It is now well recognized that the Northern Hemisphere extratropical climate is distinguished by prominent two-way dynamical coupling between the stratosphere and troposphere during winter. This coupling is synoptically manifested by intraseasonal variability in the Northern Annular Mode (NAM). At upper levels the NAM modulates the strength of the stratospheric polar vortex while at tropospheric levels the NAM projects strongly upon the Arctic and North Atlantic Oscillations (AO and NAO, respectively). Recent statistical analyses show that lower stratospheric NAM variability can, in fact, provide a useful predictor of subsequent tropospheric NAM variability. Further, recent observational studies have discovered a strong link between interannual climate variability during spring onset and concomitant NAO variability. We study the hypothesis that regional tropospheric climate variability during spring onset is influenced by interannual variations in the breakup of the stratospheric polar vortex.

Stratospheric final warming (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 composited with respect to SFW dates to identify coherent anomaly structures linked to SFW events. In the high latitude stratosphere distinct westerly (easterly) zonal wind anomalies are observed during the 2 weeks prior to (following) SFW, indicating a rapid breakdown of the stratospheric polar vortex (compared to the climatological trend). The high latitude stratospheric decelerations are accompanied by opposing zonal wind accelerations in the subtropical stratosphere and both of these stratospheric zonal wind tendency features have coherent downward extensions into the troposphere. The tropospheric circulation is further marked by persistent positive (negative) episodes of the NAO in the weeks prior to (following) SFW events. The composite analyses indicate that SFW events are driven by precursor 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 composite results indicate that SFW events exhibit a coupled evolution with distinct stratospheric and tropospheric precursors. In particular, SFW events are directly linked to subsequent tropospheric NAO climate variability. We suggest that SFW events provide an important new phenomenological paradigm for studying both (a) stratosphere-troposphere dynamical coupling (b) the dynamics and predictability of Spring onset in the Northern Hemisphere