Large Ozone Intrusions during Sudden Stratospheric Warmings Enhance Ozone Radiative Forcing over South Asia

Abstract. Tropospheric ozone pollution in South Asia is mainly blamed on anthropogenic emissions. However, this study highlights the contribution of stratospheric ozone intrusions into the troposphere associated with sudden stratospheric warming (SSW) events in enhancing tropospheric ozone over the South Asian region using ERA-5 reanalysis data. We report that specifically split-downward propagating SSWs (dSSWs) cause enormous ozone enhancement in the upper troposphere and lower stratosphere (UTLS) over South Asia around the dSSW-onset, with a maximum of ~290 % within ±30 days. The ozone intrusions propagate deep into the troposphere, causing near-surface maximum ozone increase by 43 % within ±30 days around the SSW-onset. The ozone enhancement increases ozone radiative forcing in the troposphere by 0.04±0.03 W.m^-2 and UTLS by 0.08±0.06 W.m^-2 over South Asia. Frequent SSW events in a warming climate will thus likely increase stratospheric ozone intrusions and ozone radiative forcing over South Asia, potentially exacerbating regional climate warming. The elevated tropospheric ozone amounts due to stratospheric intrusions are posing threat to humans and vegetation.