Was the strengthening of the Northern polar vortex in 2024/2025 associated with the Hunga Tonga eruption?

Abstract. 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 (up to 2 ppmv above climatology) descending from the mesosphere into the polar stratosphere, collocated with ozone reductions and persistent cold anomalies of about 5–15 K, alongside a record-strong SPV. Our model can reproduce the main structure of the descending plume, its effects on chemistry, and the SPV strengthening, albeit with underestimated amplitudes 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, sharpening meridional temperature gradients and thereby intensifying the vortex. The observed 2024/2025 winter thus represents a plausible manifestation of HT-induced vortex variability, with simulations also indicating a shift toward sudden stratospheric warmings in the winter 2025/2026 as the WV forcing declines.