Long-term evolution of stratospheric water vapor from the Hunga eruption

Abstract. The Hunga underwater volcanic eruption in January 2022 injected 150 Tg of water vapour (H₂O) into the stratosphere, increasing the total stratospheric H₂O mass by 10 %. This study investigates the transport of the Hunga H₂O within, and out of, the stratosphere from 2022–2025, using H₂O observations from the Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS), along with model simulations from the Whole Atmosphere Community Climate Model (WACCM) and the FLEXible PARTicle dispersion model (FLEXPART). The Hunga H₂O is isolated by using the tropical cold point temperature to account for H₂O that entered the stratosphere through the tropical tropopause, rather than via the eruption. The resulting residuals show the detailed evolution of the Hunga H₂O over time while it moves to higher latitudes and lower altitudes. Including the lower stratosphere, approximately two-thirds of the Hunga H₂O remains in the stratosphere in late 2025. There is good agreement between the observed and modelled H₂O above 20 km. However, in the lower stratosphere observations show substantially more H₂O compared to the model during 2024 and 2025, suggesting that the modelled transport across the lower stratosphere is too fast. Observations show excess H₂O in the tropics in 2023 and 2024, suggesting evidence of recirculation from SH mid-latitudes back to the tropics. This mixing signature is found in WACCM simulations and confirmed in FLEXPART transport calculations. Including the lower stratosphere in the calculation increases the decay time scale of the Hunga stratospheric water vapor by 1–2 years.