In Press
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. In Press.
Han X, Stewart AL, Chen D, Lian T, Liu X, Xie X. Topographic Rossby Wave-modulated oscillations of dense overflows. Geophysical Research Letters. Submitted.
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. Submitted.
Zhao KX, Stewart AL, McWilliams JC. Linking Overturning, Recirculation, and Melt in Glacial Fjords. Geophysical Research Letters. Submitted.
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. Submitted.
Moscoso JE, Bianchi D, Stewart AL. Emergent Quantization in Size-Structured Planktonic Ecosystems. Ecological Modelling. Submitted.
Jagannathan A, Srinivasan K, McWilliams JC, Molemaker MJ, Stewart AL. Evolution of bottom boundary layers on three dimensional topography - Buoyancy adjustment and instabilities. Journal of Physical Oceanography. Submitted.
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. Submitted.
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. 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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Stewart AL, Chi X, Solodoch A, Hogg AM. High-frequency fluctuations in Antarctic Bottom Water transport driven by Southern Ocean winds. Geophysical Research Letters [Internet]. 2021;8 :e2021GL094569. Publisher's VersionAbstract
Northward flow of Antarctic Bottom Water (AABW) across the Southern Ocean comprises a key component of the global overturning circulation. Yet AABW transport remains poorly constrained by observations and state estimates, and there is presently no means of directly monitoring any component of the Southern Ocean overturning. However, AABW flow is dynamically linked to Southern Ocean surface circulation via the zonal momentum balance, offering potential routes to indirect monitoring of the transport. Exploiting this dynamical link, this study shows that wind stress (WS) fluctuations drive large AABW transport fluctuations on time scales shorter than grl62918-math-00012 years, which comprise almost all of the transport variance. This connection occurs due to differing time scales on which topographic and interfacial form stresses respond to wind variability, likely associated with differences in barotropic versus baroclinic Rossby wave propagation. These findings imply that AABW transport variability can largely be reconstructed from the surface WS alone.
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Stewart AL. Mesoscale, tidal and seasonal/interannual drivers of the Weddell Sea overturning circulation. Journal of Physical Oceanography [Internet]. 2021;51 :3695–3722. Publisher's VersionAbstract
The Weddell Sea supplies 40–50% of the Antarctic BottomWaters that fill the global ocean abyss, and therefore exerts significant influence over global circulation and climate. Previous studies have identified a range of different processes that may contribute to dense shelf water (DSW) formation and export on the southern Weddell Sea continental shelf. However, the relative importance of these processes has not been quantified, which hampers prioritization of observational deployments and development of model parameterizations in this region. In this study a high-resolution (1/12°) regional model of the southern Weddell Sea is used to quantify the overturning circulation and decompose it into contributions due to multi-annual mean flows, seasonal/interannual variability, tides, and other sub-monthly variability. It is shown that tides primarily influence the overturning by changing the melt rate of the Filchner-Ronne Ice Shelf (FRIS). The resulting ~0.2 Sv decrease in DSW transport is comparable to the magnitude of the overturning in the FRIS cavity, but small compared to DSW export across the continental shelf break. Seasonal/interannual fluctuations exert a modest influence on the overturning circulation due to the relatively short (8-year) analysis period. Analysis of the transient energy budget indicates that the non-tidal, sub-monthly variability is primarily baroclinically-generated eddies associated with dense overflows. These eddies play a comparable role to the mean flow in exporting dense shelf waters across the continental shelf break, and account for 100% of the transfer of heat onto the continental shelf. The eddy component of the overturning is sensitive to model resolution, decreasing by a factor of ~2 as the horizontal grid spacing is refined from 1/3° to 1/12°.
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Jagannathan A, Srinivasan K, McWilliams JC, Molemaker MJ, Stewart AL. Boundary layer-mediated vorticity generation in currents over sloping bathymetry. Journal of Physical Oceanography [Internet]. 2021;51 :1757-1778. Publisher's VersionAbstract
Current–topography interactions in the ocean give rise to eddies spanning a wide range of spatial and temporal scales. The latest modeling efforts indicate that coastal and underwater topography are important generation sites for submesoscale coherent vortices (SCVs), characterized by horizontal scales of O(0.1−10)km. Using idealized, submesoscale and bottom boundary layer (BBL)-resolving simulations and adopting an integrated vorticity balance formulation, we quantify precisely the role of BBLs in the vorticity generation process. In particular, we show that vorticity generation on topographic slopes is attributable primarily to the torque exerted by the vertical divergence of stress at the bottom. We refer to this as the bottom stress divergence torque (BSDT). BSDT is a fundamentally nonconservative torque that appears as a source term in the integrated vorticity budget and is to be distinguished from the more familiar bottom stress curl (BSC). It is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom and is in fact shown to be the dominant component of BPT in solutions with a well-resolved BBL. This suggests an interpretation of BPT as the sum of a viscous, vorticity-generating component (BSDT) and an inviscid, “flow-turning” component. Companion simulations without bottom drag illustrate that although vorticity generation can still occur through the inviscid mechanisms of vortex stretching and tilting, the wake eddies tend to have weaker circulation, be substantially less energetic, and have smaller spatial scales.
