Advective Transport Between the Stratosphere and Mesosphere

Abstract The Brewer‐Dobson circulation (BDC) characterizes the large‐scale meridional overturning mass circulation influencing the composition of the whole middle atmosphere. The BDC consists of two separate parts—a shallow branch in the lower stratosphere and a deep branch higher in the middle atmosphere. Climate models robustly project the advective BDC part to accelerate due to greenhouse gas‐induced climate change and this acceleration strongly influences middle atmospheric chemistry and physics in the projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwelling across pressure levels, particularly in the lower stratosphere. The upper branch of the BDC has received considerably less research attention than its shallow part, although, together with the mean mesospheric pole‐to‐pole circulation, it couples the stratosphere and mesosphere and is responsible for a large portion of the interhemispheric transport in the middle atmosphere. Aiming to fill this gap, we here study climatology and trends in advective mass transport across the stratopause. Results based on the analysis of seven CCMI models include quantification of the climatological cross‐stratopause advective transport, characterization of its interannual variability and long‐term trend as well as detailed analysis of inter‐model differences using a novel decomposition methodology. Our results demonstrate that the changes in circulation speed as well as changes in horizontal and vertical structure in the upper stratosphere and mesosphere jointly shape the projected increasing advective mass flux across the stratopause due to increasing greenhouse‐gas emissions.