A document by Alexander Thomas Bradley. Click on the document to view its contents.

We use the United Kingdom Earth System Model simulations from the Coupled Model Intercomparison Project 6 to analyse the dynamics of the Ross Gyre, West Antarctica, under historical and projected climate-change scenarios. During the historical period, the modelled Ross Gyre is relatively stable, with an extent and strength that are in reasonable agreement with observations. The projections exhibit an eastward gyre expansion into the Amundsen and Bellingshausen seas that starts during the 2040s. The associated cyclonic ocean circulation enhances the onshore transport of warm Circumpolar Deep Water into the eastern Amundsen Sea, a regime change that increases the subsurface shelf temperatures by up to 1.2◦C and is independent of future forcing scenario. The Ross Gyre expansion is generated by a regional surface stress curl intensification associated with anthropogenic forcing. If realised in reality, such a warming would strongly influence the future stability of the West Antarctic Ice Sheet.

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.