Andrew McC. Hogg

Antarctic ice shelf basal melt is projected to increase in the future in response to a warming ocean, with implications for ice sheet buttressing, global sea level, and the overturning circulation. Regional ice shelf responses depend on interacting processes on a range of spatial scales, but previous modelling studies have been limited by their horizontal resolution. We use a new high-resolution pan-Antarctic ocean sea-ice model with interactive ice shelf melt and a realistic representation of Dense Shelf Water (DSW) to understand transient ice shelf cavity responses to atmospheric warming. In response to a step-change to end-of-twenty-first-century atmospheric warming, the total Antarctic basal melt rate increases rapidly. Increased Circumpolar Deep Water in the Amundsen and Bellingshausen Seas accelerates basal melt fivefold. Shallow melt near ice fronts also increases continent-wide, driven by warming summer surface waters. Elsewhere, DSW export, the precursor to Antarctic Bottom Water, halves within 15 years. Freshening from decreased sea ice formation and export drives this DSW reduction, with an additional contribution from ice shelf meltwater. DSW changes influence ice shelf melt locally and downstream. In cold cavities, such as the Filchner-Ronne, Ross and Amery ice shelves, reduced DSW circulation decreases deep melting. Ice shelf cavities downstream of DSW formation sites also cool as cold, dense water that was previously exported offshore is redirected along the shelf. The contrasting regional responses point to multiple processes occurring within ice shelf cavities in response to future warming, and highlight the importance of simulating high-resolution ocean processes when making basal melt projections.

Mesoscale eddies in the Southern Ocean extend from the surface to the deep ocean and contribute to regional and global ocean heat budgets. However, sparse in situ observations have limited estimates of eddy meridional heat transport (EMHT) to the near surface, leaving deep-ocean processes poorly understood. Here we quantify deep eddy kinetic energy (EKE) and EMHT using Argo observations. We find that deep EMHT reaches at least the same order of magnitude as surface transport, despite deep EKE being only one-fifth of surface levels. Float observations and sensitivity experiments reveal that, relative to mean-flow transport alone, deep EMHT extends the meridional movement of subtropical warm waters towards Antarctica by over ten degrees of latitude. About 60% of this deep transport originates from the Indian Ocean sector. These findings provide observational evidence that mesoscale eddies can modulate deep Southern Ocean heat redistribution, with potential implications for Antarctic climate.

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 passive 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 characterized by high seasonal variability, reaching about 0.16 tracer units (equivalent to 100% of its local time‐mean tracer concentration) along the continental slope. By contrast, seasonal variability of AABW formed on the wide shelves (Ross and Weddell Seas) remains below 50% of its local time‐mean concentration. Interannual variability is strongest in the Weddell Sea, where it reaches 0.12–0.15 tracer units (40%–50% of its local time‐mean concentration) along the continental slope. Advective timescales show that AABW from narrow‐shelf sources ventilates the abyss on much shorter time scales (1 year) than AABW from wide shelves (2 years). These findings provide new context for interpreting observed deep‐ocean changes, helping to distinguish anthropogenic trends from natural variability and sampling limitations.

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.

A comparison of simulated and observed overturning transports and related properties across the Overturning in the Subpolar North Atlantic Program (OSNAP) sections for the 2014–2022 period is presented, considering both depth and density space transports. The effort was motivated by the observational transport estimates at both OSNAP-West (OW) and OSNAP-East (OE) sections which show a minor role for the Labrador Sea (LS) in setting the mean and variability of the overturning in the subpolar North Atlantic. There are 9 participating groups from around the world, contributing a total of 18 ocean – sea-ice simulations with 6 different ocean models. The simulations use a common set of interannually varying atmospheric forcing datasets. The horizontal resolutions of the simulations range from nominal 1° to eddy-resolving resolutions of 0.1–0.05°. While there are many differences between the simulations and observations as well as among the individual simulations in terms of transport properties, the simulations show significantly larger transports at OE than at OW in agreement with the observations. Analyzing overturning circulations in both depth and density space together provides a more complete picture of the overturning properties and features. This analysis also reveals that, in both the simulations and observations, northward and southward flows substantially cancel each other at a given depth or density, producing much smaller residual (total) transports. Such cancellations tend to be much more prominent in depth space than in density space. In general, the observed transport features are captured better at OE than OW. The simulations generally show larger (smaller) transports with positive (negative) temperature and salinity biases in the upper ocean near the OSNAP sections, but with no such relationship with density biases. In high-resolution simulations, the transport profiles agree better with the observations, but challenges remain in some other metrics considered in our analysis. When transports are calculated using a density referenced to 2000 m depth, rather than the ocean surface, the relative contributions of transports at OW increase modestly.

