Adele K. Morrison

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.

Antarctic ice sheet and ice shelf meltwater affect all climate system components. Most current climate models omit accelerating Antarctic meltwater discharge, resulting in projections missing important climate change impacts. New multi-model climate experiments show Antarctic meltwater induces local atmospheric cooling, strongest near its source around Antarctica and diminishing with distance. In climate models, an increase in Antarctic melting of 0.1 Sv, as projected by ~2070 under a high emissions scenario, partially reduces projected anthropogenic warming over the Southern Hemisphere land masses by -0.62°C on average, when assuming a linear response between anthropogenic warming and meltwater-induced cooling. Antarctic meltwater may also alter precipitation over land, with some models indicating that substantial changes are possible. However, the significance of the precipitation signal varies within model ensembles and across different models. Accounting for changes in Antarctic meltwater and advancing precipitation representation in climate models is crucial to improve future climate projections.

The Antarctic Slope Current (ASC) is often considered a barrier to the poleward transport of Circumpolar Deep Water (CDW), yet its vast extent has hindered circumpolar analysis of this relationship. Here, we use a high‐resolution global ocean‐sea ice model that resolves the slope current to assess the link between variability in the ASC and poleward CDW heat transport, circumpolarly and across varying timescales. We find that the ASC velocity and CDW heat transport have weak correlations, with an average 0.3, over the continental slope. In fact, less than 25% of the continental slope has a correlation of over either seasonal or interannual timescales. Isolated regions of strong covariability occur where the isopycnal depth at the shelf break varies substantially in time. Overall, our results suggest that circumpolarly, the ASC is weakly associated with poleward CDW heat transport, cautioning against the general assumption of a consistent dynamical link between the two around Antarctica.

Freshening of Antarctic shelf waters over the past five decades has reduced Antarctic Bottom Water (AABW) volume. Several periods of salinity increase have, however, occurred in the Ross Sea, most recently since the mid-2010s. Using a high-resolution ocean-sea ice model, we show that an increase in salinity, comparable to the observed increase since the mid-2010s, is simulated when either a perturbation in coastal zonal winds or meltwater is applied in the Amundsen Sea. These perturbations excite baroclinic coastal trapped waves that propagate to the Ross Sea within 2–3 months. In response to the coastal trapped waves, the coastal current weakens and isopycnals shoal near the coast. This leads to enhanced surface salinity in the western Ross Sea and thereby preconditions the water column for increased AABW formation. These results highlight the response of the Ross Sea to remote changes on fast timescales.

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 m 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 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.

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.

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.