West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability

Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high

Oceanographic fieldwork in the Amundsen Sea: an overview of cruise JR141

Introduction This report provides an overview of a 2006 cruise to the Amundsen Sea, the oceanographic data collected, and how we intend to use these data to further our understanding of the physical oceanography of this region. The Amundsen Sea, located in the eastern Pacific sector of the Southern Ocean, is a region where ice shelves are rapidly thinning (Shepherd et al., 2004). The widespread, coherent nature of the thinning suggests a reaction to external forcing. Given that air temperatures are below freezing year-round, the most likely instigator of rapid change is the ocean. The Amundsen and Bellingshausen Seas have water properties unlike anywhere else around Antarctica. Circumpolar Deep Water (CDW), which elsewhere is found only within the Antarctic Circumpolar Current (ACC), floods the continental shelves, resulting in the highest water temperatures to be found around the continent. CDW is characterised by temperatures in excess of 1ºC, which is around 3°C warmer than the surface freezing point. When this water mass has access to the base of an ice shelf, melting is one to two orders of magnitude higher than it would otherwise be. Previous studies have shown the link between rapid melting and the presence of CDW beneath ice shelves (Potter and Paren, 1985; Jacobs et al., 1996), and sub-ice shelf melt rates have been found to be sensitive to relatively minor changes in water temperature (Hellmer et al., 1998). Understanding the mechanisms of the on-shelf transport of CDW and quantifying the associated heat fluxes are clearly key to understanding the oceanographic impact on the ice shelve