Morphological Classification of the Lower Stratospheric Polar Vortex Edge as a Finite-Width Potential Vorticity Band

Abstract. The stratospheric polar vortex (SPV) is a dominant feature of the winter stratosphere, characterized by circumpolar westerly winds. Its lowermost edge is a key region of troposphere–stratosphere coupling. Shaped by Rossby wave breaking (RWB), the morphology of the lower-SPV edge reflects changes in jet-stream orientation, storm-track activity, and atmospheric blocking, and is linked to surface weather and the upward propagation of wave activity. The nonlinear potential-vorticity (PV) dynamics in this region motivate approaches that capture its evolving structure. However, existing classifications typically represent the SPV edge as a pointwise boundary, overlooking the gradual transition between the stratosphere troposphere suggested by tracer observations. Here, we treat the lower-SPV edge as a finite-width boundary region whose width and morphology define distinct circulation regimes. We develop a daily, year-round classification based on recurring configurations of a lower-stratospheric PV band, defined by the area between two PV contours. We focus on the 6–7 PVU band on the 350-K isentropic surface, which remains in the stratosphere year-round, closely follows the maximum PV gradient and subpolar jet, and provides a measure of jet intensity through its width. A self-organizing map (SOM) identifies 12 regimes. These regimes span configurations associated with regional blocking and hemispheric annular modes, including variations in SPV core strength and edge sharpness. Each regime exhibit distinct lower-stratospheric PV anomalies, surface temperature and pressure patterns, and tropospheric heat, momentum, and transient-wave activity fluxes. Transition and persistence statistics reveal preferred pathways toward SPV weakening, with Scandinavian, North Pacific, and Greenland blocking emerging as prominent precursors of negative PV anomalies in the lower-SPV core. Changes in 10-hPa wind speed across regime transitions further reveal systematic SPV preconditioning and responses. Finally, anomalous regime occupancies before and after sudden stratospheric warmings (SSWs) highlight the potential of this framework for understanding and predicting SSW events.