Simulated modulation of stratospheric polar vortices by the Hunga Tonga-Hunga Ha’apai eruption

The submarine eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) in January 2022 represents a novel geophysical event due to the injection of large amounts of water vapor (WV) into the stratosphere. Following the eruption, the injected WV was transported from the tropics to higher latitudes via stratospheric circulation. Approximately one year after the eruption, the WV anomalies were spread throughout the global stratosphere, including both polar regions.

Previous studies have shown that the excess stratospheric WV was associated with significant anomalies in atmospheric circulation, particularly a weakening of the Northern Hemisphere (NH) stratospheric polar vortex (SPV). However, the observed 2024/2025 winter with an exceptionally strong NH SPV may represent a plausible manifestation of HTHH-induced modulation of vortex variability.

Here we diagnose the chain of processes linking the HTHH eruption to the exceptional behavior of the SPV observed in recent years using satellite observations, reanalyses data, and ensemble model simulations with the SOCOLv4 Earth system model, as well as model data from the HTHH Impact Model Observation Comparison project.

Our preliminary multi-model results show a seasonally recurring transport of WV in the stratosphere and lower mesosphere, accompanied by changes in composition, radiation, and dynamics. We propose mechanisms whereby excess WV from HTHH and associated ozone changes induce radiative perturbations that precondition the SPV. Furthermore, we examine how the underlying mechanisms depend on the model-projected WV forcing and how this relates to known biases of chemistry-climate models.