Andrew L. Stewart

University of California, Los Angeles

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Andrew F. Thompson

California Institute of Technology

James C. McWilliams

University of California, Los Angeles

Yidongfang Si

NOAA Oceanic and Atmospheric Research

Ian Eisenman

University of California, San Diego

Aviv Solodoch

Hebrew University of Jerusalem

Paul J. Dellar

University of Oxford

Andrew McC. Hogg

Australian National University

Hyo‐Seok Park

Hanyang University

Ken Zhao

Oregon State University

Alessandro Silvano

National Oceanography Centre

Cooperative Institutions

Centre National de la Recherche Scientifique

University of California, Los Angeles

Scripps Institution of Oceanography

University of California, San Diego

California Institute of Technology

University of Tasmania

Université Grenoble Alpes

University of Washington

Woods Hole Oceanographic Institution

Australian Antarctic Division

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Circumpolar Transport and Overturning Strength Inferred From Satellite Observables Using Deep Learning in an Eddying Southern Ocean Channel Model

Journal of Advances in Modeling Earth Systems (2024)

Shuai Meng Andrew L. Stewart Georgy E. Manucharyan

Abstract The Southern Ocean connects the ocean's major basins via the Antarctic Circumpolar Current (ACC), and closes the global meridional overturning circulation (MOC). Observing these transports is challenging because three‐dimensional mesoscale‐resolving measurements of currents, temperature, and salinity are required to calculate transport in density coordinates. Previous studies have proposed to circumvent these limitations by inferring subsurface transports from satellite measurements using data‐driven methods. However, it is unclear whether these approaches can identify the signatures of subsurface transport in the Southern Ocean, which exhibits an energetic mesoscale eddy field superposed on a highly heterogeneous mean stratification and circulation. This study employs Deep Learning techniques to link the transports of the ACC and the upper and lower branches of the MOC to sea surface height (SSH) and ocean bottom pressure (OBP), using an idealized channel model of the Southern Ocean as a test bed. A key result is that a convolutional neural network produces skillful predictions of the ACC transport and MOC strength (skill score of 0.74 and 0.44, respectively). The skill of these predictions is similar across timescales ranging from daily to decadal but decreases substantially if SSH or OBP is omitted as a predictor. Using a fully connected or linear neural network yields similarly accurate predictions of the ACC transport but substantially less skillful predictions of the MOC strength. Our results suggest that Deep Learning offers a route to linking the Southern Ocean's zonal transport and overturning circulation to remote measurements, even in the presence of pronounced mesoscale variability.

Circumpolar star

10.1029/2024ms004262

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Sensitivity of the ocean's deep overturning circulation to easterly Antarctic winds

Geophysical Research Letters (2012)

Andrew L. Stewart Andrew F. Thompson

We investigate the sensitivity of the deep meridional overturning circulation (MOC) in the Southern Ocean to changes in the easterly wind stress over the Antarctic continental slope. The deep MOC is driven by export of dense Antarctic Bottom Water from the Antarctic continental shelf, and exerts a strong influence on climate by ventilating the deep ocean with oxygen and storing carbon dioxide. The possibility that inter‐climatic modifications of the deep overturning may have been driven by changes in the atmospheric circulation has motivated the recent interest in evaluating the sensitivity of the Southern Ocean MOC to modifications of the mid‐latitude westerlies. Using a high‐resolution eddy‐resolving model of a sector of the Southern Ocean, we show that the deep cell of the MOC is highly sensitive to the strength of the polar easterlies, and relatively insensitive to the strength of the mid‐latitude westerlies. Our results highlight that determining the deep ocean ventilation in past and future climates demands an accurate evaluation of the concurrent surface wind stress throughout the Southern Ocean.

Westerlies

Antarctic Bottom Water

Wind Stress

10.1029/2012gl053099

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Citations (50)

Stirring of interior potential vorticity gradients as a formation mechanism for large subsurface-intensified eddies in the Beaufort Gyre

Journal of Physical Oceanography (2022)

