Advective transport between the stratosphere and mesosphere

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. The BDC is analytically usually defined as consisting of a diffusive part and an advective residual mean. Climate model simulations robustly show that the advective BDC part accelerates due to greenhouse gas-induced climate change and this acceleration strongly influences middle atmospheric chemistry and physics in climate model projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwellling, commonly at the tropopause level or in the lower stratosphere. The upper branch of the BDC received considerably less research attention than its shallow part, although it features important atmospheric mechanisms. It couples the stratosphere and mesosphere and is responsible for a large portion of interhemispheric transport in the middle atmosphere. Aiming to fill this gap, we present a multi-model study of climatology and trends in advective mass transport across the vertically shifting stratopause. Results based on ensembles of 7 CCMI models include decomposition of long-term changes in cross-stratopause transport into individual terms such as acceleration of the residual circulation itself, vertical shift of the stratopause, changes in width of the upwelling region and changes in the shape of the stratopause.