The connection between extreme stratospheric polar vortex events and tropospheric blockings

Abstract

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 of WV events and this is not emphasized in previous literature.

Classifying the tropospheric precursor patterns of sudden stratospheric warmings

“…The results bring up the question whether the WH pattern is a precursor of extreme PVIs. Compositing tropospheric patterns during the growth stage of PVIs shows a wave train that projects positively onto the WH pattern [ Limpasuvan et al , ; Huang et al , ]. This suggests that a pattern associated with the WH pattern could be a major precursor of PVIs.…”

“…It is known that the occurrence of SSWs is preceded by the enhancement of upward wave fluxes of planetary waves from the troposphere [Limpasuvan et al, 2004;Polvani and Waugh, 2004;Charlton and Polvani, 2007]. Recent multiyear statistical studies on the precursors of SSWs and polar vortex intensification events (PVIs) have noted that tropospheric blocking in some specific regions and tropospheric variability in the North Pacific can enhance or suppress upward planetary wave propagation and give rise to anomalous stratospheric polar vortices, including SSWs and PVIs [Martius et al, 2009;Garfinkel et al, 2010;Woollings et al, 2010;Castanheira and Barriopedro, 2010;Nishii et al, 2010Nishii et al, , 2011Barriopedro and Calvo, 2014;Dai and Tan, 2016;Huang et al, 2017]. However, these studies have not yet agreed on the tropospheric precursors.…”

A thermodynamic climatology of the disturbed stratospheric polar vortex

“…Those blocking events were associated with significantly larger upper tropospheric polewards eddy heat fluxes, stratospheric thermally forced geopotential height rises near the polar vortex, and height falls equatorwards of the polar vortex than those blocking events not associated with SSWs. Surface pressure lows and decreased blocking in the northern Pacific, along with surface highs and increased blocking in Europe are connected to vortex disturbances by constructive interference with climatological planetary waves (Garfinkel et al ., ; Huang et al ., ). Furthermore, Bao et al .…”

Duration and decay of Arctic stratospheric vortex events in the ECMWF seasonal forecast model

“…Blocking Highs (BHs) play a crucial role in exerting this tropospheric forcing. They play, in fact, a dual role as the geographical location of BHs determines their stratospheric influence through their interaction with upper‐tropospheric background planetary waves, adequately described in terms of quasi‐linear interference (Orsolini et al, ; Nishii et al, ; ; Woollings et al, ; Dai and Tan, ; Díaz‐Durán et al, ; Huang et al, ). BHs associated with the Western Pacific (WP) pattern were found to be precursors of strong vortex events, while BHs over the North Atlantic lead to long weak vortex events (Nishii et al, ; ; Dai and Tan, ).…”