The Hunga Tonga-Hunga Ha'apai Hydration of the Stratosphere.

Following the 15 January 2022 Hunga Tonga-Hunga Ha'apai eruption, several trace gases measured by the Aura Microwave Limb Sounder (MLS) displayed anomalous stratospheric values. Trajectories and radiance simulations confirm that the H 2 O, SO 2 , and HCl enhancements were injected by the eruption. In comparison with those from previous eruptions, the SO 2 and HCl mass injections were unexceptional, although they reached higher altitudes. In contrast, the H 2 O injection was unprecedented in both magnitude (far exceeding any previous values in the 17-year MLS record) and altitude (penetrating into the mesosphere). We estimate the mass of H 2 O injected into the stratosphere to be 146 ± 5 Tg, or ∼10% of the stratospheric burden. It may take several years for the H 2 O plume to dissipate. This eruption could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the radiative forcing from the excess stratospheric H 2 O.