The surface waters around the Antarctic margins are freshening as mass loss from the Antarctic ice sheets and shelves increases. This mass loss, and resulting freshening, is projected to accelerate over the coming century. However, coupled climate models generally lack an interactive ice sheet component and so underestimate the volume of freshwater entering the Southern Ocean. This is a key uncertainty in future projections and so it is critically important to understand the climate response to freshwater forcing. In particular, model sensitivity to different freshwater forcing parameters remains poorly constrained. Here we present results from two different idealised freshwater forcing experiments using ACCESS-ESM1.5 to explore the model’s response to different horizontal freshwater distributions. We demonstrate significant differences in the temporal and spatial response of both the Southern Ocean, and global climate. Our findings demonstrate that ACCESS-ESM1.5 is sensitive to the distribution of freshwater forcing. This sensitivity is likely relevant to other climate models and has important implications for interpreting previous studies and designing methods of freshwater addition.

The Southern Ocean carbon sink reflects a balance between natural CO2 outgassing from upwelled deep waters, and uptake of anthropogenic CO2 from the atmosphere. How this balance depends on water-mass transformation remains unclear across climate models. Here we use 11 CMIP6 models to examine Southern Ocean air--sea CO2 flux in a density-space framework. We show that natural carbon outgassing is concentrated in density classes characterized by water-mass destruction and that stronger destruction is associated with enhanced outgassing on interdecadal timescales within most models. Historical changes in air--sea CO2 flux are strongly density dependent, with enhanced uptake in lighter density classes and more model-dependent changes in denser upwelling classes. Anthropogenic carbon uptake is also concentrated in destruction-sensitive density classes, indicating that ventilation helps define the pathway through which anthropogenic carbon enters the ocean. However, interdecadal variability in water-mass destruction play a secondary role in setting the magnitude of anthropogenic uptake, which is more strongly driven by monotonically increasing atmosphere--ocean CO2 disequilibrium. These results identify water-mass destruction as a unifying density-space framework for interpreting Southern Ocean carbon exchange while highlighting distinct controls on natural outgassing and anthropogenic uptake.

The Southern Ocean circulation is a critical regulator of ocean climate at the Antarctic margins, where there is an interplay of multiple coupled physical mechanisms spanning a wide range of spatial scales. These interactions are especially important in the subpolar Southern Ocean, where the Antarctic Slope Current, subpolar gyres, and cross-slope exchanges influence the delivery of heat toward the Antarctic continental shelf. However, many of the processes controlling these pathways occur at scales that are unresolved in conventional hydrostatic ocean models and must therefore be parameterized. In our study, we use dynamic scale similarity to perform boundary-layer-resolving, non-hydrostatic simulations of a pan-Antarctic-style Southern Ocean domain with semi-realistic bathymetry. By solving the full incompressible Navier–Stokes equations under the Boussinesq approximation, the model explicitly resolves turbulent convection, bottom and surface boundary-layer processes, and their interactions with larger-scale circulation, without relying on turbulence or boundary-layer parameterizations. The simulations are forced by surface density gradients and wind stress, allowing the separate and combined roles of buoyancy, wind, and topography to be examined. These simulations reveal how convective overturning, boundary-layer dynamics, and bathymetric steering shape the regional flow pathways, highlighting the importance of directly resolving small-scale dynamics in Southern Ocean circulation studies.

Antarctic ice sheet and shelf meltwater is adding freshwater to the Southern Ocean, altering ocean circulation and sea ice formation, which in turn influence atmospheric temperature and precipitation. Most current climate models do not account for the impacts of increasing Antarctic meltwater, resulting in projections missing important climate change impacts. New multi-model climate experiments that incorporate meltwater effects show that Antarctic meltwater induces a cooling effect that is strongest near its source around Antarctica and diminishes in magnitude with distance away from Antarctica, suggesting that an increase in Antarctic melting (of 0.1 Sv), as expected by 2070 under a high emissions scenario, may partially reduce projected anthropogenic warming over the Southern Hemisphere land masses by on average 0.62°C. 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.

Understanding recent Southern Ocean warming and freshening requires separating direct anthropogenic forcing from dynamic redistribution. We apply a water-mass-based decomposition — passive heave, dynamic heave, and spice — to EN4 objective analysis (2002−2015). Globally, passive heave captures coherent anthropogenic signals: the fingerprint of heat uptake in temperature and water cycle amplification in salinity. In the Southern Ocean, passive heave and dynamic heave contribute comparably to warming, the former reflecting anthropogenic heat added to the ocean, the latter interannual-to-decadal adiabatic redistribution, with spice playing a minimal role. For freshening, spice dominates at twice the passive heave magnitude, while passive heave delivers the forced freshening signal linked to water cycle amplification and ice-ocean interactions, and dynamic redistribution partially opposes freshening. Together, dynamic heave reinforces passive heave thermosteric sea level rise through heat redistribution, while opposing passive heave halosteric rise through salt redistribution, demonstrating that Southern Ocean sea level change is shaped by both forced anthropogenic signals and internal redistribution. This framework offers a physically rigorous tool for precision climate tracking of the ocean's response to emissions.

Antarctic sea ice has undergone a dramatic transformation in recent years, with record-low extents since 2016 and growing evidence for a shift in the sea ice system. While atmospheric forcing contributes to these changes, increasing attention is turning to the role of the ocean and the pathways through which subsurface heat reaches the surface.

Ocean mixing regulates the transfer of heat, salt, and freshwater through the water column and is therefore a key process linking the Southern Ocean heat reservoir to Antarctic sea ice. Yet direct observations of turbulent mixing near sea ice remain exceptionally sparse. Recent turbulence observations near Dronning Maud Land reveal hotspots of enhanced mixing over the continental slope, where tidal processes drive substantial upward heat transport beneath sea ice. These observations demonstrate how localized mixing can influence upper-ocean heat content and sea ice growth.

We place these observations in the broader context of historical turbulence measurements collected around Antarctica over the past several decades. By synthesizing available microstructure observations, we can assess where and under what conditions enhanced mixing occurs, and discuss how the changing mixing landscape impacts stratification, water masses and heat fluxes toward the surface. Capturing these poorly observed processes requires better use of the few turbulence observations available to improve model parameterizations of mixing and constrain the ocean's role in the Antarctic sea ice decline.

Basal melting beneath Antarctic ice shelves is the primary driver of recent Antarctic ice loss, yet most global ocean models, including ACCESS, do not explicitly represent ice shelf cavities. Instead, they prescribe meltwater input at the ocean surface, neglecting the two-way coupling between ice shelf melt and ocean circulation. Recent model development has produced a high-resolution pan-Antarctic ACCESS model that explicitly resolves ice shelf cavities and thermodynamic interactions at the ice – ocean interface.

Using this new capability, we investigate Antarctic ice shelf melt and ocean responses to future climate change. We find that ocean warming on the continental shelf is driven primarily by atmospheric forcing and occurs even when meltwater input is fixed. However, representing ice shelf – ocean interactions substantially alters other aspects of the response. The decline in dense water formation is weaker than in many previous studies, in part because latent heat is extracted from the ocean during ice shelf melting. Responses also vary strongly between regions: grounding line melt decreases in cold cavities associated with reduced dense water circulation, while West Antarctica experiences strong ocean warming and a fivefold increase in melt rate.

This model configuration provides a powerful new capability for the Australian research community to investigate Antarctic ice shelf – ocean interactions and their role in future climate change.