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

Abstract

Twenty six years of MF radar wind measurements made from 1994 to 2019 at Davis Station (68.6*S, 77.9*E) are used to study the mean re-sponse 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 (MERRA)-2 data with a systematic search for reductions in the zonal-mean circulation at 60*S and corresponding increases in polar temperatures. Some 38 events were identified, including the major warmings of 2002 and 2019, with an average of 1 to 2 warmings per year. At the 10 hPa level, the polar cap temperature increases ranged from 5 to 30 K, with a mean value of 11 K, while the zonal wind speed reductions varied between -7 to -43 ms-1, with -1 a mean value of -15 ms . Peak values occurred near 40 km. Warmings occurred mainly between August and October, with a small peak in oc-currence 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 some 3-4 days prior to the changes in the stratosphere. Warming events were driven by episodic intensifications in planetary waves amplitudes, with quasi-stationary PW 1 being especially important. Plane-tary wave 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.