Southern Hemisphere Stratospheric Warmings and Coupling to the Mesosphere-Lower Thermosphere

Twenty six years of medium frequency (MF) radar wind measurements made from 1994 to 2019 at Davis Station (68.6 degrees S, 77.9 degrees E) are used to study the mean response of the mesosphere-lower thermosphere to stratospheric warmings in the Southern Hemisphere. Warming events were detected using Modern-Era Retrospective Analysis for Research and Applications (MERRA2) data with a systematic search for reductions in the zonal-mean circulation at 60 degrees S and corresponding increases in polar temperatures. Some 37 events were identified, including the 2002 major warming and the large event of 2019, with an average of 1-2 warmings per year. At the 10 hPa level, the polar cap temperature increases ranged from 3 to 28 K, with a mean value of 12 K, while the zonal wind speed reductions varied between -6 and -43 ms(-1), with a mean value of -15 ms(-1). Peak values occurred near 40 km. Warmings occurred mainly between August and October, with a small peak in occurrence in April/May. The MF radar data showed an average reduction in the mesospheric eastward winds of about 5-7 ms(-1) at heights near 75 km that occurred 3-4 days prior to the changes in the stratosphere. Warming events were driven by episodic intensifications in planetary wave amplitudes, with quasi-stationary planetary scale waves (PW) 1 being especially important. PW Eliassen-Palm flux divergences show a systematic behavior with time and height that is consistent with a poleward residual circulation and downwelling over the pole prior to the warming events and an equatorward flow and upwelling after the peak of the events.