Examining the Influence of Stratospheric Polar Vortex Variability and Tropopause Polar Vortices on Rossby Wavebreaking Regimes

Variability in the high-latitude wintertime stratosphere is known to influence the tropospheric flow and Rossby wave dynamics on subseasonal-to-seasonal (S2S) timescales. This analysis focuses on these impacts to stratospheric variability on the dynamic tropopause region. The analysis first considers the influence of stratospheric variability on the high-latitude tropopause features known tropopause polar vortices (TPVs). TPVs are long-lived, (i.e., weeks), tropopause-based cyclonic features are known to initiate cyclogenesis and influence the extratropical jet stream. Given the S2S timescales of both the TPV lifespans and high-latitude stratospheric variability, the interaction between the two phenomena is hypothesized to represent a potential mechanism for shifts in Rossby wavebreaking regimes.

The second part of the analysis explores and considers the mechanisms by which the anomalous stratospheric flow influences shifts in Rossby wavebreaking regimes, through the consideration of the associated variability in TPVs. To quantify this relationship, a robust TPV tracking algorithm was applied to ERA-Interim wind and temperature data between 1979-2017. During the same period, the strength of the winter-time stratospheric polar vortex was defined by the daily and zonally-averaged 10-hPa zonal winds at 60N. TPVs were then categorized as occurring during extremely strong (>90% percentile), extremely weak (<10% percentile) and neutral (45–55% percentile) stratospheric polar vortex conditions. A Rossby wavebreaking detection algorithm is employed to establish how variability in stratospheric flow and TPVs are connected to anomalous wavebreaking conditions near the tropopause. This algorithm detects potential vorticity streamers, overturns, and cutoffs, and can track the progression of wavebreaking through time and space. Using this tool, we will be able to establish how variations in stratospheric flow promote shifts in the type, location, and frequency of Rossby wavebreaking regimes, and understand how TPVs may form from or promote the occurrence of Rossby wavebreaking.