Linkages of Subtropical Stratospheric Intraseasonal Intrusions with Indian Summer Monsoon Deficit Rainfall

Abstract

The authors investigate the life cycle of a strong subtropical stratospheric intrusion event and propose a hypothesis through which it might reduce the intensity of the Indian summer monsoon (ISM) rainfall (ISMR) after the monsoon onset during June 2014. The diagnostic analysis of ERA-Interim data revealed that stratospheric intrusion occurs in the region of the subtropical westerly jet (SWJ) as a result of Rossby wave breaking (RWB). The RWB event is associated with eddy shedding. These eddies transport extratropical stratospheric mass and energy fluxes downward and southward to north India (NI). As a result, the intrusion spreads dry, cold, and ozone-rich air deep into the troposphere (~500 hPa) over the NI. It enhanced the static stability and weakens the north–south upper-tropospheric temperature gradient. The intrusion of cold and dry air persisted for the entire June, which might have inhibited northward propagation of ISM convection and could be responsible for prolonged hiatus in northward phase propagation of the ISM after onset. The relation between stratospheric intrusion events and ISMR from long-term data (1979–2007) is also investigated. The analysis shows that the stronger negative anomalies of rainfall are associated with stratospheric intrusions during break spells. Thus, the study reveals that stratospheric intrusion is an important factor that may influence ISMR deficit.