Regional structure of the polar vortex via planetary wave phase diagnostics and its link to cold air outbreaks npj Climate and Atmospheric Science

Extreme winter cold-air outbreaks (CAOs) impose substantial societal and economic impacts across the Northern Hemisphere, yet the stratospheric processes that modulate these events-and shape their regional expression-remain incompletely understood. Previous studies relied on cluster-based classifications of polar vortex states, which discretize continuous variability and often emphasize North American impacts. Here, we introduce a, phase-based diagnostic framework that characterizes the evolving geometry of the lower stratospheric polar vortex (LSPV) using amplitude, percentage of longitudinal spatial variance explained by wave-1 and wave-2 components, and their longitudinal phase in 100 hPa geopotential height anomalies. This approach distinguishes displacement (wave-1) and stretching (wave-2) configurations and identifies four recurrent trough orientations over North America, Atlantic, Asia, and Far East. Composite analysis for January-February 1979-2024 shows that these phase-locked configurations are frequently associated with vertically coherent anomaly patterns between 100 hPa and 500 hPa and with distinct surface temperature anomalies. A transition-probability matrix indicates that LSPV trough configurations often reorganize between preferred longitudinal sectors than propagate gradually in longitude. These results show that wave-1 (displacement) and wave-2 (stretching) geometries represent recurrent modes of LSPV variability during January-February, with sector-dependent surface temperature anomalies, providing a framework for understanding the regional structure of CAO-related midlatitude winter circulation patterns. Grant no. NA20OAR4320271