The stratospheric ozone rich cold intrusion during El-Niño over the Indian region: Implication during the Indian summer monsoon

Abstract:

Ozone in the upper troposphere is a dominant radiative constituent. In this study, we investigate ozone variability due to stratospheric intrusions in the upper troposphere over India and associated radiative impacts during monsoon breaks co-occurring with El Niño using the ECHAM5-HAMMOZ, Global-Chemistry-climate model simulations, and ERA-Interim reanalysis data. Our analysis shows that during El Niño, deep stratospheric intrusions occur at the North India-Tibetan Plateau (NI-TP) region and the eastern edge of the monsoon anticyclone. These intrusions lead to an enormous increase in ozone amounts (~100 ppb) in the upper troposphere over India. The intrusions frequently penetrate deep into the troposphere which enhances the surface ozone levels by ~10–20 ppb and augments radiative forcing by ~0.33 W m−2 at the top of the atmosphere. The stratospheric intrusions are associated with the eastward-moving wave train (WT; composed of cyclonic and anticyclonic circulations) in the upper troposphere emanating from the anomalously warm east Pacific, traversing towards NI-TP locale. This wave train transport extra-tropical cold air mass, producing an anomalous cooling of ~1.6–2 K in the upper troposphere over NI-TP. The upper tropospheric anomalous cooling seems to act as a dominating factor opposing the heating due to enhanced ozone. The wave train likewise intensifies Rossby wave breaking, facilitating stratospheric intrusions, enhancing the anomalous subsidence over the NI-TP region. Our analysis shows that break days with El Niño substantiate with simultaneous occurrence of (a) RWB, (b) transport of cold air mass via WT-1 and (c) subsidence by El Niño circulation. These factors lead to exacerbation of deficit rainfall.