Abstract The accurate computation of pressure gradients in ocean models is essential for simulating ocean and climate processes. Inaccurate computation can result in spurious velocities on the order of real ocean flows, particularly in quiescent ice shelf cavities. We introduce improvements to a finite volume discretization of the pressure gradient force, required with a sloped ocean surface boundary beneath ice shelves and when model layers intersect with ice shelves and bathymetry. We demonstrate the sensitivity of pressure gradient force calculations to choices of sub‐grid cell pressure, density and geopotential distributions. Our method results in a marked improvement in the simulation of pressure gradient forces in idealized ice shelf cavities, with spurious velocities reduced to order 10−9 m s−1 or smaller in quiet, linear stratification test cases. These velocities are substantially smaller than the magnitude of real ocean flows beneath ice shelf cavities, providing confidence in their simulated flows. Though we provide methods to reduce spontaneous motion from very thin ocean layers subject to numerical errors, these methods could be further improved, as could the small spurious flows that remain with nonlinear stratification.

Antarctic Bottom Water (AABW) is a cold, dense water mass formed around Antarctica that ventilates the global abyssal ocean. Recent studies have reported widespread warming, freshening, and thinning of AABW, yet its temporal variability remains poorly constrained because of sparse observations. This limits our ability to distinguish trends induced by climate change from natural variability. We address this limitation by examining seasonal and interannual AABW variability using dye-like tracers in the high-resolution ocean-sea ice model ACCESS-OM2-01. The tracers are released in four key AABW formation regions: the Ross and Weddell Seas, Prydz Bay and the shelf adjacent to Adélie Land. We find pronounced differences in the magnitude and time scales of AABW variability across formation regions. AABW from the narrow-shelf regions (Prydz Bay and Adélie Land coast) is characterised by high seasonal variability (0.16 tracer units) that can be traced at depths greater than 3500 m. Seasonal variability of AABW formed on the wide shelves is below 0.08 tracer units. Interannual variability of AABW is strongest (0.16 tracer units) along the continental slope in the Weddell Sea. Advective timescales show that AABW from narrow-shelf sources ventilate the abyss on much shorter time scales (≤1 year) than AABW from wide shelves (≥2years). These findings provide new context for interpreting observed deep-ocean changes, helping to distinguish anthropogenic trends from natural variability and sampling limitations.

Ocean-driven basal melting of Antarctic ice shelves plays an important role in the mass loss of the Antarctic Ice Sheet. Ice shelf cavity-resolving ocean models are a valuable tool for understanding ice shelf-ocean interactions and for simulating projections of ice shelf and ocean states under future climate. Designed to assess the current state of ice shelf-ocean modelling, the second Ice Shelf-Ocean Model Intercomparison Project, ISOMIP+, consists of 12 ocean model configurations submitted with a common, idealised experimental setup. Here, we focus on the experiments Ocean0-2 , which are ocean models with idealised, static ice shelf geometries, but where the ocean reaches a balance with prescribed far-field ocean conditions. Different thermal transfer coefficient values (ranging from 0.011 to 0.2) are used for each model in the melting parameterisation to achieve a common, tuned melt rate since the models cover a range of types of vertical coordinates, ice-ocean boundary layer treatments, and numerical schemes. These model differences lead to spread in the resultant ocean properties, circulation, boundary-layer structure and spatial distribution of melting. We also highlight similarities between models, such as a shared linear relationship across most models between melt rate and overturning and barotropic streamfunctions during the spin-up and spin-down, demonstrating a robust relationship between melt and circulation across models and forcing conditions. The ISOMIP+ results provide a systematic comparison of ice shelf cavity-capable ocean models. However, we also demonstrate the need for realistic ice shelf-ocean model intercomparison projects (some already underway) to assess model biases and inter-model variation against sparse observations. Further research is needed to understand the differences between models and further improve our modelled representations of the ice-ocean boundary layer and ice shelf cavity circulation.

