Modification of the Quasi-Biennial Oscillation by Stratospheric Water Vapor From the Hunga Tonga–Hunga Ha'apai Eruption

Abstract. The Hunga Tonga–Hunga Ha'apai (HTHH) undersea volcano injected approximately 0.5 Tg of sulfur dioxide (SO₂) and about 150 Tg of water vapor (H₂O) into the stratosphere. Using a combination of observations and model simulations, we investigate the response of the Quasi-Biennial Oscillation (QBO) to these stratospheric SO₂ and H₂O perturbations, as well as the dynamic mechanisms underlying the QBO modification. A portion of the sulfate aerosols and H₂O remained in the tropical stratosphere for over a year. The water-rich HTHH eruption significantly influenced the QBO amplitude, with the simulated 30 hPa QBO index anomalies accounting for 11–32% of the anomalies observed in the 2022 reanalysis data. The combined effect of H₂O and sulfate aerosols on stratospheric temperature is mainly concentrated in the tropical stratosphere (south of 15°N). H₂O-induced radiative cooling reshapes the residual circulation and tropical upwelling response, reducing the upward advective tendency acting on positive zonal wind perturbations in key regions and favoring their downward propagation into the 30–100 hPa layer. This process effectively modulates the amplitude of the stratospheric QBO through the “upwelling mechanism”. Additionally, the cooling of the stratosphere caused by H₂O and sulfate aerosols generates easterly and westerly wind shear through the “thermal wind balance mechanism” between April and October, further influencing the stratospheric QBO.