Abstract
Abstract. Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the “final stratospheric warming” (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 vs. wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. The timing and classification of the FSW are sensitive to which pressure level the FSW central date is defined, particularly in the Southern Hemisphere (SH) where trends in the FSW dates associated with ozone depletion and recovery are more evident at 50 than 10 hPa. However, regardless of which FSW definition is selected, we find the wave geometry of the FSW affects total column ozone anomalies in both hemispheres and tropospheric circulation over North America. In the Southern Hemisphere, the timing of the FSW is strongly linked to both total column ozone before the event and the tropospheric circulation after the event.
Highlights
The polar stratosphere exhibits a distinct seasonal cycle featuring a wintertime polar vortex, that is, strong circumpolar westerly winds that form in late summer and decay the following spring, which is due to the seasonal cycle of incoming solar radiation
A corresponding figure for the 50 hPa final stratospheric warming (FSW) dates is shown in the Appendix (Fig. A1), but we found in both hemispheres that the differences in total column ozone (TCO) anomalies tied to wave geometry were more apparent for 10 hPa FSW dates
Late events show more variability than early events. Both sudden stratospheric warming events in the middle of winter and final stratospheric warming events that mark the end of winter in the stratosphere are characterized by a similar evolution and are often classified by the same metrics, i.e., when the zonal-mean zonal winds of the polar vortex fall below some threshold
Summary
The polar stratosphere exhibits a distinct seasonal cycle featuring a wintertime polar vortex, that is, strong circumpolar westerly winds that form in late summer and decay the following spring, which is due to the seasonal cycle of incoming solar radiation. SSW events have been classified according to a range of characteristics (Butler et al, 2015), notably with respect to the zonal wavenumber dominating the polar stratosphere at the time of or just prior to the event (Bancalá et al, 2012; Barriopedro and Calvo, 2014) or according to vortex elliptical moment diagnostics (Waugh, 1997; Charlton and Polvani, 2007; Mitchell et al, 2011; Seviour et al, 2013), that is, whether the vortex splits into two vortices or displaces off the pole. This study explores the classification of FSW events by wave geometry (Sect. 2), the connections between wave geometry and dynamical behavior in the stratosphere (Sect. 3), ozone distribution (Sect. 4), and surface impacts (Sect. 5)
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