The circulation and rainfall response in the southern hemisphere extra-tropics to climate stabilisation

Changes from transient to equilibrium climate states following stabilisation of anthropogenic forcings will determine the long-term future of our climate, including the future of rainfall and drought. However, there is a lack of research and very long climate simulations to characterise the future equilibrium climate. As forcing and global temperature stabilise, some regions will likely continue to warm, including much of the Southern Ocean. Here we explore the change in surface warming, atmospheric circulation, and rainfall in two case studies of high CO2-only and high all forcings from LongRunMIP. We find an initial delay in Southern Ocean warming, enhancing the meridional temperature gradient between the tropics and the Southern Ocean. The enhanced gradient peaks sooner and with lower magnitude over the Indian Ocean relative to the gradient over the Pacific Ocean. Coincident with the peak meridional temperature gradient there is clear stabilisation and then reversal of mid-latitude annual drying. The response in atmospheric circulation in one case study has the character of a shift from positive to negative Southern Annular Mode (SAM). Some regions of the mid-latitudes, such as southern Australia, become drier and do not rebound to wetter conditions during the transition to an equilibrium climate state, whereas other regions such as eastern Australia may see transient drying that then rebounds to a wetter climate as a new equilibrium is reached. This study adds to growing evidence of a time-dependent response in rainfall and water availability to greenhouse gas emissions across key regions, including Australia, and further understanding is needed about short-term and long-term changes.