PV-Based Classification of the Polar Vortex Edge: Variability and Impacts of Rossby Wave Breaking

The stratospheric polar vortex (SPV) is a dominant feature of the winter stratosphere, characterized by circumpolar westerly winds. In the mid-latitudes, the SPV edge separates the lower stratosphere from the upper troposphere and is primarily deformed by tropospheric Rossby wave breaking (RWB). The geometry of the SPV edge strongly influences jet stream orientation, storm activity, blocking patterns, and surface weather. Moreover, the SPV’s response to climate change remains a major source of inter-model spread in CMIP6 simulations.

The inconsistent responses of the SPV across CMIP6 models are linked to the large internal variability of nonlinear PV dynamics, highlighting the importance of daily, synoptic-scale patterns. Previous classification efforts of the SPV edge have typically been monthly and often rely on zonal-mean diagnostics, limiting their ability to capture synoptic-scale variability.

To address this, we develop a novel framework for daily, year-round classification of the lower SPV edge, focusing on recurring RWB patterns that dominate SPV variability. Our analysis targets the 6–7 PVU band on the 350 K isentropic surface, a region that is (i) strictly stratospheric year-round, avoiding topographic intersections, (ii) closely aligned with the maximum PV gradient approach and the sub-polar jet stream, (iii) materially conserved in the absence of diabatic processes, and (iv) indicative of jet intensity through PV-band width. Using a self-organizing map (SOM) approach, we identify 12 leading regimes of the lower SPV edge.

Applied to ERA5 reanalysis data (1979–2024), the PV-band classification captures meaningful PV configurations and anomalies across the 350 K surface, consistent with well-established weather-scale patterns (e.g., Greenland and Scandinavian blocking) and SPV modes (stronger/weaker SPV and SPV edge). These modes exhibit immediate impacts both downward to the surface and upward into the stratosphere. This framework provides a powerful tool for evaluating inter-model spread in climate simulations and analyzing SPV responses to climate change, by quantifying deviations from observed SPV-edge configurations and their temporal characteristics.