The Role of the Stratosphere in Driving Uncertainties in the Southern Hemisphere Future Climate

The strength and latitudinal position of the extratropical eddy-driven jet (EDJ) in the Southern Hemisphere (SH) winter and summer is naturally forced by the conditions of the tropical oceans and by the variability of the SH stratospheric polar vortex (SPV). Uncertainty in the responses of these remote drivers (RDs) of extratropical circulation to anthropogenic forcing leads to uncertainty in the future strength and latitude of the EDJ. In turn, these changes in tropospheric circulation can lead to changes at the regional scale. During this century, the combined effect of ozone recovery and the increase in greenhouse gases (GHGs) will influence the SPV. Therefore, understanding the ‘tug-of-war’ between these two anthropogenic forcings is crucial to understand future projections. Moreover, the influences of the stratosphere will be combined with the influences of forced changes in the tropics. This complex interplay between RDs and the magnitude of each RD’s response to anthropogenic forcings differs among Global Climate Models (GCMs), which leads to different responses of the EDJ and regional climate. In this work we analyze an ensemble of CMIP6 models to study the role of forced responses in the SPV in driving changes in climate projections, and how the influence of the stratospheric changes combines with the influence of the changes in a small set of tropical RDs. In particular, we find that a strengthening of the SPV leads to a strengthening and small poleward shift of the EDJ in winter, and that a delay in the SPV breakdown date leads to a strong poleward shift of the EDJ in summer. The evolution of the summer circulation response in CMIP6 models during the twenty-first century can be explained from the combined effect of ozone recovery and GHG increase. At the regional scale, in winter, a strengthening of the SPV leads to drying in Southeastern South America and wetting in Tierra del Fuego, in the south of South America, while in summer, a SPV breakdown delay leads to wetting in the west coast of all three large extratropical continental sectors and the coasts of Antarctica. Finally, we develop storylines of future circulation and precipitation changes in both summer and winter which help understand the relative role of the SPV among other dynamical drivers of change in the SH.