Coupling of the residual circulation and static stability in the lowermost extratropical stratosphere

Recent observational studies have revealed the climatological existence of a layer of strongly increasing temperature that lies just above the extratropical tropopause (the tropopause inversion layer, TIL). Associated with this TIL is a distinct static stability maximum just above the tropopause and decreasing static stability towards typical lower stratospheric values aloft. Here, it is shown that the TIL is linked to the stratospheric residual circulation. Output from the Canadian Middle Atmosphere Model (CMAM) - a comprehensive global chemistry climate model - and from ERA40 is analyzed. Both, CMAM and ERA40 exhibit a more or less realistic TIL. In mid-latitudes residual mean vertical velocities are close to zero in the uppermost troposphere and decrease towards significant negative values in the lower stratosphere. By analyzing the heat budget in detail it is shown that this vertical gradient in residual vertical velocity in the lowermost stratosphere is associated with a maximum in static stability just above the tropopause and decreasing static stability aloft - as is observed. Moreover, it is shown that this static stability structure can be reproduced when radiative heating rates are replaced by simple Newtonian cooling in the heat budget. Conclusions are drawn concerning the link between tropical and extratropical tropopause through the stratospheric residual circulation. Variations in tropical tropopause temperature are found to be significantly correlated with variations in static stability just above the extratropical tropopause.