How the Hunga Tonga—Hunga Ha'apai Water Vapor Cloud Impacts Its Transport Through the Stratosphere: Dynamical and Radiative Effects

The eruption of the Hunga Tonga—Hunga Ha'apai (HTHH) volcano on 15 January 2022 injected about 150 Tg of water vapor (H2O), roughly 10% of the background stratospheric H2O content, to altitudes above 50 km. Simulations of the spatial evolution of the H2O cloud with the ICON‐Seamless model are very close to observations from the Aura Microwave Limb Sounder. The vertical transport of the H2O cloud had three phases: an initial subsidence phase, a stable phase, and a rising phase. Radiative cooling of H2O clearly affects the transport of the H2O cloud, as demonstrated with passive tracers, and is the main driver within the subsidence phase. It also counteracts the large‐scale rising motion in the tropics, leading to the stable phase, and modulates the large‐scale transport of the H2O cloud for about 9 months. This holds for different QBO phases, where the H2O cloud differs mainly in its vertical extent. Plain Language Summary The eruption of the Hunga Tonga—Hunga Ha'apai (HTHH) volcano on 15 January 2022 injected about 150 Tg water vapor (H2O) to an altitude above 50 km. This is more than 10% of the total stratospheric H2O content. We study the distribution of the H2O cloud and its ascent into the mesosphere using observations from the Aura Microwave Limb Sounder satellite and by performing simulations of the cloud with the ICON‐Seamless model. The vertical transport of the H2O cloud had three phases: a subsidence, a stable, and a rising phase. The temperature inside the H2O cloud is lower than outside the cloud. This causes the subsidence of the H2O cloud and has consequences for the transport during the next months. From October 2022 on, the transport is mainly determined by the large‐scale wind patterns. Key Points Radiative cooling of the H2O cloud influences the transport of the H2O cloud, stratospheric dynamics and, changes tracer transport Radiative cooling of the H2O cloud influences the cross equatorial transport of the H2O cloud The phase of the QBO modulates the large‐scale transport and the vertical extension of the HTHH cloud