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

Abstract

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.

Plain Language Summary The Brewer-Dobson circulation, responsible for mass, heat and constituents global transport in the stratosphere, is projected to accelerate in a warming climate. This circulation is driven by the momentum transferred by dissipating waves. We explore the seasonality of trends in wave dissipation in the subtropical lower stratosphere. First, we show that the largest changes in the wave dissipation take place in the summer hemisphere, opposite to the largest changes in the zonal wind, which is known to control wave dissipation conditions. We investigate this apparent contradiction and find that (a) the conditions are particularly favorable for the waves to be affected by the changing wind in summer, due to their spectral characteristics and the structure of the background zonal wind, in particular the proximity of the zero wind line; and (b) in the Northern Hemisphere the changes are primarily associated with stationary waves triggered by enhanced deep convection in a warmer climate.