Observational perspective on sudden stratospheric warmings and blocking from Eliassen-Palm fluxes

In this study, we examine eight major boreal sudden stratospheric warming (SSW) events between 2007 and 2019 to understand the vertical coupling between the troposphere and stratosphere as well as the relationship between SSWs and blocking events using global navigation satellite system (GNSS) radio occultation (RO) observations. Our study covers the main aspects of SSW events, including the vertical structure of planetary-wave propagation, static stability, geometry of the polar vortex, and occurrence of blocking events. To analyze wave activity and atmospheric circulation, we compute the quasi-geostrophic Eliassen-Palm (EP) flux and geostrophic winds. The results show that the observations agree with theory and previous studies in terms of the primary dynamic features and provide a detailed view of their vertical structure. We observe a clear positive peak of upward EP flux in the stratosphere prior to all SSW events. In seven out of eight events, this peak is preceded by a clear peak in the troposphere. Within the observed timeframe, we identify two types of downward dynamic interactions and the emergence of blocking events. During the 2007 and 2008 "reflecting" events, we observe a displacement of the polar vortex along with a downward propagation of wave activity from the stratosphere to the troposphere during vortex recovery, coinciding with the formation of blocking in the North Pacific region. Conversely, in the other six SSW "absorbing" events from 2009 to 2019, which were characterized by a vortex split, we observe wave absorption and the subsequent formation of blocking in the Euro-Atlantic region. The analysis of the static stability demonstrates an enhancement of the polar tropopause inversion layer as the result of SSWs, which was stronger for the absorbing events. Overall, our study provides an observational view of the synoptic and dynamic evolution of the major SSWs, their link to blocking, and the impact on the polar tropopause.