The Climate Response to Antarctic Sea Ice Loss

Antarctic sea ice is projected to decrease in response to increasing greenhouse gas concentrations. Antarctic sea ice and its changes are known to strongly modulate the climate of the high-latitude Southern Hemisphere, but sea-ice loss may also affect the large-scale atmospheric and oceanic circulation. Limited studies so far have examined the coupled atmosphere-ocean response to Antarctic sea-ice loss. This thesis provides the first comprehensive examination of the climate response to Antarctic sea-ice loss, isolating the atmospheric and oceanic responses, and their coupled interactions, using several modelling approaches. It provides the first multi-model assessment of the atmospheric response to Antarctic sea-ice loss, and novel bespoke coupled model experiments, to assess the oceanic response and the importance of ocean-atmosphere coupling. Lower tropospheric warming over regions of sea-ice loss and the nearby Southern Ocean is robust across all models and experimental approaches. Ocean coupling allows the warming response to spread globally, including to the Antarctic interior, Arctic Ocean and tropical upper troposphere, and throughout the global oceans. A weakening and slight equatorward shift of the Southern Hemisphere tropospheric eddy-driven jet is also common across all models and experiments, but of greater magnitude in the coupled model. The oceanic response features a 20% reduction in Antarctic Circumpolar Current transport, likely due to reduced surface wind stress. Surface waters warm, becoming more stable and more stratified in the Southern Ocean, with similar changes, but of lesser magnitude, in the Arctic Ocean, where sea ice declines. These results demonstrate that the ocean governs the widespread effects of Antarctic sea-ice loss, and is also important in determining the magnitude of its effects over the Southern Hemisphere high latitudes.