Antarctic Bottom Water (AABW) is a key component of the global overturning circulation, yet variability in its formation remains poorly understood at many source regions. Cape Darnley, East Antarctica, is the most recently identified source of AABW. Despite its recognised contribution to global abyssal overturning, sparse observations have left its long-term variability and sensitivity to climate forcing largely unknown.

Here, we use a high-fidelity Regional Ocean Modelling System (ROMS) simulation spanning 1992–2017 to provide the first multi-decadal assessment of Cape Darnley Bottom Water (CDBW) formation and export. We quantify variability in dense water properties and export, identify dominant timescales of variability, and examine the processes controlling year-to-year and multi-year changes. Particular focus is placed on the influence of polynya sea ice production, upstream ocean preconditioning, atmospheric forcing, and interactions with the Amery Ice Shelf system.

Our results establish the natural range of variability in this AABW source region and provide insight into the mechanisms governing dense shelf water formation along the East Antarctic margin. By identifying the drivers and stability of CDBW production, this work contributes to improved understanding of Southern Ocean overturning, ice-ocean interactions, and the response of Antarctic coastal systems to a changing climate.

Antarctic Bottom Water (AABW) supplies 40% of the ocean volume and maintains the deep overturning circulation. AABW is produced when Dense Shelf Water overflows from the continental shelf into the deep ocean and mixes with surrounding water. Despite strong seasonality on the shelf, the seasonality of AABW is largely unknown. We use 7 years of full-depth profiles collected every ten days by Deep Argo floats to quantify the seasonal variability of AABW in the Australian Antarctic Basin. While seasonality is weak at basin-scale, it becomes significant in the deep ocean downstream of the Adelie Land and Ross Sea sources of AABW. Adelie-sourced AABW around 140°E is coldest (-0.02°C) and densest (+0.003 kg/m3) in austral summer. Ross-sourced AABW around 150°E is most saline, densest, and thickest in austral winter. The arrival time of the densest AABW at each longitude can be used to estimate the speed of AABW. The speed of AABW from the Adélie Land source to 140°E (0.04 – 0.06 m/s) is about twice the average speed of AABW from the Ross Sea source to 150°E (0.025 m/s). Further, the seasonal variability of AABW in the abyssal ocean can help us better understand the controls on Dense Shelf Water export.

Because North Atlantic Deep Water acts as a precursor of Circumpolar Deep Water (CDW) in the Atlantic Meridional Overturning Circulation (AMOC), shifts in the AMOC can be expected to impact CDW properties in the Southern Ocean. To investigate the Southern Ocean response to a weakened AMOC, we conduct a meltwater perturbation experiment using a high-resolution global ocean-sea ice model. The anomalous meltwater (2500 Gt/yr) around Greenland is applied in the meltwater experiment, which triggers the slowdown of the AMOC in the entire Atlantic basin. In the Southern Ocean, our results from the meltwater forcing experiment show cooling and freshening of CDW. Further, CDW becomes warmer and saltier in the Indo-Pacific sector, which is consistent with a see-saw pattern between the Atlantic and Indo-Pacific Sectors. These results suggest that AMOC weakening can substantially reorganize Southern Ocean deep-water pathways and CDW properties.

The Southern Ocean lower overturning circulation is poorly represented in most global ocean and coupled climate models. Australia’s ACCESS-OM2 flagship model at 1/10° resolution successfully simulates realistic Antarctic bottom water formation, driving recent advancements in understanding Southern Ocean dynamics. However, the 0.25° version failed to capture these processes effectively. In developing the next-generation ACCESS-OM3 global ocean-sea ice model and its coupled counterpart ACCESS-CM3, key insights from ACCESS-OM2 are incorporated to improve the representation of lower overturning dynamics, even at coarser grid resolutions (25 km). This study examines the promising progress made to date in representing Antarctic Bottom Water formation and reduced deep convection in the offshore Southern Ocean in ACCESS-OM3/-CM3 compared to their predecessors. Technical improvements include an entirely new ocean dynamical core (MOM6 rather than MOM5.1), grid resolution refinements, revised GM parameterisation, spreading of Antarctic meltwater at the coast and through a pseudo iceberg meltwater parameterisation with latent heat extraction. As development remains ongoing, we welcome feedback and ideas to further improve the representation of the Southern Ocean lower overturning circulation.

Antarctic Bottom Water (AABW) forms as dense water sinks from the Antarctic continental shelf to the abyss. Sinking of AABW is balanced by a return flow of lighter water; the resulting overturning circulation sets the deep stratification, ventilates the abyss, and stores heat and carbon. Observations show that AABW has warmed, freshened, and contracted, and the deep overturning circulation has slowed. The past and future behaviour of AABW is determined by the freshwater balance on the continental shelf. Freshening by glacial melt (Ross sector) and changes in sea ice (Weddell sector) has reduced the density of water exported from the Antarctic continental shelf to form AABW. Loss of the densest varieties of AABW has driven contraction of the AABW layer, descent of isopycnals, and warming below 3000 m depth as cold AABW is replaced by warm Circumpolar Deep Water (CDW). Salinity first declines as fresher AABW is exported to the deep sea, but continued freshwater input eventually makes shelf water too light to sink, severing the connection between the shelf and the abyss. Deep salinity then increases as mixing and entrainment with saltier water is no longer compensated by freshwater input. Feedbacks associated with contraction of AABW and poleward expansion of CDW may explain rapid rates of Antarctic ice-loss and sea level rise inferred during the last interglacial.

The Southern Ocean has some of the highest rates of deep warming, which are especially extreme in the abyssal Antarctic basins. These high warming rates have been linked to a slowdown of Antarctic Bottom Water formation and export, yet climate models do not agree on the drivers and magnitude of this change. We assess how model resolution and atmospheric coupling shape the deep overturning and abyssal heat response to projected SSP5-8.5 and glacial meltwater forcing, using three models within the ACCESS framework: a high-resolution (1/10°) ocean-sea ice model (OM2-01) that resolves Dense Shelf Water (DSW) formation, a coarse-resolution (1°) ocean-sea ice model (OM2), and a fully coupled earth system model (ESM1.5).

Under meltwater forcing, only OM2-01 produces a large overturning slowdown and abyssal warming consistent with observed patterns, driven by near-complete shutdown of DSW formation. Coarse-resolution models, which do not resolve DSW export but instead experience high levels of open ocean convection, are largely insensitive to coastal meltwater. Instead, ESM1.5 responds primarily to sea ice and precipitation-driven surface freshening under SSP5-8.5. These results suggest that CMIP6-class model will systematically underestimate both the sensitivity of Southern Ocean overturning to glacial melt and the magnitude of future abyssal warming, with direct implications for the interpretation of future climate scenarios.