Impact of Hunga Tonga-Hunga Ha’apai water vapour on polar vortex dehydration and ozone depletion: Antarctic 2023 and Arctic 2024

The January 2022 eruption of Hunga Tonga-Hunga Ha’apai (HTHH) injected a huge amount (~150 Tg) of water vapour (H2O) into the stratosphere, along with small amount of sulfur dioxide (SO2). Following slow transport in the meridional Brewer-Dobson circulation, the additional H2O is now distributed throughout the stratosphere. Here we use an off-line 3-D chemical transport model (CTM) to study the residence time of this excess H2O and its impact on polar ozone depletion. The model results are compared to satellite data from the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), and Infrared Atmospheric Sounding Interferometer (IASI), and to balloon-borne measurements from Scott Base (77.8oS). Simulations with the TOMCAT/SLIMCAT CTM successfully reproduce the spread of the injected H2O through late 2023 (at time of writing) as observed by MLS. Dehydration in the 2023 Antarctic polar vortex caused the first substantial (~20 Tg) removal of HTHH H2O from the stratosphere. The CTM indicates that this process will dominate removal of HTHH H2O for the coming years, giving an overall e-folding timescale of 4 years; around 25 Tg of the injected H2O is predicted to still remain in the stratosphere by 2030. We have diagnosed the additional H2O chemical impacts on stratospheric ozone throughout the simulation, with a focus on the 2023 Antarctic ozone hole. Following relatively low Antarctic column ozone in midwinter 2023 due to transport effects, additional springtime depletion due to H2O-related chemistry was small and maximised at the vortex edge (10 DU in column). Effective dehydration in the core of the vortex limited the impact of the additional H2O. We will also discuss the HTHH-H2O impacts on ozone depletion in the forthcoming 2024 springtime Arctic vortex. This will be the first Arctic winter with likely substantial HTHH enhancement of lower stratospheric H2O. As dehydration is rare in the Arctic, there is the possibility of differing impacts compared to the Antarctic through the persistence of the enhanced H2O at the pole.