Georgy E. Manucharyan Andrew L. Stewart

Abstract The Beaufort Gyre (BG) is hypothesized to be partially equilibrated by those mesoscale eddies that form via baroclinic instabilities of its currents. However, our understanding of the eddy field’s dependence on the mean BG currents and the role of sea ice remains incomplete. This theoretical study explores the scales and vertical structures of eddies forming specifically due to baroclinic instabilities of interior BG flows. An idealized quasi-geostrophic model is used to show that flows driven only by the Ekman pumping contain no interior potential vorticity (PV) gradients and generate weak and large eddies, ℴ(200km) in size, with predominantly barotropic and first baroclinic mode energy. However, flows containing realistic interior PV gradients in the Pacific halocline layer generate significantly smaller eddies of about 50 km in size, with a distinct second baroclinic mode structure and a subsurface kinetic energy maximum. The dramatic change in eddy characteristics is shown to be caused by the stirring of interior PV gradients by large-scale barotropic eddies. The sea ice-ocean drag is identified as the dominant eddy dissipation mechanism, leading to realistic sub-surface maxima of eddy kinetic energy for drag coefficients higher than about 2×10 −3 . A scaling law is developed for the eddy potential enstrophy, demonstrating that it is directly proportional to the interior PV gradient and the square root of the barotropic eddy kinetic energy. This study proposes a possible formation mechanism of large BG eddies and points to the importance of accurate representation of the interior PV gradients and eddy dissipation by ice-ocean drag in BG simulations and theory.

Barotropic fluid

Eddy

10.1175/jpo-d-21-0040.1

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Dynamic and Thermodynamic Impacts of the Winter Arctic Oscillation on Summer Sea Ice Extent

Journal of Climate (2017)

Hyo‐Seok Park Andrew L. Stewart Jun‐Hyeok Son

Arctic summer sea ice extent exhibits substantial interannual variability, as is highlighted by the remarkable recovery in sea ice extent in 2013 following the record minimum in the summer of 2012. Here, the mechanism via which Arctic Oscillation (AO)-induced ice thickness changes impact summer sea ice is explored, using observations and reanalysis data. A positive AO weakens the basin-scale anticyclonic sea ice drift and decreases the winter ice thickness by 15 and 10 cm in the Eurasian and the Pacific sectors of the Arctic, respectively. Three reanalysis datasets show that the upward surface heat fluxes are reduced over wide areas of the Arctic, suppressing the ice growth during the positive AO winters. The winter dynamic and thermodynamic thinning preconditions the ice for enhanced radiative forcing via the ice–albedo feedback in late spring–summer, leading to an additional 10 cm of thinning over the Pacific sector of the Arctic. Because of these winter AO-induced dynamic and thermodynamics effects, the winter AO explains about 22% (r = −0.48) of the interannual variance of September sea ice extent from 1980 to 2015.

Arctic geoengineering

Ice-albedo feedback

Arctic dipole anomaly

Sea ice concentration

Arctic oscillation

10.1175/jcli-d-17-0067.1

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Citations (36)

The Mann Eddy: formation and interaction with the North Atlantic Current

Ocean Sciences Meeting 2020 (2020)

Aviv Solodoch Andrew L. Stewart James C. McWilliams

Source

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An analytical model for wind-driven Arctic summer sea ice drift

H.-S. Park Andrew L. Stewart

Abstract. The authors present an approximate analytical model for wind-induced sea-ice drift that includes an ice–ocean boundary layer with an Ekman spiral in the ocean velocity. This model provides an analytically tractable solution that is most applicable to the marginal ice zone, where sea-ice concentration is substantially below 100%. The model closely reproduces the ice and upper-ocean velocities observed recently by the first ice-tethered profiler equipped with a velocity sensor (ITPV). The analytical tractability of our model allows efficient calculation of the sea-ice velocity provided that the surface wind field is known and that the ocean surface geostrophic velocity is relatively weak. The model is applied to estimate intraseasonal variations in Arctic sea ice cover due to short-timescale (around 1 week) intensification of the southerly winds. Utilizing 10 m surface winds from ERA-Interim reanalysis, the wind-induced sea-ice velocity and the associated changes in sea-ice concentration are calculated and compared with satellite observations. The analytical model captures the observed reduction of Arctic sea-ice concentration associated with the strengthening of southerlies on intraseasonal time scales. Further analysis indicates that the wind-induced surface Ekman flow in the ocean increases the sea-ice drift speed by 50% in the Arctic summer. It is proposed that the southerly wind-induced sea-ice drift, enhanced by the ocean's surface Ekman transport, can lead to substantial reduction in sea-ice concentration over a timescale of one week.

Ekman layer

Sea ice concentration

Ekman transport

10.5194/tcd-9-2101-2015

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Citations (3)

Ice-Ocean Interactions and Circulation Around the Antarctic Margins IV

Andrew L. Stewart Andrew F. Thompson Louise C. Biddle Matthew H. England

Circulation (fluid dynamics)

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Final results from McCoy et al., 'Global Observations of Submesoscale Coherent Vortices in the Ocean', submitted to Progress in Oceanography.

