Stratosphere-troposphere coupling during polar vortex breakdown

We examine how interannual variations in the timing of stratospheric final warming (SFW) events affect the stratosphere-troposphere system. Specifically, we isolate the coupled stratosphere-troposphere dynamical evolution observed to occur in association with SFW events in the Northern and Southern Hemispheres. In approaching this problem, we implicitly address the hypothesis that the tropospheric circulation is influenced by interannual variations in the breakup of the stratospheric polar vortex. SFW events are defined in terms of the variability of the zonal-mean zonal wind in the subpolar lower stratosphere. Three-dimensional circulation anomalies, taken as departures from long term seasonal trend values, are then composited with respect to SFW onset dates to identify anomaly structures linked to SFW events. The analyses are performed for SFW events in both the Northern and Southern Hemispheres. SFW events are found to provide a strong organizing influence on the large-scale circulation of the stratosphere and troposphere, acting to accelerate seasonal transitions in comparison to climatological seasonal trends. A coherent pattern of significant westerly (easterly) zonal wind anomalies extends from the stratosphere to the Earth's surface at high latitudes in the weeks prior to (after) SFWs, indicating rapid breakdowns in the stratospheric and tropospheric westerly jets. The high latitude stratospheric decelerations are accompanied by opposing zonal wind accelerations in the subtropical stratosphere with downward extensions into the troposphere. The near surface manifestation of SFW events consists of anomalously low (high) sea level pressure over the poles in the weeks prior to (following) SFW events. The composite analyses indicate that SFW events are driven by planetary wave anomaly patterns observed in the extratropical lower troposphere a few days prior to SFW. Eliassen-Palm flux diagnoses confirm the presence of an anomalous upward flux of Rossby wave activity into the stratosphere during this time, which acts to decelerate the stratospheric polar vortex and precipitate the final warming. Our study indicates that polar vortex breakdown is distinguished by a robust large-scale dynamical coupling of the troposphere and stratosphere. Although this coupling is strongest in the Northern Hemisphere, parallel dynamical signatures are also observed in the Southern Hemisphere circulation.