Atmosphere-Ocean Forcing of Ice-Sheet Change in the Amundsen Sea Sector of Antarctica

Abstract:

Thinning of the ice sheet that drains into the Amundsen Sea is currently the major contribution of Antarctica to sea level rise. Acceleration of the outflow glaciers and inland thinning of the ice are now well-documented, but our understanding of the underlying causes remains rudimentary. The observations that thinning is most rapid at the coast and propagates inland suggest that changes in ocean-forced melt of the ice shelves are responsible. However, a critical question that remains unanswered is whether current changes are a continuing response to an earlier trigger, or are driven by current ocean variability.

It has been argued that atmospheric circulation changes associated with sea surface temperature variability over the central tropical Pacific Ocean could drive changes in ocean conditions along the Amundsen Sea coast. That conclusion was based on a coarse-resolution model, some results of which now appear questionable. However, observational evidence is supportive of such a link with the tropical Pacific, a pronounced cooling of the Amundsen Sea in 2012 being linked with cool La Nina conditions in the cental Pacific.

We critically re-evalute the evidence for such a link with large-scale atmospheric forcing and explore the implications using an ice sheet/shelf model forced by a new parematerisation of melting in response to prescribed ocean temperatures. In particular, we test the hypothesis that initial retreat of Pine Island Glacier from a prominent seabed ridge coincided with a period of exceptional warmth in the central tropical Pacific that had a major impact on West Antarctic atmospheric circulation in the early 1940s. We investigate the magnitude and duration of the 1940’s warm event required to start the retreat of the grounding line, and address the question of whether that trigger is the primary cause of current change, or whether more recent warmth has been critical in driving continuing retreat of the grounding line.