Zenodo (CERN European Organization for Nuclear Research) (2020)

Daniel McCoy Daniele Bianchi Andrew L. Stewart

Submesoscale coherent vortices (SCVs) are small-scale, subsurface eddies that are ubiquitous in the ocean. Observations suggest that they efficiently trap and transport water, nutrients, and other properties thousands of kilometers away from their formation regions. However, the weak sea-surface signature restricts SCV observations to mostly chance encounters with shipboard subsurface instrumentation. Thus, the global occurrence, properties, and generation frequency of SCVs remain poorly constrained. Here we present results from a new algorithm used to identify SCVs from Argo float data, applied to roughly 2 million profiles conducted globally from August 1997 to January 2020. After application of the SCV detection algorithm to the global Argo array, we identify 2501 casts piercing spicy-core SCVs (those with anomalously hot and salty water mass characteristics), and 1583 casts piercing minty-core SCVs (anomalously cold and fresh cores) over more than 20 years of available data. The Matlab file 'final_individual_scvs.mat' contains various data for each SCV identified. By grouping detections from consecutive Argo casts, we are also able to record 383 spicy-core SCV time-series and 169 minty-core SCV time-series. The Matlab file 'final_timeseries_scvs.mat' contains the data for these time-series. For a more detailed description of each Matlab file, please see 'README.rtf'. Reach out to Daniel McCoy (dmccoy801@gmail.com) or Daniele Bianchi (dbianchi@atmos.ucla.edu) for inquiries.

Marine geology

10.5281/zenodo.4037442

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Comments on “horizontal gravity disturbance vector in atmospheric dynamics” by Peter C. Chu

Dynamics of Atmospheres and Oceans (2023)

Edmund K. M. Chang Christopher L. Wolfe Andrew L. Stewart James C. McWilliams

In a recent paper (Chu23), the author formulated the equations governing atmospheric motion in a spheroidal coordinate system. Since the mass distribution of the Earth is not exactly spheroidal, the true gravity is not vertical in that coordinate system. Chu23 compared the magnitude of the horizontal component of gravity in that system to those of the dynamically active forces and concluded that the horizontal components of gravity should not be neglected. In recent papers by the authors (CW22 and SM22), we explained that the actual interpretation of the approximation made in atmospheric and oceanic modeling is not neglecting the horizontal component of the true gravity, but is a geometrical approximation, approximating nearly spheroidal geopotential surfaces with bumps on which the true gravity is vertical by exactly spheroidal surfaces. We showed that under such an interpretation, the errors due to the geometrical approximation are small. Chu23 claimed that CW22 and SM22 erroneously neglected the gravity perturbations in their analyses. Here, we explain further the differences between these approaches, in the process showing that the criticisms of Chu23 on CW22 and SM22 are invalid, further supporting our conclusion that the horizontal component of the true gravity is not relevant in ocean and atmospheric dynamics.

Geopotential

Component (thermodynamics)

Horizontal and vertical

10.1016/j.dynatmoce.2023.101382

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Citations (2)

Multilayer shallow water equations with complete Coriolis force. Part 2. Linear plane waves

Journal of Fluid Mechanics (2011)

Andrew L. Stewart Paul J. Dellar

Abstract We investigate the behaviour of linear plane waves in multilayer shallow water equations that include a complete treatment of the Coriolis force. These equations improve upon the conventional shallow water equations, based on the traditional approximation, that include only the part of the Coriolis force due to the locally vertical component of the rotation vector. Including the complete Coriolis force leads to dramatic changes in the structure of long linear plane waves. It allows subinertial waves to exist with frequencies below the inertial frequency, the minimum frequency for which waves exist under the traditional approximation. These subinertial waves are characterized by a distinguished limit in which the horizontal pressure gradient becomes comparable to the upwellings and downwellings driven by the non-traditional Coriolis term in the vertical momentum equation. The subinertial waves connect wave modes that remain separate in the conventional multilayer shallow water equations, such as the surface and internal waves in a two-layer system. Eastward-propagating surface waves in a two-layer system connect with westward-propagating internal waves, and vice versa, via the long subinertial waves. The long subinertial waves cannot be classified as either surface or internal waves, due to the phase difference between the disturbances to the interfaces in these waves.

Inertial wave

Love wave

Boussinesq approximation (buoyancy)

10.1017/jfm.2011.364

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Citations (12)