Circumpolar Deep Water (CDW) is a warm water mass in the deep Southern Ocean and its transport onto the Antarctic continental shelf is the main contributor of heat to the region. Observations suggest that CDW has warmed and shifted poleward recently, potentially contributing to increased poleward heat transport and the accelerated melting of Antarctic ice shelves. However, due to the sparsity of observations and the complex dependencies of CDW on changes in both the ocean interior and surface conditions, the processes driving its warming in the Southern Ocean remain unclear. In this study, we employ a high-resolution global ocean-sea ice model to investigate the mechanisms of CDW change, focusing on future change at the end of 21st century. We conduct separate perturbation experiments to isolate the effects of projected surface buoyancy forcing (from heat and salt fluxes) and wind stress. Our findings reveal a warming signal of CDW driven by surface buoyancy forcing, contrasted by a reduced warming signal when wind stress changes are also included. A heat budget analysis shows that the warming of CDW is due to reduced winter cooling from mixing with warmed waters above and below the thermocline. These changes in mixing processes include weaker convective mixing due to increased stratification, and less heat loss by vertical diffusion because of the warmer ocean surface. In addition to changes in the mixing processes, the warming of deeper CDW is partly driven by a reduced Antarctic Bottom Water formation, which causes isopycnal descent and thickening of CDW.

Abstract The Weddell and Ross Gyres influence climate‐relevant processes including poleward heat transport, nutrient upwelling and carbon sequestration. However, long‐term tracking of changes to their circulation is limited by the sparsity of observations characteristic of the region. Here we present observational estimates of gyre strength and area over 2003–2023 using satellite altimetry and gridded hydrographic products. We find that strength and area co‐vary across different timescales. The Weddell Gyre has an annual cycle with an amplitude of 20 Sv and a late‐autumn maximum dominating month‐to‐month changes. This gyre also displays a strengthening trend of 0.9 Sv per year until 2015. The Ross Gyre has an annual cycle peaking in late autumn with an amplitude of 10 Sv, but its variability is dominated by interannual changes (±7 Sv). Analysis using an ocean‐sea ice model suggests our estimates likely capture >87% and >66% of the variability of the Weddell and Ross Gyres.

Today, the Southern Ocean (SO) is the primary sink of anthropogenic heat and carbon, yet anthropogenic warming and ozone depletion have altered its circulation and stratification, creating uncertainty about the persistence of this sink. Evidence from the last glacial period and deglaciation shows that the SO played a central role in past climate transitions by releasing heat and carbon to the atmosphere through sea-ice retreat and enhanced ventilation of deep and abyssal waters. Antarctic warming during weak Atlantic Meridional Overturning Circulation (AMOC) phases was likely amplified by SO feedbacks, such as increased deep-ocean convection, and a strengthening/poleward shift of Southern Hemisphere westerlies. However, changes in the abyssal circulation, its interaction with North Atlantic Deep Water, and its impact on oceanic carbon during the deglaciation remain debated. Finally, although sediment and modeling evidence suggests that Antarctic Ice Sheet (AIS) discharge can significantly affect climate and biogeochemistry, clear signals remain scarce in existing proxy records-either masked by dominant AMOC-driven variability or unresolved due to limited temporal resolution. Improved model representation and proxy records are needed to clarify the role of abyssal circulation and the interaction between the ocean and AIS.

Tides can modify sea surface height by several meters, yet their representation in models remains imperfect, largely owing to errors in simulating tidal energy loss. In the open ocean, barotropic tides lose energy to internal tides; this process is typically parameterized using a wave drag scaled by a scaling factor greater than one to compensate for processes not captured by the parameterization. Here, we implement a novel wave drag parameterization consistent with linear internal tide theory in MOM6 and run a global, barotropic, -tide-only model to examine the sensitivity of modeled tides to resolution. We apply physics-based, spatially varying tuning to explore the role of missing dissipative processes in simulating tides. Decreasing grid spacing from 100 to 4 km reduces the global tidal elevation error from 10.21 to 4.37 cm and improves the simulated dissipation. Modeled tidal solutions converge at 4-8 km resolution, with the scaling factor converging to similar to 6. The scaling factor remaining larger than one suggests that linear internal tide generation alone cannot explain all open-ocean dissipation. Correcting linear theory for supercritical topography and applying spatially varying tuning reduce tidal elevation error by a further 0.41 cm, but lead to larger regional disagreement in dissipation compared with observations. These results suggest that barotropic tide models should be run with grid spacing of 8 km or less, and that minimizing global tidal elevation error may not, by itself, be a sufficient tuning criterion, as it does not always guarantee an improved distribution of tidal energy loss.

