The impact of the 2022 Hunga water-rich eruption on polar stratospheric clouds, chlorine activation and ozone depletion

The Hunga eruption, January 2022, injected ~150 Tg of water vapour into the sub-tropical mid-stratosphere, unprecedented in the satellite era. The effects of the Hunga water vapour on the heterogeneous chemistry in the Antarctic stratosphere did not occur in the 2022 season, with the vortex edge a barrier to transport to high southern latitudes, until vortex break-up. Here, we analyse chlorine activation and ozone loss impacts starting with the 2023 vortex, and find these effects were not uniform throughout the vortex, and were limited by widespread mid-winter dehydration that occurs in the Antarctic each year, mainly in the colder “vortex core” region, via ice-containing polar stratospheric clouds (PSCs).  

Heterogeneous, chlorine-activating reactions on PSC surfaces play a key role in polar stratospheric ozone depletion and the formation of the seasonal Antarctic ozone hole. Stratospheric water vapour is one of the key factors in PSC formation; thus, the water vapour enhancement from Hunga is likely to impact PSC occurrence and therefore may increase the ozone depletion. However, the effects may vary between different regions of the vortex. The core region experiences the lowest temperatures, frequently reaching the threshold for ice PSCs, and the most extensive dehydration (in the lower stratosphere) over much of the vortex season. In contrast, the edge region is more sunlit, less cold, and experiences less extensive dehydration than the core, meaning there is scope for different chemical impacts in this region. Overall, the Hunga 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.

Here we use the TOMCAT three-dimensional chemical transport model to investigate the impacts of the Hunga water vapour on Antarctic stratospheric ozone and associated heterogeneous chlorine reactions, comparing the 2023 and 2024 Antarctic vortex seasons. We find that the enhanced water vapour raised PSC formation temperatures, resulting in earlier formation and, consequently, an earlier onset of the heterogeneous chemistry and chlorine activation. However, it is evident that the vortex structure has some influence on the impact. Compared to a control simulation without Hunga, the effect on the edge region occurs throughout the PSC season, whereas the core, due to effective dehydration, experiences large differences at the beginning and end of the season, with minimal differences in between.

We will also briefly summarise the impact of Hunga water vapour on Arctic springtime ozone in recent years, including 2025. The effect of Hunga in the Arctic has so-far been limited by the timing of the water transport (2022/23) and a series of stratospheric warming events (SSWs) (2023/24). However, the 2024/25 Arctic winter began colder than usual in December/January. Therefore, effects of the Hunga water vapour may be more pronounced during this Arctic winter subject to any warming events still to come.