A chemistry-climate model study of past changes in the Brewer-Dobson circulation

Model simulations indicate an acceleration of the Brewer-Dobson circulation (BDC) in the past with a direct impact on the exchange of air masses between the troposphere and the stratosphere. However, most observational data sets do not confirm the model results. Bonisch et al. (2011) and Ray et al. (2010) indicate a strengthening of the BDC in its shallow branch and a weakening in its deep branch. In contrast, balloon-borne measurements and Michelson Interferometer for Passive Atmospheric Sounding satellite observations show no significant acceleration of the BDC at Northern Hemisphere (NH) midlatitudes. In our study, the change of the BDC in the recent past is analyzed from simulations with the chemistry-climate model ECHAM/MESSy Atmospheric Chemistry. As the sign of change considerably depends on the underlying forcings, namely, changes in concentrations of greenhouse gases (GHGs) and ozone-depleting substances (ODSs), as well as their interactions, we separate their contributions by using sensitivity simulations. The changes in tropical upward mass flux indicate a strengthening of the BDC between 1960 and 2000 in NH winter season in the lower and a weakening in the upper stratosphere with a change in sign at 10hPa. While the lower stratospheric increase of about 2%/decade is caused by equal contributions from rising GHGs and ODSs, rising ODS concentrations counteract the GHG effect in the middle and upper stratosphere with a total decrease of about 0.5%/decade. Changes in mean age of air show a decrease of about 0.13yr/decade in the lower and middle stratosphere and a slight increase in the Arctic upper stratosphere and lower mesosphere, which is induced by interactions between the forcings.