Satellite microwave observations of Antarctic sea ice started in 1973, just in time to capture a massive open water area enclosed in winter sea ice in the Weddell Sea, known as the Weddell Polynya. This polynya was roughly the size of the United Kingdom and it lasted through the winters of 1974-76 with observed ocean mixed layer depths exceeding 3000 m. This study evaluates the impacts of the 1970s Weddell Polynya on Antarctic Bottom Water trends and volume transports. We use two global ocean simulations at eddying resolutions to create polynyas similar in size and duration to the Weddell Polynya. These are initiated with a brief, localized wind perturbation near Maud Rise. A comparison of simulated water-mass trends to available full-depth hydrographic data reveals that warming and oxygen decline in the bottom layers in the Weddell and Scotia Seas can be explained by a multidecadal recovery from the Weddell Polynya that continues today. The observed decline of Antarctic Bottom Water (AABW) outflow from the Scotia Sea since the 1990s can also be attributed to the multidecadal recovery from a large spike in abyssal transport created by the polynya. However, the model simulations do not show a substantial change in the Antarctic Circumpolar Current transport or lower cell overturning transports north of the South Scotia Ridge, and the water-mass trends do not substantially propagate north of ;55 degrees S in the Atlantic sector. Recently observed bottom water trends in the Pacific and East Indian sector of the Southern Ocean are likely controlled by factors other than the Weddell Polynya, such as increased glacial melt.

Warm water intrusions onto Antarctica's continental shelves have the potential to rapidly melt ice shelves and destabilize glaciers. However, uncertainties remain about the mechanisms driving these intrusions and their variability, in part because of limited ocean observations. Here, we use a high-resolution ocean-sea ice model to investigate the drivers of warm modified Circumpolar Deep Water (mCDW) intrusions in Vincennes Bay, where limited observations show that mCDW accesses the continental shelf and rapid retreat of the grounding line has been reported. We find large interannual variability in the simulated mCDW intrusions, with the vertical average temperature over the shelf below 400 m varying from to C on interannual timescales. The simulations show that onshore heat transport is facilitated by weakening of easterly winds, which drives a shoaling of isopycnals, allowing mCDW to access the continental shelf. The isopycnal shoaling also weakens the Antarctic Slope Current but mCDW intrusions are more highly correlated with change in isopycnal depth than strength of the Antarctic Slope Current, suggesting opening of an isopycnal pathway between offshore and shelf waters is the key process enhancing onshore heat transport. Ekman pumping over a narrow band of the continental slope is correlated with temperature on the shelf, suggesting that wind-driven upwelling can also enhance intrusions of mCDW. Given the projected weakening of easterlies and strengthening of westerlies around Antarctica, results suggest that warm intrusions into Vincennes Bay will become more frequent, leading to increased basal melt rates and a heightened contribution of the region to future sea-level rise.

Around the margins of Antarctica, dense waters formed on the continental shelf are exported to oceanic depths. This overflow of dense waters to the abyss ventilates the ocean, and is vital to the global overturning circulation. Accurately quantifying the variability in the transport of dense waters exported from the Antarctic continental shelf poses substantial challenges, due to the reliance on costly, carbon-emitting, and sparse observations or on models that do not capture complete dynamics. Here, we demonstrate that Antarctic dense water overflows can be monitored from space, using year-round sea surface height observations from satellite altimetry. We employ high-resolution simulations to characterize the sea surface height signature of the dense waters crossing the Ross Sea continental shelf break. This allows us to find a sea surface height proxy that captures the dense water transport variability, even when model outputs are subsampled to the sparse satellite observation coverage. When applied to the existing satellite record, this proxy reveals interannual variability that aligns with changes in dense water properties measured from hydrographic surveys. Our findings suggest that satellite-based monitoring can effectively complement and enhance existing in situ observing systems, by providing long-term and extensive spatial coverage of Antarctic dense water transports.

