The evolution of stratospheric water vapour in the years since the Hunga Tonga eruption

The Hunga Tonga-Hunga Ha'apai underwater volcanic eruption in January 2022 injected 150 Tg of water vapour (H2O) into the stratosphere, increasing the total stratospheric H2O mass by 10%. The goal of this study is to investigate the transport of the Hunga H2O within, and out of, the stratosphere in the four years since the eruption, using H2O observations from the Microwave Limb Sounder (MLS) and model simulations from WACCM and 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 residual H2O shows that the Hunga water vapor has moved to higher latitudes and lower altitudes over time. There is excess H2O in the tropics in 2023 and 2024, providing evidence of mixing from mid-latitude back to the tropics. As of mid-2025, approximately half of the 150 Tg of H2O that was injected by Hunga has either been removed or transported to the lowermost stratosphere.