Modulation of the Northern polar vortex by the Hunga Tonga-Hunga Ha'apai eruption and associated surface response

Abstract. The January 2022 Hunga Tonga-Hunga Ha'apai (HT) eruption injected sulfur dioxide and unprecedented amounts of water vapor (WV) into the stratosphere. Given the manifold impacts of previous volcanic eruptions, the full implications of these emissions are a topic of active research. This study explores the dynamical implications of the perturbed upper atmospheric composition using an ensemble simulation with the Earth System Model SOCOLv4. The simulations replicate the observed anomalies in the stratosphere and lower mesosphere's chemical composition and reveal a novel pathway linking water-rich volcanic eruptions to surface climate anomalies. We show that in early 2023 the excess WV caused significant negative anomalies in tropical upper-stratospheric/mesospheric ozone and temperature, forcing an atmospheric circulation response that particularly affects the Northern Hemisphere polar vortex (PV). The decreased temperature gradient leads to a weakening of the PV, which propagates downward similarly to sudden stratospheric warmings (SSWs) and drives surface anomalies via stratosphere-troposphere coupling. These results underscore the potential for HT to create favorable conditions for SSWs in subsequent winters as long as the near-stratopause cooling effect of excess WV persists. Our findings highlight the complex interactions between volcanic activity and climate dynamics and offer crucial insights for future climate modeling and attribution.