Role of Extratropical Stratospheric air in the Genesis of Downstream Monsoon Low Pressure Systems

Abstract

Low-pressure systems (LPS) are convectively coupled vortices that contribute nearly half of summer monsoon rainfall over the Indian subcontinent. About one-third of boreal summer monsoon LPS are caused by downstream amplification of propagating systems from the Western North Pacific (WNP). The relation between vorticity-rich external systems and downstream LPS has been overlooked, and the thesis focuses on exploring the role of extratropical stratospheric air intrusions in the genesis of downstream vortex seeds. The analysis of summer downstream LPS events from 1979 to 2017 revealed that 43% of them are associated with extratropical stratospheric air intrusions over the WNP. The stratospheric air intrusions lead to high tropospheric potential vorticity (PV), and the downstream vortex seeds are observed to initiate and intensify to the southwest of the PV anomalies. The PV anomaly could deform the local temperature column and cause adiabatic lifting, which in turn induces or intensifies low-level cyclonic vortices. The subsequent intensification of the low-level vortex is aided by deep convection, observed southwest of the PV anomaly, through vortex stretching and low-level PV generation induced by diabatic heating. The background environment conditions and the vertical extent and magnitude of these intrusions determine the effectiveness of the associated processes. These results illustrate the potential predictability of downstream LPS initiated by extratropical stratospheric intrusions and may also be relevant for TC and other disturbances in WNP as well as over the other tropical regions where these conditions are met.