Twenty-first century Southern Hemisphere impacts of ozone recovery and climate change from the stratosphere to the ocean

Abstract. Changes in stratospheric ozone concentrations and increasing concentrations of greenhouse gases (GHGs) alter the temperature structure of the atmosphere and drive changes in the atmospheric and oceanic circulation. We systematically investigate the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the Southern Hemisphere during the twenty-first century using a unique coupled ocean-atmosphere climate model with interactive ozone chemistry and enhanced oceanic resolution. We use the high emission scenario SSP5-8.5 for GHGs under which the springtime Antarctic total column ozone returns to 1980s levels by 2048 in our model, warming the lower stratosphere and strengthening the stratospheric westerly winds. Novel results of this study include the springtime stratospheric circulation response to GHGs, which is characterized by changes of opposing sign over the Eastern and Western Hemispheres, the opposing responses of the Agulhas leakage to ozone recovery and increasing GHGs, and large uncertainties in the prediction of atmospheric and oceanic circulation changes related to whether the ozone field is prescribed or calculated interactively. By performing a thorough spatial analysis of the predicted changes in the stratospheric dynamics, we find that the GHG effect during spring exhibits a strong dipole pattern, which contrasts the GHG effect during the rest of the year and which was previously not reported, as it cannot be detected when zonal means are considered. Over the Western Hemisphere, GHGs drive a warming of the lower stratosphere and a weakening of the westerlies, while over the Eastern Hemisphere they drive a cooling and a strengthening of the westerlies. Associated with these changes, planetary waves break higher up in the stratosphere over the Eastern Hemisphere, strengthening the polar downwelling and inducing dynamical warming in the upper stratosphere, while weakening the downwelling and inducing dynamical cooling in the lower stratosphere. The opposite occurs over the Western Hemisphere. Because the changes in the Western Hemisphere dominate during November in our model, we find that during this month GHGs lead to a weakening of the lower branch of the Brewer-Dobson Circulation, reinforcing the weakening caused by ozone recovery. At the surface, the westerly winds weaken and shift equatorward due to ozone recovery, driving a weak decrease in the transport of the Antarctic Circumpolar Current and in the Agulhas leakage, which transports warm and saline waters from the Indian into the Atlantic Ocean. The increasing GHGs drive changes in the opposite direction that overwhelm the ozone effect. The total changes at the surface and in the oceanic circulation are nevertheless weaker in the presence of ozone recovery than those induced by GHGs alone, highlighting the importance of the Montreal Protocol in mitigating some of the impacts of climate change. We additionally compare the combined effect of interactively calculated ozone recovery and increasing GHGs with their combined effect in an ensemble in which we prescribe the CMIP6 ozone field. This second ensemble simulates a weaker ozone effect in all the examined fields. The magnitude of the difference between the simulated changes at the surface and in the oceanic circulation in the two ensembles is as large as the ozone effect itself. This shows that the choice between prescribing or calculating the ozone field interactively can affect the prediction of changes not only in the atmospheric, but also in the oceanic circulation.