Effects of prescribed CMIP6 ozone on simulating the SouthernHemisphere atmospheric circulation response to ozone depletion

Abstract. The Antarctic ozone hole has led to substantial changes in the Southern Hemisphere atmospheric circulation, such as the strengthening and poleward shift of the mid-latitude westerly jet. Ozone recovery during the twenty-first century is expected to continue to affect the jet's strength and position, leading to changes in the opposite direction compared to the twentieth century and competing with the effect of increasing greenhouse gases. Simulations of the Earth's past and future climate, such as those performed for the Coupled Model Intercomparison Project Phase 6 (CMIP6), require an accurate representation of these ozone effects. Climate models that use prescribed ozone fields lack the important feedbacks between ozone chemistry, radiative heating, dynamics, as well as transport. These limitations ultimately affect their climate response to ozone depletion. This study investigates the impact of prescribing the ozone field recommended for CMIP6 on the simulated effects of ozone depletion in the Southern Hemisphere. We employ a new, state-of the-art coupled climate model, FOCI, to compare simulations in which the CMIP6 ozone is prescribed with simulations in which the ozone chemistry is calculated interactively. At the same time, we compare the roles played by ozone depletion and by increasing concentrations of greenhouse gases in driving changes in the Southern Hemisphere atmospheric circulation, using a series of historical sensitivity simulations. FOCI reliably captures the known effects of ozone depletion, simulating an austral spring and summer intensification of the mid-latitude westerly winds and of the Brewer-Dobson circulation in the Southern Hemisphere. Ozone depletion is the primary driver of these historical circulation changes in FOCI. These changes are weaker in the simulations that prescribe the CMIP6 ozone field. We attribute this weaker response to the missing ozone-radiative-dynamical feedbacks and to a prescribed ozone hole that is displaced compared to the simulated polar vortex, altering the propagation of planetary wave activity. As a result, the dynamical contribution to the ozone-induced austral spring lower stratospheric cooling is suppressed, leading to a weaker cooling trend. Consequently, the intensification of the polar night jet is also weaker in the simulations with prescribed CMIP6 ozone. In addition, the persistence of the Southern Annular Mode is shorter in the prescribed ozone chemistry simulations. These results suggest that climate models which prescribe the CMIP6 ozone field still underestimate the historical ozone-induced dynamical changes in the Southern Hemisphere, while models that calculate the ozone chemistry interactively simulate an improved response to ozone depletion.