The West Antarctic Ice Sheet Response to Tropical Forcing

Abstract:

Glaciers terminating in the Amundsen Sea in West Antarctica have all accelerated over the past decades, leading to significant global sea level rise. Oceanic melting of the ice shelves, the main driver of ice loss, is highly variable at interanual to decadal timescales, yet the ice response is thought to be largely due to an ice dynamic instability tied to the glacier bed geometry, triggered by an initial multi-decadal ocean warming anomaly with potential anthropogenic origins, and leaving a relatively minor role to the ocean variability regarding the pace of ice retreat and acceleration. Here, we use historical hydrographic observations to construct a robust climatology of ocean properties over the entire Amundsen Sea, allowing analysis of the ocean heat content distribution and its variability since 1994. Instead of a continuous warming, a spatially coherent decadal cycle prevails from the continental shelf break to the ice fronts. Satellite observations of sea surface height demonstrate that barotropic variability of the Antarctic slope front undercurrent, largely connected to zonal wind anomalies arising from changes in atmospheric convection in the tropical Pacific like El Niño, modulates deep heat flux from the Southern Ocean to the continental margin, and explains most of the ocean heat content variations. In turn, ocean heat variability is associated with basin-wide melt variability of long enough duration to significantly impact ice velocity. The amplitude and the spatial coherency of the ice response despite a variety of geometrical constraints indicate a consistent and important role for the ocean in modulating ice dynamics, now and in the future.