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.