The Dynamics of Northern Hemisphere Stratospheric Final Warming Events

Abstract

A lag composite analysis is performed of the zonal-mean structure and dynamics of Northern Hemisphere stratospheric final warming (SFW) events. SFW events are linked to distinct zonal wind deceleration signatures in the stratosphere and troposphere. The period of strongest stratospheric decelerations (SD) is marked by a concomitant reduction in the high-latitude tropospheric westerlies. However, a subsequent period of tropospheric decelerations (TD) occurs while the stratospheric circulation relaxes toward climatological conditions. During SFW onset, a wavenumber-1 disturbance at stratospheric altitudes evolves into a circumpolar anticyclonic circulation anomaly. Transformed Eulerian-mean dynamical diagnoses reveal that the SD period is characterized by an anomalous upward Eliassen–Palm (EP) signature at high latitudes extending from the surface to the middle stratosphere. The associated wave-driving pattern consists of zonal decelerations extending from the upper troposphere to the midstratosphere. Piecewise potential vorticity tendency analyses further indicate that zonal wind decelerations in the lower and middle troposphere result, at least in part, from the direct response to latitudinal redistributions of potential vorticity occurring in the lower stratosphere. The TD period exhibits a distinct dynamical behavior with anomalous downward EP fluxes in the high-latitude stratosphere as the zero zonal wind line descends toward the tropopause. This simultaneously allows the stratospheric polar vortex to radiatively recover while providing anomalous upper-tropospheric zonal decelerations (as tropospheric Rossby wave activity is vertically trapped in the high-latitude troposphere). The tropospheric decelerations that occur during the TD period are regarded as a subsequent indirect consequence of SFW events.