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Bai Y, Wang Y, Stewart AL. Does Topographic Form Stress Impede Prograde Ocean Currents?. Journal of Physical Oceanography [Internet]. 2021;51 :2617-2638. Publisher's VersionAbstract
Topographic form stress (TFS) plays a central role in constraining the transport of the Antarctic Circumpolar Current (ACC), and thus the rate of exchange between the major ocean basins. Topographic form stress generation in the ACC has been linked to the formation of standing Rossby waves, which occur because the current is retrograde (opposing the direction of Rossby wave propagation). However, it is unclear whether TFS similarly retards current systems that are prograde (in the direction of Rossby wave propagation), which cannot arrest Rossby waves. An isopycnal model is used to investigate the momentum balance of wind-driven prograde and retrograde flows in a zonal channel, with bathymetry consisting of either a single ridge or a continental shelf and slope with a meridional excursion. Consistent with previous studies, retrograde flows are almost entirely impeded by TFS, except in the limit of flat bathymetry, whereas prograde flows are typically impeded by a combination of TFS and bottom friction. A barotropic theory for standing waves shows that bottom friction serves to shift the phase of the standing wave’s pressure field from that of the bathymetry, which is necessary to produce TFS. The mechanism is the same in prograde and retrograde flows, but is most efficient when the mean flow arrests a Rossby wave with a wavelength comparable to that of the bathymetry. The asymmetry between prograde and retrograde momentum balances implies that prograde current systems may be more sensitive to changes in wind forcing, for example associated with climate shifts.
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Cohanim K, Zhao KX, Stewart AL. Dynamics of Eddies Generated by Sea Ice Leads. Journal of Physical Oceanography [Internet]. 2021;51 :3071–3092. Publisher's VersionAbstract
Interaction between the atmosphere and ocean in sea ice–covered regions is largely concentrated in leads, which are long, narrow openings between sea ice floes. Refreezing and brine rejection in these leads inject salt that plays a key role in maintaining the polar halocline. The injected salt forms dense plumes that subsequently become baroclinically unstable, producing submesoscale eddies that facilitate horizontal spreading of the salt anomalies. However, it remains unclear which properties of the stratification and leads most strongly influence the vertical and horizontal spreading of lead-input salt anomalies. In this study, the spread of lead-injected buoyancy anomalies by mixed layer and eddy processes are investigated using a suite of idealized numerical simulations. The simulations are complemented by dynamical theories that predict the plume convection depth, horizontal eddy transfer coefficient, and eddy kinetic energy as functions of the ambient stratification and lead properties. It is shown that vertical penetration of buoyancy anomalies is accurately predicted by a mixed layer temperature and salinity budget until the onset of baroclinic instability (~3 days). Subsequently, these buoyancy anomalies are spread horizontally by eddies. The horizontal eddy diffusivity is accurately predicted by a mixing-length scaling, with a velocity scale set by the potential energy released by the sinking salt plume and a length scale set by the deformation radius of the ambient stratification. These findings indicate that the intermittent opening of leads can efficiently populate the polar halocline with submesoscale coherent vortices with diameters of ~10 km, and they provide a step toward parameterizing their effect on the horizontal redistribution of salinity anomalies.
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Solodoch A, Stewart AL, McWilliams JC. Formation of anticyclones above topographic depressions. Journal of Physical Oceanography [Internet]. 2021;51 :207-228. Publisher's VersionAbstract
Long-lived anticyclonic eddies (ACs) have been repeatedly observed over several North Atlantic basins characterized by bowl-like topographic depressions. Motivated by these previous findings, the authors conduct numerical simulations of the spindown of eddies initialized in idealized topographic bowls. In experiments with one or two isopycnal layers, it is found that a bowl-trapped AC is an emergent circulation pattern under a wide range of parameters. The trapped AC, often formed by repeated mergers of ACs over the bowl interior, is characterized by anomalously low potential vorticity (PV). Several PV segregation mechanisms that can contribute to the AC formation are examined. In one-layer experiments, the dynamics of the AC are largely determined by a nonlinearity parameter ϵ that quantifies the vorticity of the AC relative to the bowl’s topographic PV gradient. The AC is trapped in the bowl for low ϵ≲1, but for moderate values (0.5≲ϵ≲1) partial PV segregation allows the AC to reside at finite distances from the center of the bowl. For higher ϵ≳1, eddies freely cross the topography and the AC is not confined to the bowl. These regimes are characterized across a suite of model experiments using ϵ and a PV homogenization parameter. Two-layer experiments show that the trapped AC can be top or bottom intensified, as determined by the domain-mean initial vertical energy distribution. These findings contrast with previous theories of mesoscale turbulence over topography that predict the formation of a prograde slope current, but do not predict a trapped AC.
