In Press
Wei H, Wang Y, Stewart AL, Mak J. Scalings for eddy buoyancy fluxes across prograde shelf/slope fronts. Journal of Advances in Modeling Earth Systems. In Press.
Zhao KX, Stewart AL, McWilliams JC, Fenty IG, Rignot EJ. Standing Eddies in Glacial Fjords and their Role in Fjord Circulation and Melt. Journal of Physical Oceanography. In Press.
Manucharyan GE, Stewart AL. Stirring of interior potential vorticity gradients as a formation mechanism for subsurface-intensified eddies in the Beaufort Gyre. Journal of Physical Oceanography. In Press.
Han X, Stewart AL, Chen D, Liu X, Lian T. Controls of topographic Rossby wave properties and downslope transport in dense overflows. Journal of Physical Oceanography. Submitted.
Jeong H, Lee S-S, Park H-S, Stewart AL. Future changes in Antarctic coastal polynyas and bottom water formation simulated by a high-resolution coupled model. Submitted.
Solodoch A, Stewart AL, Hogg AMC, Manucharyan G. Machine Learning-Derived Inference of the Meridional Overturning Circulation from Satellite-Observable Variables in an Ocean State Estimate. Journal of Advances in Modeling Earth Systems. Submitted.
Silvano A, Holland P, Naughten K, Dragomir O, Dutrieux P, Jenkins A, Si Y, Stewart AL, Pena-Molino B, Janzing G, et al. Baroclinic ocean response to climate forcing regulates decadal variability of ice-shelf melting in the Amundsen Sea. Geophysical Research Letters. Submitted.
Stewart AL, Neumann NK, Solodoch A. "Eddy" saturation of the Antarctic Circumpolar Current by standing waves. Journal of Physical Oceanography. Submitted.
Schmidgall CR, Si Y, Stewart AL, Thompson AF, Hogg AMC. Dynamical Controls on Bottom Water Transport and Transformation across the Antarctic Circumpolar Current. Journal of Physical Oceanography. Submitted.
Si Y, Stewart AL, Eisenman I. Coastal freshening enhances eddy-driven heat transfer toward the Antarctic margins. Submitted.
Jagannathan A, Srinivasan K, McWilliams JC, Molemaker MJ, Stewart AL. Evolution of bottom boundary layers on three dimensional topography - Buoyancy adjustment and instabilities. Submitted.
Stewart AL, McWilliams JC. Matters Arising: Gravity is Vertical in Geophysical Fluid Dynamics. Scientific Reports [Internet]. 2022;12 :6029. Publisher's Version Download PDF
Han X, Stewart AL, Chen D, Lian T, Liu X, Xie X. Topographic Rossby Wave-modulated oscillations of dense overflows. Journal of Geophysical Research: Oceans [Internet]. 2022;127 :e2022JC018702. Publisher's VersionAbstract
The global supply of Antarctic Bottom Water (AABW) is sourced from a handful of dense overflows. Observations from the Weddell Sea indicate that the overflow there exhibits prominent oscillations accompanied by dense eddies, while the Ross Sea overflow shows no significant oscillations other than tides, yet the genesis of these oscillations and their role in mediating AABW export remain poorly understood. Here idealized model simulations are used to investigate the dynamics of these oscillations. It is shown that the dominant oscillations result from the formation of Topographic Rossby waves (TRWs) associated with baroclinic instability of the dense overflow. A key finding is that the TRWs can feed back onto the dense overflow, producing coherent subsurface eddies of the same frequency. A series of sensitivity experiments reveal that these behaviors depend strongly on the local environment: steep topographic slopes suppress the baroclinic growth of TRWs, while strong downstream along-slope flows suppress the upstream propagation of TRW energy and genesis of subsurface eddies. These results explain the varying prevalence of different oscillatory phenomena observed across different dense overflow regimes.
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Solodoch A, Stewart AL, Hogg AMC, Morrison AK, Kiss AE, Thompson AF, Purkey SG, Cimoli L. How does Antarctic Bottom Water Cross the Southern Ocean?. Geophysical Research Letters [Internet]. 2022;49 :e2021GL097211. Publisher's VersionAbstract
Antarctic Bottom Water (AABW), which fills the global ocean abyss, is derived from dense water that forms in several distinct Antarctic shelf regions. Previous modeling studies have reached conflicting conclusions regarding export pathways of AABW across the Southern Ocean and the degree to which AABW originating from distinct source regions are blended during their export. This study addresses these questions using passive tracer deployments in a 61-year global high-resolution (0.1°) ocean/sea-ice simulation. Two distinct export “conduits” are identified: Weddell Sea- and Prydz Bay-sourced AABW are blended together and exported mainly to the Atlantic and Indian Oceans, while Ross Sea- and Adelie Land-sourced AABW are exported mainly to the Pacific Ocean. Northward transport of each tracer occurs almost exclusively (>90%) within a single conduit. These findings imply that regional changes in AABW production may impact the three-dimensional structure of the global overturning circulation.
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Zhao KX, Stewart AL, McWilliams JC. Linking Overturning, Recirculation, and Melt in Glacial Fjords. Geophysical Research Letters [Internet]. 2022;49 :e2021GL095706. Publisher's VersionAbstract
Fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit both overturning and horizontal recirculations, which are driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about how various fjord characteristics influence the interaction between 3D fjord circulation and glacial melt. In this study, high-resolution numerical simulations of idealized glacial fjords demonstrate that recirculation strength controls melt, which feeds back on overturning and recirculation. The relationships between overturning, recirculation, and melt rate are well predicted by vorticity balance, reduced-order melt parameterizations, and empirical scaling arguments. These theories allow us to take into account the near-glacier horizontal velocities, which yield improved predictions of fjord overturning, recirculation, and glacial melt.
