The dominant role of the summer hemisphere in subtropical lower stratospheric wave drag trends

It is well established that the shallow branch of the Brewer-Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen-Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection.