How does the northern-winter wave driving of the Brewer-Dobson circulation increase in an enhanced-CO₂ climate simulation?

Recent climate studies show that the northern-winter wave driving of the Brewer-Dobson circulation is enhanced if greenhouse gas concentrations increase. An explanation for this enhancement does not yet exist. In this study, the enhanced wave driving, as simulated in a doubled-CO2 experiment with the MA-ECHAM4 climate model, is analyzed in detail. The extratropical poleward eddy heat flux increases (decreases) in the stratosphere (troposphere) mainly due to the stationary (transient) heat-flux component. The heat flux at 100 hPa is a measure of the stratospheric wave driving, and is found to increase by 12% in the doubled-CO2 climate. This increase is dominated by the stationary-wave 1 heat flux, which is also enhanced in the midlatitude troposphere. The heat flux increase at 100 hPa is almost entirely due to an increase in the longitudinal temperature variability. The latter increase is mainly due to the well-understood sharpening of the lower-stratospheric meridional temperature gradient