Antarctic Ice Sheet projections show the highest sensitivity to increased basal melting in the Amundsen Sea sector. However, little is known about the processes controlling future increase in melt rates. We build an ensemble of three ocean–sea-ice–ice-shelf simulations for both the recent decades and the late 21st century, constrained by regional atmosphere simulations and the multi-model mean climate change of the 5th Climate Model Intercomparison Project under the RCP8.5 scenario. The ice shelf melt rates are typically multiplied by 1.4 to 2.2 from present day to future, for a total basal mass loss increased by 347Gt/yr. This is approximately equally explained by advection of warmer water from remote locations and by regional changes in Ekman downwelling and in the ice-shelf melt-induced circulation, while increased iceberg melt plays no significant role. Our simulations suggest that high-end melt projections previously used to constrain recent sea level projections may have been significantly overestimated.

The importance of resolving mesoscale air-sea interactions to represent cyclones impacting the East Coast of Australia, the so-called East Coast Lows (ECLs), is investigated using the Australian Regional Coupled Model based on NEMO-OASIS-WRF (NOW) at $1/4^\circ$ resolution. The fully coupled model is shown to be capable of reproducing correctly relevant features such as the seasonality, spatial distribution and intensity of ECLs while integrating more physical processes, including air-sea feedbacks over ocean eddies and fronts. The thermal feedback (TFB) and the current feedback (CFB) are shown to influence the intensity of tropical ECLs (north of $30^\circ S$), with the TFB modulating the pre-storm sea surface temperature and the CFB modulating the wind stress. By fully uncoupling the atmospheric model of NOW, the intensity of tropical ECLs is increased due to the absence of the cold wake that provides a negative feedback to the cyclone. The number of ECLs might also be affected by the air-sea feedbacks but large interannual variability hamper significant results with short term simulations. The TFB and CFB modify the climatology of sea surface temperature (mean and variability) but no direct link is found between these changes and those noticed in ECL properties. These results show that the representation of ECLs, mainly north of $30^\circ S$, depend on how air-sea feedbacks are simulated, with significant effects associated with mesoscale eddies. This is particularly important for atmospheric downscaling of climate projections as small-scale sea surface temperature interactions and the effects of ocean currents are not accounted for.

Following observations of a drop in the West Antarctic Ice Sheet (WAIS) mass balance over the last few decades with the possibility to reach a tipping point leading to ineluctable glaciers outlets instability in the region, understanding the driving processes has become a priority. In particular, the Circumpolar Deep Water (CDW) intrusion onto the continental shelf in the Amundsen Sea is, nowadays, in the spotlight, and gathers the attention of both the observers and the modellers. This modelling study presents the analysis of a 1/12° simulation of the Amundsen Sea sector reproducing well the interannual-to-decadal variability of the ice-shelves basal melt rates. The development of a methodology to study the ocean state in the reference frame of the continental shelf break enables us to distinguish and characterize a western fresh shelf zone and an eastern warm shelf zone in the region. Connecting it with the more regional circulation, we try to shed light on the different mechanisms driving the CDW inflow onto the continental shelf in the region. In particular, we draw attention to the sea ice effect in terms of Ekman pumping along the shelf break, and we point out the possible initiation of a southern Antarctic Circumpolar Current (ACC) branch to the south-east of the Ross Gyre, which could control part of the variability along the Amundsen Sea shelf. Finally, we discuss correlations between the ocean variability at the shelf break and the one of the melt activity underneath the ice-shelves.

Projections of the sea level contribution from the Greenland and Antarctic ice sheets rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous CMIP5 effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

Understanding the driving processes at stake for the Circumpolar Deep Water (CDW) intrusion onto the Amundsen shelf is crucial. We use a multi-decadal ocean simulation at 1/12° to revisit the ocean dynamics at the Amundsen shelf break, distinguishing a western fresh shelf and an eastern warm shelf. While the prevailing presence of the Antarctic Slope Current - fed to the east of Russel Bay through vortex stretching of an outflow of melted waters - blocks CDW intrusions in the west, the contact of Antarctic Circumpolar Current (ACC) branches along the shelf in the east favors this inflow. Of particular importance is a southern ACC branch initiated to the south-east of the Ross Gyre, which interacts with the topography at the entry of the western Pine Island-Thwaites trough. Then, we link the ocean interannual-to-decadal variability at the shelf break with the ice-shelf basal melting and create a Fresh-Warm Boundary Index (FWBI) to follow the oscillation of the fresh-warm shelves limit through time in Russel Bay, which could be a focal point to understand the low frequency fluctuations of the basal melt. We suggest that not only a wind-induced Ekman pumping could favor the CDW inflow at the shelf break, but also topographic interactions, a bottom Ekman transport, a sea-ice–induced Ekman pumping resulting from strong surface currents, and the baroclinicity of the eastward along-shelf current in the west. Finally, we highlight that El Niño-Southern Oscillation has no strong correlation with the ice-shelf basal melt variability, except for the very recent years.

Ocean warming around Antarctica has the potential to trigger marine ice-sheet instabilities. It has been suggested that abrupt and irreversible cold-to-warm ocean tipping points may exist, with possible domino effect from ocean to ice-sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice-shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater or momentum fluxes, and we characterize the key physical processes at play in warm-to-cold and cold-to-warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, i.e., with ocean temperatures close to the surface freezing point and very low ice-shelf melt rate. The transitions are reversible, i.e., cancelling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes over the continental shelf, as a direct consequence of the freshwater flux perturbation, or through changes in net sea-ice production resulting from either heat flux perturbations or from changes in sea-ice advection for the momentum flux perturbation. These changes affect the vertical ocean stratification and thereby ice-shelf basal melting. For warmer climate conditions than presently, the surface buoyancy forcing becomes less important as there is a decoupling between the surface and subsurface layers, and ice-shelf melt rates appear less sensitive to climate conditions.