Winds and Meltwater Together Lead to Southern Ocean Surface Cooling and Sea Ice Expansion

Southern Ocean surface cooling and Antarctic sea ice expansion from 1979 through 2015 have been linked both to changing atmospheric circulation and melting of Antarctica's grounded ice and ice shelves. However, climate models have largely been unable to reproduce this behavior. Here we examine the contribution of observed wind variability and Antarctic meltwater to Southern Ocean sea surface temperature (SST) and Antarctic sea ice. The free‐running, CMIP6‐class GISS‐E2.1‐G climate model can simulate regional cooling and neutral sea ice trends due to internal variability, but they are unlikely. Constraining the model to observed winds and meltwater fluxes from 1990 through 2021 gives SST variability and trends consistent with observations. Meltwater and winds contribute a similar amount to the SST trend, and winds contribute more to the sea ice trend than meltwater. However, while the constrained model captures much of the observed sea ice variability, it only partially captures the post‐2015 sea ice reduction. Plain Language Summary While most of the globe has warmed in recent decades, the Southern Ocean around Antarctica cooled at the surface and its sea ice expanded from the beginning of satellite observations in 1979 through 2015. This unexpected behavior has been linked to changes in winds and to the addition of cold, fresh water from the melting of Antarctic's ice sheet and ice shelves. However, the importance of these two potential drivers has been unclear, partly because global climate models have often struggled to reproduce the observed changes. Here, we modify a climate model, constraining it to simulate observed winds and adding in realistic amounts of meltwater. With these changes, the model can simulate changes in SST and sea ice that are similar to observations. Winds and meltwater both play an important role. However, they cannot fully explain the large Antarctic sea ice reductions that were observed after 2015, suggesting that other factors may be at play. Key Points We nudge winds to observations and add estimates of observed freshwater from ice sheet and ice shelf melt in a coupled climate model Southern Ocean sea surface temperature trends and variability better match observations, with both winds and meltwater being important The constrained model simulates strong Antarctic sea ice expansion and only partially captures recent sea ice lows