Water vapor impacts from the 2022 Hunga eruption on the Arctic stratospheric polar vortex and surface temperatures

The January 2022 Hunga Tonga–Hunga Ha’apai eruption caused an unprecedented injection of water vapor into the stratosphere. The excess water vapor from this event stayed in the stratosphere for several years, but whether it influenced surface climate conditions remains unclear. Here, we aim to investigate the impacts of the anomalous water vapor on the variability of the Arctic stratospheric polar vortex and its downward influence on extratropical surface climate. Using the coupled high-top Community Earth System Model Version 2 (CESM2/WACCM6), we conduct a 12-member ensemble of 3-year-long idealized water vapor perturbation simulations that mimic the eruption. Our ensemble simulations demonstrate that water vapor-induced upper-stratospheric cooling weakens the Arctic stratospheric polar vortex in the first post-eruption winter of 2022/2023, with a weaker influence in the second post-eruption winter. The weakening of the polar vortex is driven by the reduced equator-to-pole temperature gradient in the winter stratosphere and is accompanied by pronounced polar stratospheric warming episodes that propagate into the troposphere. We identify more frequent occurrences of the negative Arctic Oscillation and colder-than-normal winters over the northern Eurasian continent in individual perturbation simulations. Our simulations suggest that the Hunga water vapor forcing increases the frequency of a weakened Arctic stratospheric polar vortex and slightly increases the chance for Eurasian winter cooling, although with a weak signal-to-noise ratio.