Cross-hemispheric transport of the Hunga aerosol plume: in situ evidence and radiative effects from the northern hemisphere

Abstract. The Hunga Tonga–Hunga Ha’apai eruption (20° S) in January 2022 injected a substantial amount of water vapor and aerosols into the stratosphere, primarily impacting the Southern Hemisphere and tropics. Using a combination of satellite observations and in situ measurements with optical particle counters, we show that a significant portion of the aerosol plume was transported into the Northern Hemisphere (NH) mid-latitudes. This cross-hemispheric transport occurred within the tropically controlled transition zone, within the shallow branch of the Brewer–Dobson circulation. By October 2022, enhanced aerosol concentrations were observed up to 50° N, at altitudes between 17–23 km with some dense plumes at around 21–22 km. In situ observations reveal an effective radius of around 330 nm, comparable to what was observed in the Southern Hemisphere (SH). Aerosol extinction coefficients in the mid-latitudes (30–50° N) increased by ∼50 % over background levels, corresponding to an aerosol optical depth (AOD) increase of (1–2) × 10−3. These enhancements led to a modest, but not negligible, shortwave top-of-atmosphere (TOA) radiative forcing of −0.05 ± 0.01 W m−2 between October 2022 and April 2023. Our results show that the moderate aerosol impact of the Hunga eruption in the SH produced non-negligible radiative impacts in the NH, emphasizing the importance of considering both hemispheres when analysing the total impact.