Examining the Influence of Stratospheric Polar Vortex Variability on Rossby Wave Breaking Regimes

Variability in the high-latitude wintertime stratospheric circulation is known to promote the likelihood of extreme cold spells across the northern hemisphere and shifts in storm tracks on subseasonal-to-seasonal timescales. While multiple theories exist to explain the downward influence of stratospheric variability on the troposphere, these impacts are not fully understood. To help address the research gap in explaining the pathway for the impacts of stratospheric variability on surface weather, we investigate the response of Rossby wave breaking events (RWBs) to stratospheric variability. RWBs occur at the final stages of baroclinic lifecycles, often promoting large blocking anticyclones or cyclogenesis further downstream, both of which can lead to extreme temperature and precipitation anomalies. Additionally, previous studies have explored the role of lower stratospheric shear on baroclinic instability and note the sensitivity of baroclinic lifecycles to lower stratospheric conditions. We aim to expand upon this work with a modern Rossby wave breaking detection algorithm and reanalysis dataset to explore the zonally-asymmetric responses of stratospheric variability on baroclinic lifecycles.

The Rossby wave breaking detection algorithm developed by Kaderli (2023) traces the 2 potential vorticity unit (or 2 PVU) contours, representative of the dynamic tropopause, on isentropic surfaces. Then, regions with horizontally overturning 2 PVU contours are identified as wave breaking events, which are further classified into streamers, overturns, and cutoffs based on the shape and size of each wave break. Each wave breaking event can be tracked through time and space, including the sign and magnitude of momentum flux, size of the wave break, and the orientation (i.e. anticyclonic or cyclonic) of the wave break. Using this tool, we establish how variations in stratospheric flow promote shifts in the type, location, and frequency of Rossby wave breaking regimes. We employ ERA-I reanalysis data to identify RWB events and identify the strength of the wintertime stratospheric polar vortex. The strength of stratospheric polar vortex is denoted by percentiles of the daily and zonally-averaged 10-hPa zonal winds at 60oN, classified into extremely strong (>90% percentile), extremely weak (<10% percentile) and neutral (45–55% percentile) conditions. The distributions of each Rossby wave breaking type shift substantially in the entrance and exit regions to the Atlantic and Pacific storm track regions in each stratospheric regime.