Feedbacks between basal melt and cavity geometry in coupled ice+ocean simulations of the Amundsen Sea glaciers

Ice-shelf cavities in the Amundsen Sea are expanding as the ice thins and grounding lines retreat. To sustain ice-shelf thinning, whilst accommodating a (up to) 2-fold increase in ice flux across the grounding line, basal melt and calving rates must respond. Changes in far-field ocean temperature are often evoked to explain the sustained thinning, but there is no indication of significant trends in ocean properties over the observational period. On the other hand, internal feedbacks between changes in ice-shelf geometry and basal melt could play a role in driving glacier retreat, but these processes remain poorly understood. Here we explore such melt-geometry feedbacks using a coupled ice+ocean model of the Amundsen Sea glaciers. Under present-day ocean conditions, all glaciers, including Pine Island and Thwaites glaciers, continue to retreat, and we find significant trends in ice-shelf melt rates despite the absence of trends in the far-field ocean forcing. For example, melt rates for Pine Island Glacier double over a 50-year period under constant ocean conditions. For all cavities, the trend in melt can be attributed to a reconfiguration of the ocean circulation beneath the ice shelf, in response to changes in cavity geometry. We argue that these melt-geometry feedbacks may play an important role in the evolution of the Amundsen Sea glaciers, and they should be adequately captured in numerical simulations – something simple basal-melt parameterizations are currently unable to do.