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Stewart AL, McWilliams JC, Solodoch A. On the role of bottom pressure torques in wind-driven gyres. Journal of Physical Oceanography [Internet]. 2021;51 :1441-1464. Publisher's VersionAbstract
Previous studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the winds only when integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments.
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Moscoso JE, Stewart AL, Bianchi D, McWilliams JC. The Meridionally Averaged Model of Eastern Boundary Upwelling Systems (MAMEBUSv1.0). Geoscientific Model Development [Internet]. 2021;14 :763-794. Publisher's VersionAbstract
Eastern boundary upwelling systems (EBUSs) are physically and biologically active regions of the ocean with substantial impacts on ocean biogeochemistry, ecology, and global fish catch. Previous studies have used models of varying complexity to study EBUS dynamics, ranging from minimal two-dimensional (2-D) models to comprehensive regional and global models. An advantage of 2-D models is that they are more computationally efficient and easier to interpret than comprehensive regional models, but their key drawback is the lack of explicit representations of important three-dimensional processes that control biology in upwelling systems. These processes include eddy quenching of nutrients and meridional transport of nutrients and heat. The authors present the Meridionally Averaged Model of Eastern Boundary Upwelling Systems (MAMEBUS) that aims at combining the benefits of 2-D and 3-D approaches to modeling EBUSs by parameterizing the key 3-D processes in a 2-D framework. MAMEBUS couples the primitive equations for the physical state of the ocean with a nutrient– phytoplankton–zooplankton–detritus model of the ecosystem, solved in terrain-following coordinates. This article defines the equations that describe the tracer, momentum, and biological evolution, along with physical parameterizations of eddy advection, isopycnal mixing, and boundary layer mixing. It describes the details of the numerical schemes and their implementation in the model code, and provides a reference solution validated against observations from the California Current. The goal of MAMEBUS is to facilitate future studies to efficiently explore the wide space of physical and biogeochemical parameters that control the zonal variations in EBUSs.
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Zhao KX, Stewart AL, McWilliams JC. Geometric Constraints on Glacial Fjord-Shelf Exchange. Journal of Physical Oceanography [Internet]. 2021;51 :1223-1246. Publisher's VersionAbstract
The oceanic connections between tidewater glaciers and continental shelf waters are modulated and controlled by geometrically complex fjords. These fjords exhibit both overturning circulations and horizontal recirculations, driven by a combination of water mass transformation at the head of the fjord, variability on the continental shelf, and atmospheric forcing. However, it remains unclear which geometric and forcing parameters are the most important in exerting control on the overturning and horizontal recirculation. To address this, idealized numerical simulations are conducted using an isopycnal model of a fjord connected to a continental shelf, which is representative of regions in Greenland and the West Antarctic Peninsula. A range of sensitivity experiments demonstrate that sill height, wind direction/strength, subglacial discharge strength, and depth of offshore warm water are of first-order importance to the overturning circulation, while fjord width is also of leading importance to the horizontal recirculation. Dynamical predictions are developed and tested for the overturning circulation of the entire shelf-to-glacierface domain, subdivided into three regions: the continental shelf extending from the open ocean to the fjord mouth, the sill-overflow at the fjord mouth, and the plume-driven water mass transformation at the fjord head. A vorticity budget is also developed to predict the strength of the horizontal recirculation, which provides a scaling in terms of the overturning and bottom friction. Based on these theories, we may predict glacial melt rates that take into account overturning and recirculation, which may be used to refine estimates of ocean-driven melting of the Greenland and Antarctic ice sheets.
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McCoy DE, Bianchi D, Stewart AL. Global Observations of Submesoscale Coherent Vortices in the Ocean. Progress in Oceanography [Internet]. 2020;189 :102452. Publisher's VersionAbstract
Subsurface-intensified anticyclones are ubiquitous in the ocean, yet their impact on the large-scale transport of heat, salt and chemical tracers is poorly understood. These submesoscale coherent vortices (SCVs) can trap and advect waters thousands of kilometers away from the formation region, providing a transport pathway that is unresolved by low-resolution Earth System Models. However, knowledge of the importance of these eddies for the large scale circulation is hindered by the lack of systematic observations. Here, we take advantage of the global network of Argo floats to identify occurrences of these eddies, which appear as weakly stratified anomalous water masses with Gaussian-shaped vertical structures. We develop a general algorithm to detect subsurface eddies that have propagated away from their source region, and apply it to the database of Argo float profiles, resulting in roughly 4000 detections from more than 20 years of observations. We further group detections into regional populations to identify hot-spots of generation and mechanisms of formation. Analysis of regional SCV statistics reveals important sites of SCV generation in Eastern Boundary Upwelling Systems, marginal sea overflows, and mode water formation regions along major open-ocean fronts. Because of the heat and salt anomaly contained within their cores, SCV could leave a significant imprint on the hydrographic properties of water masses in regions of high SCV density.
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