We calculate the climate forcing for the two years after the January 15, 2022, Hunga Tonga-Hunga Ha’apai (Hunga) eruption. We use satellite observations of stratospheric aerosols, trace gases and temperatures to compute the tropopause radiative flux changes relative to climatology. Overall, the net downward radiative flux decreased compared to climatology. Although the Hunga stratospheric water vapor anomaly increases the downward infrared radiative flux, the solar flux reduction due to Hunga aerosol shroud dominates the net flux over most of the two-year period. Decreases in temperature produced by the Hunga stratospheric circulation changes contributes to the decrease in downward flux; however, the Hunga induced decrease in ozone increases the net short-wave downward flux creating small sub-tropical net flux increase in late 2022. Coincident with the aerosols settling out, the water vapor anomaly disperses, and circulation changes disappear so that the contrasting forcings all decrease together. By the end of 2023, most of the Hunga induced radiative forcing changes have disappeared. There is some disagreement in the satellite stratospheric aerosol optical depth (SAOD) which we view as a measure of the uncertainty; however, SAOD uncertainty does not alter our conclusion that, overall, aerosols dominate the radiative flux changes followed by temperature and ozone.

On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai (HT) eruption injected SO2 and water into the middle stratosphere. Shortly after the eruption, the water vapor anomaly moved northward toward and across the equator. This northward movement appears to be due to a Rossby wave forced by the excessive IR water vapor cooling. Following the early eruption stage, persistent mid-stratospheric water vapor and aerosol layers were mostly confined to Southern Hemisphere (SH) tropics (Eq. to 30°S). However, during the spring of 2022, the westerly phase of the tropical quasi-biennial oscillation (QBO) descended through the tropics. The HT water vapor and aerosol anomalies were observed to again split across the equator coincident with the descent of the QBO shear zone. This split occurred because of the enhanced meridional transport circulation associated with the QBO. Neither transport event can be reproduced using MERRA2 assimilated winds.

On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai eruption injected SO2 and H2O into the middle stratosphere. The eruption produced a persistent mid-stratospheric sulfate aerosol layer, mostly below 26 km, confined to Southern Hemisphere (SH) tropics. Coincident with, and slightly above the aerosol layer, an enhanced H2O layer is also observed. The SH tropical confinement is simulated using a trajectory model. Measurements over several months following the eruption show that the H2O layer is slowly rising while the aerosol layer is descending. The H2O layer upward movement is consistent with the vertical velocity at these altitudes. Gravitationally settling explains the descent of the aerosol layer. A cold anomaly coincident with the H2O enhancement is observed and is caused by thermal adjustment to the additional H2O IR cooling. A simple model of volcanic water injection at the time of the eruption simulates the observed H2O.