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Abstract:
The supply of warm circumpolar deep water to the west Antarctic continental shelves is a key driver of the rapid melting and retreat of the surrounding glaciers. Future changes in Antarctic glacial melt are the largest source of uncertainty in sea level rise projections, so are crucial to understand. Here, we use high-resolution coupled climate models to demonstrate that the ocean forcing of ice shelf melt can both increase and decrease under high-emission scenarios, revealing deep uncertainty about the sign of these changes. HadGEM3-HH, with a 1/12° ocean model, faithfully captures the circulation and hydrography of the Amundsen Sea in the present-day climate. However, under the HighRes-Future scenario, the model undergoes a sudden shift to cold conditions on the Amundsen Sea continental shelf during 2040-2046, with temperatures at 500 m depth decreasing by around 2 °C, despite steady warming at the surface. We demonstrate that this sudden shift can be traced back to increased sea ice melting driven by surface warming, especially in the Bellingshausen Sea. The resultant fresh anomalies are advected into the Amundsen Sea, where they disrupt the advection of warm deep water onto the continental shelf. This contrasts with HadGEM3-MM (1/4° ocean model), which exhibits substantial cold biases in the present day but substantial deep warming from 2070-2100. These simulations imply that there exists an alternative stable state for the west Antarctic continental shelves, with cold deep waters and low ice shelf melt rates, that could plausibly be reached under future climate change.
