Spring onset in the Northern Hemisphere: A role for the stratosphere?

The annual transition from winter to spring (spring onset) and its interannual variability are key elements of Northern Hemisphere climate affecting the hydrological cycle, vegetative growing season, and ecosystem productivity. Interannual variability in spring onset is linked to concomitant variability in the Arctic Oscillation (AO) or closely related North Atlantic Oscillation (NAO). It is also now well recognized that a robust two-way dynamical coupling between the stratosphere and troposphere is observed in the boreal extratropics during winter in association with intraseasonal variability in the Northern Annular Mode (NAM). During winter the NAM jointly influences (a) the polar vortex strength and (b) the sign and magnitude of the AO/NAO. 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 then composited with respect to SFW dates to identify anomaly structures linked to SFW events.

SFWs are found to provide a strong organizing influence on the large-scale circulation of the stratosphere and troposphere, acting to sharpen spring onset compared to the climatological seasonal trend. A coherent pattern of significant westerly (easterly) zonal wind anomalies is observed to extend from the stratosphere to the Earth's surface at high latitudes in the two weeks prior to (after) SFWs, indicating rapid breakdowns in both the stratospheric and tropospheric westerly jets. The high latitude stratospheric decelerations are accompanied by opposing zonal wind accelerations in the subtropical stratosphere with 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 study indicates that spring onset is (a) typically more rapid than suggested by the climatological trend and (b) distinguished by a robust large-scale dynamical coupling of the troposphere and stratosphere. We suggest that an improved understanding and simulation of this coupled evolution will likely lead to enhancements in medium-range forecasts of spring onset.