Sudden Stratospheric Warmings (SSWs) are major disturbances in the polar region of the winter hemisphere that cause major changes in stratospheric temperature and circulation. SSWs are characterized by a temperature increase of tens of degrees Kelvin, averaged over 60°-90° latitude, and a weakening of the polar vortex that persists for the order of a week at the 10 hPa level (roughly 32 km) [Labitzke and Naujokat, 2000]. The polar vortices are cyclones centered on both of the Earth’s poles that are present from the mid-troposphere to the lower stratosphere. Eastward zonal winds define the strong polar vortices in the winter. Increased planetary wave (PW ) activity in the winter hemisphere leads to increased PW breaking in the polar stratosphere and the deposition of the PW’s westward momentum in the polar vortex. This weakens, and in the case of major SSWs, can reverse the zonal wind direction to westward. The reversal of the stratospheric jet allows more eastward propagating gravity waves (GWs) to travel up into the mesosphere where, in normal winter conditions, westward propagating GWs dominate [Liu and Roble, 2002]. The atypical wintertime GW filtering and the resulting dominating westward GWs induce an equatorward circulation in the mesosphere, which leads to the cooling of the upper mesosphere. While these mesospheric coolings have been observed in the polar regions for several decades [Labitzke, 1972], they have only recently been observed at mid-latitudes [Yuan et al., 2012].
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