Evidence for Hunga-Tonga eruption influence on the anomalously strong Northern-Hemisphere stratospheric polar vortex in 2024/2025

The 2022 Hunga Tonga (HT) eruption injected an unprecedented amount of water vapour (WV) into the stratosphere and mesosphere, emerging as a potential multi-year driver of variability in those layers and associated climate feedbacks. Using satellite and reanalysis datasets, and ensemble simulations with the SOCOLv4 model, with and without the HT forcing, we diagnose the chain of processes linking the eruption to the exceptionally strong Northern Hemisphere stratospheric polar vortex (SPV) observed in winter 2024/2025. Satellite data show “tongues” of enhanced WV descending from the mesosphere into the polar stratosphere, collocated with ozone reductions and persistent cold anomalies, alongside a record-strong SPV. Our model with the HT forcing can reproduce the main structure of the descending plume, its effects on chemistry, and the SPV strengthening, albeit with underestimated magnitudes and an earlier onset than observed. Offline radiative transfer calculations indicate that the WV and ozone anomalies drive net radiative cooling over the polar stratosphere, which steepens meridional temperature gradients and thereby intensifies the Arctic polar vortex. The observed 2024/2025 winter thus represents a plausible manifestation of HT-induced vortex variability. Our simulations also predicted the observed shift towards more favorable conditions for sudden stratospheric warmings in the winter 2025/2026 as the WV forcing declines.