A Impacts of Subseasonal-to-Seasonal Stratospheric Variability on Tropopause Polar Vortices

Abstract:

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 to 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 pathway for high-latitude variability on S2S timescales.

First, a climatological analysis of TPVs is presented in the context of stratospheric variability using data from the ERA-Interim Reanalysis. Extreme stratospheric conditions are quantified using the 60˚N zonal-mean zonal winds at 10 hPa and the tracks and amplitude of TPVs are identified using a TPV tracking algorithm applied to the dynamic tropopause represented by the 2-PVU isosurface. In examining strong and weak stratospheric vortex states, there are significant differences in TPV track frequency and amplitude over high-latitude North America and the North Atlantic. Next, the climatological relationships are examined in the context of a case study of the tropospheric impacts of the 2018 sudden stratospheric warming event. The role of TPVs and their associated weather is discussed in the context of facilitating the downward communication of the stratospheric flow anomalies into the troposphere on S2S timescales and the ability of S2S forecast models to capture the relationships.