Dense shelf water (DSW) formation on the Antarctic shelf plays a crucial role in our global climate system. However, possible end-of-twenty-first-century changes to ocean circulation and temperature under different climate scenarios are poorly constrained. Here, we force a 0.1 degrees global ocean-sea ice model with spatially variable anomalies derived from a multimodel mean of 22 CMIP6 models to investigate the impact of mid-[shared socioeconomic pathway (SSP) 2-4.5] and high-range (SSP5-8.5) emissions on Antarctic margin circulation at the end of this century. We perform these simulations with and without future freshwater contributions from the Antarctic Ice Sheet to assess the changes in the presence and absence of meltwater. In the experiments without anomalous meltwater, the Antarctic continental shelf warms and freshens, becoming increasingly stratified with reduced sea ice extent across all months. Reduced sea ice growth leads to freshening over the continental shelf, which drives an acceleration of the upper-ocean Antarctic Slope Current (ASC), even in the absence of future meltwater contributions. Incorporating future projections of meltwater significantly amplifies these responses, with a complete shutdown of DSW formation and export under both midand high-range scenarios. However, even under a midrange emissions scenario without additional meltwater forcing, substantial changes in Antarctic continental shelf circulation and hydrography are anticipated by the end of this century, including a 35% reduction in DSW formation. Our results further suggest that the temperature response around the Antarctic margins is sensitive to the magnitude of future freshwater forcing, highlighting a need for better constraints on projections of meltwater contributions from the Antarctic Ice Sheet under different climate scenarios.

Antarctic Bottom Water (AABW) is derived from dense water that sinks from the Antarctic continental shelf to the deep ocean. The sinking of AABW is balanced by a return flow of lighter water, and the resulting overturning circulation determines the density stratification of the deep ocean, regulates ocean storage of heat and carbon, and supplies oxygen to the deep sea. In this Review, we highlight progress in understanding how and why AABW is changing and the consequences for the deep overturning circulation. Since the mid-1980s, ocean heat content below 4,000 dbar has increased at a rate of 12.9 (±1.8) trillion watts, and the AABW has thinned by more than 50 dbar decade−1, with more rapid thinning observed closer to the sources of AABW. The abyssal overturning circulation has slowed in response to freshening of shelf waters by glacial melt and changes in sea ice formation. Numerical model simulations indicate that these trends will accelerate under projected increases in meltwater input. Future research priorities include sustained observations in the deep ocean and on the Antarctic continental shelf; exploration of feedbacks between ocean circulation, sea ice, dense water formation and ice shelf melt; and improved representation of AABW in ocean and climate models. Antarctic Bottom Water (AABW) sinks near Antarctica and fills the deep ocean. This Review discusses how AABW is formed, past changes to its properties and transport, and projects future changes in AABW and the deep overturning circulation.

Antarctic Bottom Water (AABW) formation drives the lower limb of the meridional overturning circulation, influencing the Earth's climate, carbon cycle, and marine biological productivity. Recent findings suggest that AABW across the Southern Ocean is changing, including freshening, warming and thinning. These changes motivate the need for process‐based understanding of AABW circulation and variability. However, the harsh Antarctic environment limits in‐situ observations, making high‐resolution models an essential tool for investigating AABW. This study examines AABW varieties from the Ross Sea and the continental shelf adjacent to the Adélie Land coast in the Australian‐Antarctic Basin. We explore their interactions and pathways using passive dye‐like tracers in an ocean–sea ice model. Our findings reveal that, despite higher volume transport across the shelf break, RSBW contributes less to the basin's ventilation compared to ALBW. This is due to the presence of alternative pathways that divert RSBW eastward and its significant mixing with ambient waters along the continental slope. In contrast, ALBW plays a more prominent role in ventilating the abyssal waters of the basin, a conclusion supported by recent Deep Argo observations. We also found that when RSBW first encounters ALBW along the slope, it vertically overrides ALBW due to its relatively lower density at this stage.

The formation of Antarctic Bottom Water (AABW) is a key process in the global ocean circulation, but modeling the formation and downslope flow of AABW represents an ongoing challenge for ocean and climate models due to the high horizontal resolution required. Here, we assess the formation and export of AABW to the abyss and its sensitivity to horizontal model resolution in a circumpolar ocean‐sea ice model available at horizontal resolutions of 1/10°, 1/20°, and 1/40°. The formation of Dense Shelf Water (DSW), the precursor of AABW, reduces with increasing model resolution in most formation regions due to shelf freshening. Increased eddy activity with higher model resolution flattens the isopycnals in the open Southern Ocean and enables access of lighter, fresher waters onto the continental shelf. Despite the decrease in DSW formation, the total offshore AABW transport increases with increasing model resolution, especially across the 2,500 m isobath, due to less diapycnal mixing during the downslope flow. This resolution dependency is strongest in the Ross and Weddell Seas, the two most important regions of AABW formation. We conclude that a horizontal resolution of 1/10° is sufficient to simulate AABW export from East Antarctica, in agreement with theory of the downslope flow of dense plumes, but finer resolutions of up to 1/40° increase the offshore transport and may be required to resolve the AABW export in the Weddell and Ross Seas.

