the oscillation period, resembling stratified Stokes–Ekman layer turbulence. Simulation results suggest that oscillating boundary layers on smooth slopes at low Reynolds number (Re ⩽ 840), unity Prandtl number and slope Burger numbers greater than unity do not cause significant irreversible turbulent buoyancy flux (mixing), and that flat-bottom dissipation rate models derived from the tide amplitude are accurate within an order of magnitude.https://authors.library.caltech.edu/records/h7yzn-zqn25Dynamics of eddying abyssal mixing layers over sloping rough topography
https://resolver.caltech.edu/CaltechAUTHORS:20230215-30605800.7
Authors: Drake, Henri F.; Ruan, Xiaozhou; Callies, Jörn; Ogden, Kelly; Thurnherr, Andreas M.; Ferrari, Raffaele
Year: 2022
DOI: 10.1175/jpo-d-22-0009.1
The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed watermass transformations are dominated by rough topography "hotspots", where the bottom-enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger up-welling in a thin Bottom Boundary Layer (BBL). These watermass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downwards diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough's stratification. We propose simple modifications to the 1D boundary layer model which approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the non-local coupling to the basin-scale circulation.https://authors.library.caltech.edu/records/yepqf-wsm24Coupling between Abyssal Boundary Layers and the Interior Ocean in the Absence of Along-Slope Variations
https://resolver.caltech.edu/CaltechAUTHORS:20230314-844891400.9
Authors: Peterson, Henry G.; Callies, Jörn
Year: 2023
DOI: 10.1175/jpo-d-22-0082.1
To close the overturning circulation, dense bottom water must upwell via turbulent mixing. Recent studies have identified thin bottom boundary layers (BLs) as locations of intense upwelling, yet it remains unclear how they interact with and shape the large-scale circulation of the abyssal ocean. The current understanding of this BL–interior coupling is shaped by 1D theory, suggesting that variations in locally produced BL transport generate exchange with the interior and thus a global circulation. Until now, however, this picture has been based on a 1D theory that fails to capture the local evolution in even highly idealized 2D geometries. The present work applies BL theory to revised 1D dynamics, which more naturally generalizes to two and three dimensions. The BL is assumed to be in quasi-equilibrium between the upwelling of dense water and the convergence of downward buoyancy fluxes. The BL transport, for which explicit formulas are presented, exerts an influence on the interior by modifying the bottom boundary condition. In 1D, this BL transport is independent of the interior evolution, but in 2D the BL and interior are fully coupled. Once interior variables and the bottom slope are allowed to vary in the horizontal, the resulting convergences and divergences in the BL transport exchange mass with the interior. This framework allows for the analysis of previously inaccessible problems such as the BL–interior coupling in the presence of an exponential interior stratification, laying the foundation for developing a full theory for the abyssal circulation.https://authors.library.caltech.edu/records/xy3tw-xr293Vertical-Slice Ocean Tomography With Seismic Waves
https://resolver.caltech.edu/CaltechAUTHORS:20230615-812761000.6
Authors: Callies, Jörn; Wu, Wenbo; Peng, Shirui; Zhan, Zhongwen
Year: 2023
DOI: 10.1029/2023gl102881
Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin-scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.https://authors.library.caltech.edu/records/jqxmz-ke040