Northern Hemisphere stratospheric polar vortex morphology under localized gravity wave forcing: a shape-based classification

Abstract. The Northern Hemisphere stratospheric polar vortex (SPV) response to localized gravity wave (GW) forcing remains poorly understood, particularly in terms of its detailed morphology. Here, we investigated geometry-specific impacts of enhanced orographic GW drag in three hotspot regions, the Himalayas, Northwest America, and East Asia, using ensemble simulations with the high-top UA-ICON global circulation model. By classifying daily SPV geometries into ten distinct clusters with a novel unsupervised, shape-based hierarchical clustering framework, we isolated geometry-specific responses using the class contribution method. Our results show that all hotspot forcings consistently reduce planetary wave 1 (PW1) amplitude and induce a PW1-like displacement of the SPV core, though spatial patterns vary with hotspot location. This response manifests as negative geopotential height (GPH) anomalies within the forced region and positive anomalies to the north, indicating localized SPV edge mixing. The response is also sensitive to the forcing’s latitudinal position: the Himalayas, as the southernmost hotspot, produces a deepened vortex, while the more poleward Northwest America and East Asia forcings show similar patterns with greater intrusion of positive GPH anomalies into the vortex core. The forcing reduces PW1 amplitude both by shifting the frequency of specific clusters and altering the mean structure of the most frequent classes. Our results demonstrate that shape-based clustering combined with the class contribution framework can reveal robust, spatially coherent signals that might otherwise be masked by internal variability, providing a new perspective for understanding SPV variability and its predictability.