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Antarctic sea ice cover is projected to significantly decrease by the end of the twenty-first century if greenhouse gas concentrations continue to rise, with potential consequences for Southern Hemisphere weather and climate. Here we examine the atmospheric response to projected Antarctic sea ice loss at quadrupled CO2, inferred from 11 Coupled Model Intercomparison Project phase 5 models. Our study is the first multimodel analysis of the atmospheric response to Antarctic sea ice loss. Projected sea ice loss enhances the negative phase of the Southern Annular Mode, which slightly damps the positive Southern Annular Mode response to increased CO2, particularly in spring. The negative Southern Annular Mode response largely reflects a weakening of the eddy-driven jet, and to a lesser extent, an equatorward shift of the jet. Sea ice loss induces near-surface warming over the high-latitude Southern Ocean, but warming does not penetrate over the Antarctic continent. In spring, we find multimodel evidence for a weakened polar stratospheric vortex in response to sea ice loss.

Plain Language Summary Increasing greenhouse gases through human activities are predicted to cause a decrease in Antarctic sea ice cover by the end of the century. If this happens, it is unknown what the impacts on Southern Hemisphere weather and climate could be. The aim of this study was to use simulations from 11 climate models to explore the potential consequences of future Antarctic sea ice loss. We analyzed model simulations with CO2 quadrupled from preindustrial levels, which led to large reductions in Antarctic sea ice. We found that sea ice loss led to warmer temperatures in the lowermost atmosphere over the Southern Ocean, but that this warming did not penetrate the Antarctic continent. Sea ice loss also had an impact on the predominantly westerly winds that encircle Antarctica, causing them to weaken. Climate models have some difficulties in representing Antarctic sea ice, and as a result, projections of Antarctic sea ice are highly uncertain. Our results imply that reducing uncertainties in projections of Antarctic sea ice may lead to better forecasts of future changes in Southern Hemisphere weather and climate.