Extratropical Stratospheric Air Intrusions Over the Western North Pacific and the Genesis of Downstream Monsoon Low-Pressure Systems

Low-pressure systems (LPS) are convectively coupled vortices that contribute nearly half of the summer monsoon rainfall over the Indian subcontinent. About one-third of the boreal summer monsoon LPS are caused by downstream amplification of westward propagating disturbances from the western North Pacific (WNP). Analysis of downstream LPS events from 1979 to 2017 reveals that 43% of them are caused by extratropical stratospheric air intrusions over the WNP. 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 anomalies can deform the temperature in its neighborhood and cause adiabatic lifting, which in turn can induce and intensify low-level cyclonic vortices. The subsequent intensification of the low-level vortex is aided by deep convection, observed to the southwest of the PV anomaly, through vortex stretching and low-level PV generation by diabatic heating.