Dense water formation on the Antarctic continental shelf is the main process by which Antarctic Bottom Waters form and is fundamental to the abyssal overturning circulation. However, most ocean models fail to simulate Antarctic dense water formation on the continental shelf and flow down the continental slope (i.e., overflow) due to resolution constraints. While the impact of horizontal and vertical resolution on the overflows has been previously studied, the effect of surface vertical resolution on dense water formation remains unexplored. To address this gap, we vary the surface ocean grid cell of two dense water‐forming models from 1.1 to 5.1 m thickness. We used two ocean and sea ice models, each employing a different boundary layer parameterization scheme. In one model, thickening the surface cell to 5.1 m reduced dense water formation by 64% and led to the complete cessation of the overflow after 10 years of simulation. In the other, the same thickening decreased dense water formation by 32% and overflow by 67% over the same period. The dense water formation reduction in the experiments with thicker surface grid cells is explained by a southward shift in the surface Ekman transport, which brings light offshore waters to the coast and limits dense water formation at the continental shelf. Although dense water formation responds to surface layer thickening in both models, differences in sea ice production contribute to greater sensitivity in one case, where a weaker sea ice formation in the 5.1 m configuration further decreases dense water production. These results highlight that a high vertical resolution at the ocean surface is required to form Antarctic dense waters.

Ocean-driven basal melting of Antarctic ice shelves is an important process that affects the Antarctic Ice Sheet, global climate and sea level. Basal melting is controlled by small-scale processes; therefore ice shelf–ocean models rely on parameterisations to predict basal melt. However, most currently used basal melt parameterisations represent melting as a shear-driven process and do not adequately include the effects of stratification by accumulation of buoyant meltwater beneath flat and weakly sloped ice interfaces. We implement an improved three-equation melt parameterisation that accounts for the stratified suppression of turbulence into two ocean models. This stratification feedback parameterisation is based on the results of large eddy simulation (LES) studies, which suggest a functional dependence of heat and salt transfer coefficients on the viscous Obukhov scale. Changes in melting and circulation due to the stratification feedback are regime-dependent: melt rates in idealised, quiescent simulations decrease by 80 % under warm-cavity conditions and 50 % under cold conditions. The stratification feedback also modifies melt rate patterns in a high-resolution regional Pine Island Glacier simulation. However, unconstrained boundary layer parameters, inter-model differences and unresolved processes continue to present challenges for accurately modelling basal melt in ocean models.

The delivery of ocean heat to Antarctic ice shelves is due to intrusions of waters warmer than the local freezing point temperature. Changes in the supply of ocean heat will determine how rapidly ice shelves melt at their base, which affects Antarctic Ice Sheet mass loss and future global mean sea-level rise. However, processes driving ice-shelf basal melting are still poorly understood. Here we investigate the drivers of heat convergence along the Antarctic margins by performing an ocean heat budget analysis using a high-fidelity 4 km circum-Antarctic ocean-ice-shelf model. The simulation produces high basal melting in West Antarctica associated with sustained ocean heat convergence driven by advection of relatively warm deep water intrusions, with minimal seasonality in both heat supply and basal melting. For East Antarctica, ice shelves have substantial basal melt seasonality, driven by strong air-sea winter cooling over the continental shelf depressing shallow melting, while in summer, increased heat inflow towards the ice shelves is driven by surface-warmed waters that subduct under shallow regions of ice, increasing melt. The high seasonality of basal melting in East Antarctic ice shelves is responsive to interactions between the atmospheric forcing, the local icescape, and the activity of coastal polynyas. Our results suggest that seasonal changes in future climate change scenarios are critical in determining the duration and intensity of air-sea fluxes with substantial impacts on ice-shelf basal melting and ice-sheet and sea-level budgets.