Understanding Drivers of Southern Ocean Climate Trends

Recent trends in Southern Ocean (SO) climate - of surface cooling, freshening and sea-ice expansion – are not captured in standard historical simulations of state-of-the-art coupled climate models, suggesting that there may be a ‘missing process’ which is currently not represented in our models. Here we demonstrate that the addition of realistic historical changes in Antarctic melt-water to a coupled climate model can produce a much better match to these observations. We use an ensemble of simulations of the Goddard Institute for Space Studies Earth System Model to compute ‘Climate Response Functions’ (CRFs) for the addition of Antarctic melt-water. These functions show surface Southern Ocean cooling and freshening, an expansion of winter sea ice and an increase in steric height, all consistent with observed trends since 1990's. The CRF framework allows the comparison of Antarctic melt-water, greenhouse gas forcing and ozone-induced surface winds changes as drivers in SO Sea Surface Temperature. The freshwater CRFs presented here strongly suggest that interactive Antarctic ice melt must be included in models in order to correctly hind-cast trends in the historical record and, by implication, make realistic future predictions.