Disentangling Anthropogenic Effects on Southern Hemisphere Circulation and Surface Climate: A Multi-Model Large Ensemble Approach

Southern Hemisphere (SH) circulation and surface climate changes are emerging in recent decades. For example, in most seasons, the SH Hadley cell edge and the eddy-driven jet stream have shifted polewards, and the Southern Annular Mode (SAM) has trended towards its positive phase. However, attributing these changes to specific external forcings, such as greenhouse gas (GHG) increases or stratospheric ozone depletion, has been hindered by small ensemble sizes and lack of coherent forcing methodology across multiple models. In this study, we analyzed simulations from 10 models within the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) to isolate the long-term (1850–2014) SH climate response to individual forcings (GHGs, aerosols, and ozone). We found that long-term SH climate trends are dominated by GHGs across all seasons. Over this time period, stratospheric ozone depletion exerts an influence comparable to GHGs but is restricted to austral spring and summer. Although anthropogenic aerosols show weaker effects on climate trends that oppose those induced by GHG forcing and ozone depletion, they have a noticeable impact. Notably, ozone recovery following the Montreal Protocol is currently weakening or reversing spring and summer trends in key circulation metrics. Finally, we quantify the degree to which the observed sea surface cooling of the Southern Ocean and Antarctic sea ice increase from 1980 to 2014 fall outside the models' ensemble spread. Our results highlight the need to better understand the roles of different forcings, model differences, and discrepancies between models and observations to constrain projections of future Southern Hemisphere climate change.