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Si Y, Stewart AL, Eisenman I. Coupled ocean/sea ice dynamics of the Antarctic Slope Current driven by topographic eddy suppression and sea ice momentum redistribution. Journal of Physical Oceanography [Internet]. 2022;52 :1563-1589. Publisher's VersionAbstract
The Antarctic Slope Current (ASC) plays a central role in redistributing water masses, sea ice, and tracer properties around the Antarctic margins, and in mediating cross-slope exchanges. While the ASC has historically been understood as a wind-driven circulation, recent studies have highlighted important momentum transfers due to mesoscale eddies and tidal flows. Furthermore, momentum input due to wind stress is transferred through sea ice to the ASC during most of the year, yet previous studies have typically considered the circulations of the ocean and sea ice independently. Thus, it remains unclear how the momentum input from the winds is mediated by sea ice, tidal forcing, and transient eddies in the ocean, and how the resulting momentum transfers serve to structure the ASC. In this study the dynamics of the coupled ocean–sea ice–ASC circulation are investigated using high-resolution process-oriented simulations and interpreted with the aid of a reduced-order model. In almost all simulations considered here, sea ice redistributes almost 100% of the wind stress away from the continental slope, resulting in approximately identical sea ice and ocean surface flows in the core of the ASC in a fully spun-up equilibrium state. This ice–ocean coupling results from suppression of vertical momentum transfer by mesoscale eddies over the continental slope, which allows the sea ice to accelerate the ocean surface flow until the speeds coincide. Tidal acceleration of the along-slope flow exaggerates this effect and may even result in ocean-to-ice momentum transfer. The implications of these findings for along- and across-slope transport of water masses and sea ice around Antarctica are discussed.
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Moscoso JE, Bianchi D, Stewart AL. Controls and characteristics of biomass quantization in size-structured planktonic ecosystem models. Ecological Modelling [Internet]. 2022;468 :109907. Publisher's VersionAbstract
Strong relationships between size and other traits have long motivated studies of the size structure and dynamics of planktonic food webs. Size structured ecosystem models (SSEMs) are often used to represent the behavior of these ecosystems, with organism size as a first order approximation of the axis of biological diversity. Previous studies using SSEMs have reported the emergence of localized “peaks” in the size spectrum, a phenomenon that will be referred to in this study as “quantization”. However, SSEMs that are used routinely in Earth System Models (ESMs), they tend to be too coarsely discretized to resolve quantization. Observational studies of plankton biomass have also shown qualitatively similar patterns, with localized peaks along the size spectrum. The conditions under which quantization occurs and the ecosystem parameters that control the locations of the biomass “peaks” along the size spectrum have not been systematically explored. This study serves to simultaneously advance our understanding of the constraints on quantization in size-structured ecosystems, and to suggest an approach to discretizing SSEMs that leverages quantization to select a greatly reduced number of size classes. A size-structured model of the pelagic food web, similar to those implemented in global models, is used to investigate the sensitivity of biomass peaks to predator–prey interactions, and nutrient forcing. This study shows that the location of biomass peaks along the size spectrum is strongly controlled by the size selectivity of predation, and the location of biomass peaks along the size spectrum is less sensitive to variations in nutrient supply, external ecosystem forcing, and vertical heterogeneity. Taking advantage of a robust localization of biomass peaks, the dynamics of a continuous planktonic size spectrum to be represented using a few selected size classes, corresponding to locations of the peaks along the size spectrum. These findings offer an insight on how to approach discretization of size structured ecosystem model in Earth system models.
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Wilson EA, Thompson AF, Stewart AL, Sun S. Bathymetric control of subpolar gyres and the overturning circulation in the Southern Ocean. Journal of Physical Oceanography [Internet]. 2022;52 :205-223. Publisher's VersionAbstract
The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing.
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Lim W-I, Park H-S, Stewart AL, Seo K-H. Suppression of Arctic sea ice growth in the Eurasian-Pacific Seas by winter clouds and snowfall. Journal of Climate [Internet]. 2022;35 :669-686. Publisher's VersionAbstract
The ongoing Arctic warming has been pronounced in winter and has been associated with an increase in downward longwave radiation. While previous studies have demonstrated that poleward moisture flux into the Arctic strengthens downward longwave radiation, less attention has been given to the impact of the accompanying increase in snowfall. Here, utilizing state-of-the-art sea ice models, we show that typical winter snowfall (snow water equivalent) anomalies of around 1.0 cm, accompanied by positive downward longwave radiation anomalies of ∼5 W m−2, can cause basinwide sea ice thinning by around 5 cm in the following spring over the Arctic seas in the Eurasian–Pacific seas. In extreme cases, this is followed by a shrinking of summer ice extent. In the winter of 2016/17, anomalously strong warm, moist air transport combined with ∼2.5-cm increase in snowfall (snow water equivalent) decreased spring ice thickness by ∼10 cm and decreased the following summer sea ice extent by 5%–30%. This study suggests that small changes in the pattern and volume of winter snowfall can strongly impact the sea ice thickness and extent in the following seasons.
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Stewart AL. Physical oceanography: Warming spins up the Southern Ocean. Nature Climate Change [Internet]. 2021;11 :1022-1024. Publisher's VersionAbstract
Eastward flow in the Southern Ocean is the primary conduit between ocean basins. A comprehensive study of multi-decadal observational records and model experiments reveals that warming in the upper ocean is causing this flow to accelerate.
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