Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.

The Antarctic Slope Front (ASF) is a strong gradient in water mass properties close to the Antarctic margins, separating warm water from the Antarctic ice sheet. Heat transport across the ASF is important to Earth’s climate, as it influences melting of ice shelves, the formation of bottom water, and thus the global meridional overturning circulation. Previous studies based on relatively low-resolution global models have reported contradictory findings regarding the impact of additional meltwater on heat transport toward the Antarctic continental shelf: It remains unclear whether meltwater enhances shoreward heat transport, leading to a positive feedback, or further isolates the continental shelf from the open ocean. In this study, heat transport across the ASF is investigated using eddy- and tide-resolving, process-oriented simulations. It is found that freshening of the fresh coastal waters leads to increased shoreward heat flux, which implies a positive feedback in a warming climate: Increased meltwater will increase shoreward heat transport, causing further melt of ice shelves.

A current along a sloping bottom gives rise to upwelling, or downwelling Ekman transport within the stratified bottom boundary layer (BBL), also known as the bottom Ekman layer. In 1D models of slope currents, geostrophic vertical shear resulting from horizontal buoyancy gradients within the BBL is predicted to eventually bring the bottom stress to zero, leading to a shutdown, or “arrest,” of the BBL. Using 3D ROMS simulations, we explore how the dynamics of buoyancy adjustment in a current-ridge encounter problem differs from 1D and 2D temporal spin up problems. We show that in a downwelling BBL, the destruction of the ageostrophic BBL shear, and hence the bottom stress, is accomplished primarily by nonlinear straining effects during the initial topographic encounter. As the current advects along the ridge slopes, the BBL deepens and evolves toward thermal wind balance. The onset of negative potential vorticitymodes of instability and their subsequent dissipation partially offsets the reduction of the BBL dissipation during the ridge-current interaction. On the upwelling side, although the bottom stress weakens substantially during the encounter, the BBL experiences a horizontal inflectional point instability and separates from the slopes before sustained along-slope stress reduction can occur. In all our solutions, both the upwelling and downwelling BBLs are in a partially arrested state when the current separates from the ridge slope, characterized by a reduced, but non-zero bottom stress on the slopes.

Warm ocean waters drive rapid ice-shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near-bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice-ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth-dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf-break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice-shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth-independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice-shelf response in the Amundsen Sea to climate forcing.

Depth-averaged eddy buoyancy diffusivities across continental shelves and slopes are investigated using a suite of eddy-resolving, process-oriented simulations of prograde frontal currents characterized by isopycnals tilted in the opposite direction to the seafloor, a flow regime commonly found along continental margins under downwelling-favorable winds or occupied by buoyant boundary currents. The diagnosed cross-slope eddy diffusivity varies by up to three orders of magnitude, decaying from  in the relatively flat-bottomed region to  over the steep continental slope, consistent with previously reported suppression effects of steep topography on baroclinic eddy fluxes. To theoretically constrain the simulated cross-slope eddy fluxes, we examine extant scalings for eddy buoyancy diffusivities across prograde shelf/slope fronts and in flat-bottomed oceans. Among all tested scalings, the GEOMETRIC framework developed by D. P. Marshall et al. (2012, https://doi.org/10.1175/JPO-D-11-048.1) and a parametrically similar Eady scale-based scaling proposed by Jansen et al. (2015, https://doi.org/10.1016/j.ocemod.2015.05.007) most accurately reproduce the diagnosed eddy diffusivities across the entire shelf-to-open-ocean analysis regions in our simulations. This result relies upon the incorporation of the topographic suppression effects on eddy fluxes, quantified via analytical functions of the slope Burger number, into the scaling prefactor coefficients. The predictive skills of the GEOMETRIC and Eady scale-based scalings are shown to be insensitive to the presence of along-slope topographic corrugations. This work lays a foundation for parameterizing eddy buoyancy fluxes across large-scale prograde shelf/slope fronts in coarse-resolution ocean models.

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.

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.

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.

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.

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.

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.

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 quasigeostrophic 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(200)km 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 subsurface 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.

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.

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.

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 2 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.

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°.

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.

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.

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.