Solar-Induced Stratospheric Circulation Changes

The propagation of gravity waves (GWs) and their reflection at the upper stratosphere-lower mesosphere discontinuity during maximum 11-year solar activity are studied. Solar activity is represented by a temperature increase of 2.5 K in the tropical upper stratosphere. A standard set of hydro-dynamic (HD) equations, along with the dispersion relation and the equation for the reflection coefficient, is applied to the upper stratosphere-lower mesosphere discontinuity to determine which portions of the GW spectrum are most likely to propagate across it and influence the dynamics of the middle atmosphere. We find that the GW reflection increases noticeably during maximum 11-year solar activity. Gravity waves with vh < 31ms−1 are strongly attenuated at the upper stratosphere-lower mesosphere discontinuity, while waves with 31ms−1 < vh < 279ms−1 are much more likely to enter the mesosphere. As a result, the GW flux reaching the lower mesosphere is reduced, thereby decreasing the ability of GWs to dissipate, transfer momentum to the mean flow, and drive the Brewer-Dobson Circulation (BDC). Since this is primarily a wave-driven circulation, the BDC is weaker during maximum solar activity than under normal stratospheric conditions. The connection between the lower mesosphere and BDC in the upper stratosphere is explained by the downward control principle.