Sudden stratospheric warmings and anomalous upward wave activity flux

Abrupt transitions in the polar winter stratospheric circulation such as sudden stratospheric warmings (SSWs) have profound impacts on stratospheric transport and surface weather. SSW-like events are a manifestation of strong two-way interactions between upward propagating planetary waves and the mean flow. Here we use reanalysis data to define events of extreme stratospheric mean flow deceleration (with SSWs being a subset) as well as events of extreme lower tropospheric upward planetary wave activity flux. We show that while the wave fluxes leading to SSW-like events ultimately originate near the surface, the anomalous upward wave activity fluxes associated with these events primarily originate within the lowermost stratosphere. Furthermore, the enhanced wave fluxes and mean flow decelerations appear essentially synchronously - that is, the mean flow decelerations leading to SSW-like events are as much causing enhanced upward wave activity fluxes as they are caused by these enhanced upward wave activity fluxes. SSW-like events therefore appear to be primarily forced by wave fluxes that are constructively redistributed or generated at or above the tropopause, and appear to be forced much less by anomalous wave fluxes from the troposphere below.