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Abstract:
One of the most robust circulation changes in response to GHG forcing identified from global models as well as observations is a strengthening of the overturning circulation in the lower stratosphere. The enhanced upwelling in the tropical lower stratosphere has long been recognized to cause decreases in lower stratospheric ozone. Since ozone is a radiative active gas, changes in ozone abundances feed back on local temperatures. Consequently, ozone can act as a feedback on circulation changes. In this study, we specifically focus on the feedback between the stratospheric overturning circulation (the Brewer-Dobson circulation) and ozone. By running dedicated global model simulations with an idealized 4xCO2 increase and prescribed fixed versus adapting ozone abundances, we quantify the feedback of ozone changes on the circulation. It is shown, that ozone acts to dampen the CO2-induced strengthening of the Brewer-Dobson circulation by up to 20%. The mechanism for this relative slow down of the circulation by ozone changes is a change in the equator-to-pole temperature gradient induced by ozone. In particular in summer, this leads to more easterly winds, thus inhibiting vertical wave propagation and wave driving of the circulation. Next to the effects on the BDC, we find that ozone affects the polar vortices and their breakdown dates. This can affect the tropospheric jet response to GHGs via downward coupling. Overall the results of this work together with previous studies imply that neglecting the effects of stratospheric ozone changes will lead to an overestimation of the circulation response to GHG forcing.
