Connections between Tropopause Polar Vortices and Severe Weather

Summary

Although links between tropopause polar vortices (TPVs), a distinctly Arctic phenomenon, and severe weather outbreaks in the United States may seem improbable at first, there are several physical reasons to expect such a connection. As positive potential vorticity anomalies, TPVs are surrounded by a closed, cyclonic circulation. In a sense, this circulation can be thought of as an "Arctic" jet, complementing the well-known polar and subtropical jets. So, when TPVs approach the polar jet (which happens often and predictably!), there is a potential for a jet merger event, producing a local wind maximum known as a jet streak. Jet streaks are known to impact severe weather events by generating large-scale ascent from ageostrophic circulations and enhancing vertical wind shear. Moreover, since TPVs represent an anomalously cold, dry region of the upper atmosphere, their presence can help create unstable thermodynamic profiles when paired with warm, moist air at the surface.

To investigate whether these theoretical connections are present in the real world, I conducted an analysis of major severe weather outbreaks from three different regions of the United States (the southern Great Plains, the deep south, and the Midwest). A closed PV anomaly-jet streak couplet was present in around two-thirds of these outbreaks, with around 50% of cases featuring a TPV that could be tracked back to the Arctic. The presence of these vortices and jet streaks was shown to be associated with significant enhancements of shear, instability, and quasigeostrophic ascent over the outbreak regions. Importantly, these TPVs followed a consistent pathway from the Arctic, through the north Pacific, and into the US, providing an opportunity to create long-term outlooks of enhanced severe weather potential.

Publications

Bray, M. T., S. M. Cavallo, and H. B. Bluestein, 2021: Examining the Relationship between Tropopause Polar Vortices and Tornado Outbreaks. Wea. Forecasting, 36, 1799-1814, https://doi.org/10.1175/WAF-D-21-0058.1.

Conference Presentations

Bray, M. T., S. M. Cavallo, and H. B. Bluestein, 2022: "Examining the Relationship between Tropopause Polar Vortices and Tornado Outbreaks." 102nd American Meteorological Society Annual Meeting; 31st Conference on Weather Analysis and Forecasting (WAF)/27th Conference on Numerical Weather Prediction (NWP), Virtual. Oral Presentation.