Climate models project an increase in the Brewer-Dobson circulation (BDC) with increasing future greenhouse gas (GHG) concentrations. This study identifies the causes of future changes in the BDC from sensitivity simulations with the EMAC chemistry-climate model, by changing the external forcings, like GHG concentrations, sea surface temperatures (SSTs) together with sea ice concentrations, and ozone-depleting substances (ODS), separately. The particular influence of rising tropical SSTs is assessed. Contributions of different waves to changes in the residual circulation are calculated as well as changes in mean age of stratospheric air (AoA) to account for the effect of mixing processes. We find that in boreal winter the tropical upward mass flux increases by about 1%/dec in the upper and 2%/dec in the lower stratosphere until the end of the 21st century. Mean AoA decreases by up to 60 and 30 days/dec, respectively. Changes in transient planetary and synoptic waves account for the strengthening of the BDC in the lower stratosphere, whereas upper stratospheric changes are due to improved propagation properties for gravity waves in future climate. Regarding the external forcings, the radiative impact of rising GHG concentrations is detected to affect upper stratospheric layers only, whereas lower stratospheric signals are almost entirely due to rising SSTs. Changes in tropical SSTs influence not only the shallow but also the deep branch of the BDC as confirmed from both changes in residual circulation and mixing. Declining ODS will slightly counteract the BDC increase in the Southern Hemisphere.
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