Changes in the stratospheric mean meridional circulation due to increased CO₂: Radiation- and sea surface temperature-induced effects

Mechanisms of changes in the stratospheric Brewer-Dobson circulation (BDC) due to increased CO₂ were investigated through atmospheric GCM experiments. The total effects of increased CO₂ on the BDC are separated into the radiation-induced effects (direct effects) and SST-induced effects (indirect effects). During winter, both direct and indirect effects enhance the NE BDC in the upper stratosphere much more than the SH BDC because stationary waves are more active in the NH upper stratosphere. In the NH during winter, tropospheric stationary waves are suppressed because of both direct and indirect effects, implying that wave generation in the troposphere is not related directly to the enhanced stationary waves in the upper stratosphere. In the summer stratosphere, the indirect effects mainly enhance the BDC in the stratosphere, consistent with the enhanced transient and stationary waves. The indirect effects enhance the midlatitude westerlies in the summer hemispheres and probably affect vertical wave propagation in the upper troposphere and stratosphere.