Large ozone enhancement over South Asia triggered by sudden stratospheric warming under westerly QBO phase: implications on ozone radiative forcing

Abstract. Tropospheric ozone pollution in South Asia is mainly blamed on anthropogenic emissions. However, based on ERA5 reanalysis data, this study highlights the contribution of stratospheric ozone intrusions into the Upper Troposphere and Lower Stratosphere (UTLS) associated with Sudden Stratospheric Warming (SSW) events in enhancing upper tropospheric ozone over the South Asian region. We report an enhancement in ozone in the UTLS by more than 80 % for 2018 and ∼ 30 % within ±6 d of the onset during SSW events concurrent with the westerly phase of Quasi-biennial oscillation (WQBO-SSW) compared to non-SSW years. The equatorward shift (south of 30° N) of the subtropical jet during WQBO-SSW causes lowering of the tropopause and more Rossby-wave breaking in the upper troposphere. This results in higher stratospheric ozone intrusions over the South Asian region. The ozone enhancement during WQBO-SSW events produces an instantaneous radiative forcing at the top of the atmosphere of 0.09±0.05 W m−2 due to UTLS ozone changes and 0.17±0.05 W m−2 from total-column ozone changes over South Asia.