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 ∼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.

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, M ₂ -tide-only model to examine the sensitivity of modelled tides to resolution. We apply physics-based, spatially varying tuning to explore the role of missing dissipative processes in simulating M ₂ tides. Decreasing grid spacing from 100 km to 4 km reduces the global tidal elevation error from 10.21 cm to 4.37 cm and improves the simulated dissipation. Modelled tidal solutions converge at 4–8 km resolution, with the scaling factor converging 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 minimising 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.

Complex interactions among the ocean, sea ice, and ice shelves in the Southern Ocean are critical for global climate, yet accurately simulating these processes remains challenging in climate models, such as those participating in the Coupled Model Intercomparison Project Phase 6, due to their coarse resolution and incomplete physical components. Therefore, the development of high-resolution circumpolar coupled ocean–sea ice–ice shelf models could improve our understanding of the evolution of the Southern Ocean. In this study, we use the c66m version of the Massachusetts Institute of Technology General Circulation Model, including a sea ice component and an ice shelf component, to configure the coupled Southern Ocean–Sea ice–Ice shelf Model (SOSIM v1.0). Adopting the Refined Topography dataset version 2 for the geometry of seafloor and ice draft, SOSIM features a horizontal resolution of ∼5 km and 70 vertical layers. Forced by the European Centre for Medium-Range Weather Forecasts Reanalysis v5, a long-term integration of SOSIM is run forward from 1979 to 2022, with daily outputs for estimating the oceanic state, sea ice evolution, and basal mass balance of ice shelves. A comprehensive evaluation of the performance of SOSIM has been conducted against multiple observational and reanalysis datasets. Identified biases include an underestimated Antarctic Circumpolar Current transport, an overestimated Antarctic Slope Current, a warm drift in abyssal waters, an exaggerated seasonality of sea ice extent, and an underestimated total ice shelf mass loss. Despite these limitations, SOSIM still captures large-scale hydrographic structures, the annual variability of sea ice, and cross-slope exchanges over shelf seas. Furthermore, SOSIM is set to serve as the dynamical core for the next-generation Southern Ocean Ice Prediction System being developed in China.

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.

Complex interactions among the ocean, sea ice, and ice shelves in the Southern Ocean are critical for global climate, yet accurately simulating these processes remains challenging in climate models, such as those participating in the Coupled Model Intercomparison Project Phase 6, due to their coarse resolution and incomplete physical components. Therefore, the development of high-resolution circumpolar coupled ocean–sea ice–ice shelf models could improve our understanding of the evolution of the Southern Ocean. In this study, we use the c66m version of the Massachusetts Institute of Technology General Circulation Model, including a sea ice component and an ice shelf component, to configure the coupled Southern Ocean–Sea ice–Ice shelf Model (SOSIM v1.0). Adopting the Refined Topography dataset version 2 for the geometry of seafloor and ice draft, SOSIM features a horizontal resolution of ~5 km and 70 vertical layers. Forced by the European Centre for Medium-Range Weather Forecasts Reanalysis v5, a long-term integration of SOSIM is run forward from 1979 to 2022, with daily outputs for estimating the oceanic state, sea ice evolution, and basal mass balance of ice shelves. A comprehensive evaluation of the performance of SOSIM has been conducted against multiple observational and reanalysis datasets. Identified biases include an underestimated Antarctic Circumpolar Current transport, an overestimated Antarctic Slope Current, a warm drift in abyssal waters, an exaggerated seasonality of sea ice extent, and an underestimated total ice shelf mass loss. Despite these limitations, SOSIM still captures large-scale hydrographic structures, the annual variability of sea ice, and cross-slope exchanges over shelf seas. Furthermore, SOSIM is set to serve as the dynamical core for the next-generation Southern Ocean Ice Prediction System being developed in China.