Diverse roles of water vapor trigger unusual stratosphere after 2022 Hunga Tonga eruption

Abstract The unprecedented amount (150–160 Tg) of water vapor (WV) from 2022 Hunga Tonga–Hunga Haʻapai (HT) eruption could cool the stratosphere and influence stratospheric sulfate particles formation and growth. However, it is still unclear that how much contribution from each of these diverse roles of WV to the stratospheric evolution and which role is dominant. Here, constrained by satellite observations, we develop analytical models to quantify the direct contribution of WV cooling and indirect contribution of WV affecting sulfate particles properties to stratospheric temperature modulation. For the first time, we reveal that the condensation and nucleation processes, promoted by abundant WV, contribute ~ 90% to the particle radius growth from ~ 0.2 µm to 0.35–0.45 µm after HT, accounting for observed strong aerosol extinction. This rapid growth rate is comparable to that in the first two months after the 1991 Mt. Pinatubo eruption, which emitted similar WV but ~ 80 times more sulfur dioxide. This disparity leads to stronger WV cooling than aerosols warming in the lower and middle stratosphere after HT, resulting in the strongest mid-latitude cooling since Pinatubo eruption of -8~-4 K for 4–7 months, opposite to the stratospheric warming dominated by volcanic aerosols often expected after volcanic eruptions.