The impact of planetary waves on the latitudinal and longitudinal variations of sudden stratospheric warmings

During the northern hemispheric winter occurs a large scale variability mainly caused by Planetary Waves (PWs) interacting with the mean flow, named Sudden Stratospheric Warming (SSW). The increase of the stratospheric temperature is connected with a simultaneous decrease of the mesospheric temperature and a zonal wind reversal to a westward wind from the mesosphere to the stratosphere. In most cases the SSW is strongest at polar latitudes, gets weaker toward the south and vanishes at mid-latitudes around 50° N as for example during the major warming in 2006. However, other events like in 2009, 2010 and 2012 show a similar or even stronger westward wind at mid than at polar latitudes either in the mesosphere or in the stratosphere during the SSW. To investigate the latitudinal and longitudinal dependence of SSWs, this study uses local meteor and MF-radar measurements from different stations, global satellite observations from the Microwave Limb Sounder (MLS) and assimilated model data from MERRA (Modern-ERA Retrospective analysis for research and Applications). The differences in the latitudinal structure of the zonal wind, temperature and PW activity are compared between a 'normal' event, where the event in 2006 was chosen representatively, and the latitudinal displaced events in 2009, 2010 and 2012. During the displaced events a continuous westward wind band between the pole and 20° N is observed. Furthermore, cold temperatures at mid-latitudes occur before the displaced warmings compared to 2006 as well as a southward extended stratospheric warming afterwards. These differences between the normal SSW in 2006 and the displaced events in 2009, 2010 and 2012 are linked to an increased PW activity between 30° N and 50° N and the changed stationary wave flux in the stratosphere around the displaced events compared to 2006. Finally we will discuss the longitudinal dependence of SSWs on the mesospheric circulation mainly based on comparisons of radar observations over northern Europe with corresponding results over northern America in relation to the location and form of the polar vortex.