The connection between extreme stratospheric polar vortex events and tropospheric blockings

The variations of tropospheric blockings and their connections with the stratospheric polar vortex during different stages of the life cycle of extreme stratospheric polar vortex events (i.e. strong vortex (SV) events and weak vortex (WV) events) are investigated. The blocking frequency decreases over the Euro-Atlantic sector and increases over the western North Pacific during the onset and maturation stages of SV events. There are more blocking days over the western North Pacific and weakened upward planetary wave fluxes in the stratosphere during the maturation stage of shorter time-scale SV events than during longer time-scale SV events and the weakening of the upward planetary wave fluxes mainly results from the linear wave interference effect. The blocking frequency is increased over the Euro-Atlantic sector during the decline stage of SV events. This increase is found to be related to the descending stratospheric zonal wind anomalies and the poleward displacement of the eddy-driven jet stream, which lasts for about 10 days and is associated with the descending positive stratospheric Northern Annular Mode (NAM) anomalies. The increased blocking frequency leads to an enhancement of the planetary wave fluxes in the stratosphere via both the linear wave interference (for shorter time-scale SV events) and nonlinear wave interference (for longer time-scale SV events) effect. The changes of the blocking frequency during the growth stage of WV events are almost opposite to those observed during the onset and maturation stages of SV events. The blocking frequency decreases over the Euro-Atlantic sector after the central dates of WV events, which are related to the descending stratospheric zonal wind anomalies and the equatorward displacement of the eddy-driven jet stream preceded by the descending negative stratospheric NAM anomalies about 10 days. We found that the nonlinear wave interference begins to make a considerable contribution to the weakening of the upward wave fluxes in the stratosphere after the central date ofWV events and this is not emphasized in previous literature.