Residence time of Hunga stratospheric water vapour perturbation quantified at 9 years

The January 2022 eruption of the Hunga volcano (20°S) injected 150 Tg of water vapour into the middle atmosphere, leading to an increase in the stratospheric water burden of 10%, unprecedented in the observational record. In the first two years post-eruption, the stratospheric burden hardly changed (Millán et al., 2024), except for a small decay due to Antarctic polar stratospheric cloud dehydration in 2023 (Zhou et al., 2024), leaving the residence time of volcanically injected water vapour—a key control on its climate impact—uncertain. Here we use satellite observations from the Microwave Limb Sounder (MLS) and an off-line 3-D chemical transport model (CTM), TOMCAT/SLIMCAT, with ERA5 meteorology to study the residence time of this excess H₂O.

Using MLS observations, we show a substantial decline from 2024 to early 2025, the largest drop since the eruption. Simulations with the TOMCAT/SLIMCAT CTM reproduce the observed global spread and decline of the injected H₂O through early 2025. Together, observations and model simulations indicate that the long-term removal of the Hunga water has now entered a new phase, with stratosphere-troposphere exchange playing an increasingly important role, exceeding Antarctic dehydration in 2024. We estimate that the additional stratospheric water vapour is now decaying steadily with an e-folding time of 3 years and will reach the observed pre-Hunga range of variability around 2030.

The presentation will provide an up-to-date status of observations and discuss whether the decay of the Hunga excess water is proceeding as expected.