The global stratospheric Brewer-Dobson circulation (BDC), with upwelling in the tropics and sinking motion above the poles, is expected to change with rising Greenhouse gas concentrations. A changing BDC, in turn, changes the stratospheric trace gas composition providing an important feedback via radiation and dynamics on climate change. However, trends in the BDC are largely uncertain, hitherto. Current climate models simulate a strengthening residual mean mass circulation throughout the stratosphere, resulting in decreasing mean age of air, the average transit time for an air parcel since entering the stratosphere across the tropical tropopause. Balloon-borne measurements collected over the last decades and satellite measurements with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) show show an inhomogeneous and more complicated picture of mean age trends.

We present mean age and its trends from updated balloon- and satellite-based datasets and from recent model simulation with the Lagrangian transport model CLaMS driven reanalysis meteorology. Comparison between CLaMS simulated and observed age of air shows good agreement from a climatological point of view, as well as regarding inter-annual variability related to the QBO, decadal changes and long-term trends. We discuss the trends in mean age based on the combination of satellite and balloon measurements and the CLaMS model results, and particularly relate these age trends to trends in residual circulation and mixing.