A robust mechanism for strengthening of the Brewer-Dobson circulation in response to climate change: critical-layer control of subtropical wave breaking

Climate models consistently predict a strengthened Brewer-Dobson circulation in response to greenhouse-gas (GHG) induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave drag changes remains unclear. Here we analyze simulations from a chemistry-climate model to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby-wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upwards, allowing more wave activity to penetrate into the subtropical lower stratosphere. As the subtropics represent the critical region for wave driving of the Brewer-Dobson circulation, the circulation is thereby strengthened. Transient planetary-scale waves and synoptic-scale waves generated by baroclinic instability are both found to play a crucial role in this process. Changes in stationary planetary wave drag are not so important, because they largely occur away from subtropical latitudes.