Tropical forcing of climate and ice sheet change in Antarctica: implications for the future of the West Antarctic Ice Sheet

It is now well established that rapid warming is occurring over the Pacific sector of Antarctica. Temperature trends over the last 50 years over the West Antarctica ice sheet rival or exceed those on the Antarctic Peninsula. The greatest warming has occurred in winter and spring, though temperatures have increased in all seasons. The spatial pattern of Antarctic temperature trends closely matches the spatial pattern of sea ice change: there has been little sea ice change around most of East Antarctica, but sea ice has declined nearly monotonically in the Pacific sector of the Southern Ocean (the Admundsen-Bellinghausen Seas) in all seasons. Both the sea ice and temperature changes in the Pacific sector reflect changes in atmospheric circulation that have also had a significant impact on regional ocean circulation. In particular, there has been enhanced inflow of Circumpolar Deep Water onto the Antarctic continental shelf, which is rapidly melting ice shelves, causing West Antarctic outlet glaciers to accelerate. We present evidence from ice cores that the magnitude of these changes is exceptional in the context of the last ~150 years. Although this is suggestive of a role for anthropogenic forcing, the atmospheric circulation changes observed in Antarctica are largely a response to Rossby wave propagation of anomalous convection in the tropical Pacific. The anomalous nature of recent trends in Antarctica is thus largely attributable to similarly anomalous conditions in the tropical Pacific. The uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice sheet change.