The Impacts of Stratospheric Variability on Tropopause Polar Vortices: Case Studies of the 2018 Sudden Stratospheric Warming Event and the 2020 Strong Vortex Event

Variability in the high-latitude stratospheric flow can produce anomalous conditions near the tropopause that can persist on subseasonal-to-seasonal (S2S) timescales. These anomalous conditions can impact the evolution and dynamics of weather in the troposphere. This analysis considers the mechanisms by which the anomalous stratospheric flow impacts the weather in the troposphere by considering the duration, track, and amplitude of tropopause polar vortices (TPVs). TPVs are long lived, (i.e., weeks), tropopause-based cyclonic features that exist within the high-latitude regions and are known not only to be precursors to Arctic cyclones, but also can interact with the midlatitude jet stream resulting in high-impact weather. Given the S2S timescales of both the TPV lifespan and high-latitude stratospheric variability, the interaction between the two phenomena is hypothesized to represent a potential mechanism for high-latitude variability on S2S timescales.

The analysis focuses on examining the subsets of TPV tracks that occurred in the periods after the February 2018 sudden stratospheric warming event (SSW; a weak vortex event) and the 2020 strong vortex event. Both types of events are known to modulate tropospheric flow on sub-seasonal timescales though the mechanisms of the modulation are a subject of much research attention. This analysis uses ERA-Interim reanalysis data to diagnose the impacts of both weak and strong vortex events on tracks and magnitudes of TPVs, where the TPV tracks and magnitudes are identified using an established TPV tracking algorithm. Both weak and strong vortex events are associated with anomalous wave activity flux in the tropopause region. Since a TPVs lifespan is highly correlated with near-tropopause radiative (not dynamic) forcing, it is hypothesized that changes in the dynamical wave forcing in a region associated with the strong or weak vortex event can make a region more or less favorable for TPV development and maintenance. A daily climatology of wave activity flux (WAF) created for the period of 1979-2018 is used to highlight anomalous regions of WAF relative to changes in TPV frequency and amplitude. It is hypothesized that in regions of anomalous wave activity flux divergence, TPVs are removed from dynamical influences, promoting favorable conditions for intensification of TPVs via radiative/diabatic processes. Conversely, TPVs within regions of anomalous WAF convergence are more likely to influence local dynamical processes, leading to tropospheric jet alterations and a destruction of TPVs. This analysis presents a mechanism in which TPVs are involved in the observed tropospheric flow changes following periods of anomalous stratospheric conditions.