The January 2022 eruption of Hunga Tonga - Hunga Ha’apai (Hunga eruption) injected a huge amount of water vapour (150 Tg) into the stratosphere, unprecedented in the satellite era. The eruption offers a unique opportunity to test our understanding of how a large-scale increase in water vapour impacts polar ozone, and how the vortex structure influences the timing and magnitude of the effects.
We have used an off-line 3-D chemical transport model (CTM) to study the impact of this enhanced H₂O on the ozone layer in 2022/23, 2023/24 and over the next decade. The TOMCAT/SLIMCAT CTM was run at a horizontal resolution of 2.8^o × 2.8^o and 32 levels from the surface to about 60 km forced with ECMWF ERA5 meteorology. A control simulation was integrated from 1980 to January 2025 (at time of writing). The run is extended until 2030 using repeating meteorology. Various Hunga-H₂O experiments were performed from 2022 onwards with the injection of 150 Tg of H₂O into the low-mid stratosphere at southern subtropical latitudes. The differences between the HTHH and control simulations diagnoses the direct chemical impact of the increased H₂O (and aerosol) on stratospheric ozone in the absence of dynamical feedbacks but with realistic post-eruption meteorology. The increased water vapour impacts the model through both gas-phase and heterogeneous chemistry (e.g. polar stratospheric clouds and aerosols). These differences therefore provide useful constraints on changes seen in more complex coupled radiative-dynamical-chemical models. The model results are compared to satellite data from the Microwave Limb Sounder (MLS) and Infrared Atmospheric Sounding Interferometer (IASI), and to balloon-borne measurements from Antarctic.
Once injected into the stratosphere, the excess H₂O is removed in a step-like fashion during austral spring starting in 2023 suggesting a role for polar dehydration. At this point the modelled half-life for the removal of the excess H₂O is around 3 years. Around 25 Tg of excess H₂O still remains in the model by 2030.
We diagnosed the impact of the HTHH injection on stratospheric ozone during throughout the simulation. The impact on the southern hemisphere (SH) polar lower stratosphere in 2022 was limited in the model by penetration of H₂O into the polar vortex; the model suggests an additional local depletion of up to 5% due to H₂O outside the vortex in October and less inside. The aerosol impact is larger but more uncertain. The injected H₂O spreads through the stratosphere in 2023 via the Brewer-Dobson circulation. For October 2023 the model predicts a much bigger impact of H₂O inside the SH polar vortex – additional local depletion of around 20%. By this time there is also a global reduction of ozone in the upper stratopause of around 4% due to HOx chemistry.
The presentation will give an up-to-date status of observations and modeling of the Hunga impacts on climate and chemistry and discuss if these predictions are being realised.

Explosive volcanic eruption,stratospheric water vapour,ozone depletion