KEYSERSYMP Impact of Extratropical Stratospheric Potential Vorticity Intrusions on Initiation of Low Level Vortices Over the Western North Pacific

Cyclonic vortices originating in the western north Pacific (WNP) and south China sea are rain-bearing synoptic scale systems which form an important component of the East Asian summer monsoon. A fraction of these vortices move westward and serve as precursors for the initiation of monsoon low pressure systems over the Bay of Bengal. The WNP experiences frequent intrusions of extratropical stratospheric air with high potential vorticity (PV) which occur as a consequence of Rossby wave breaking. Over an analysis period of 41 years (1979-2019) during the boreal summer, 370 PV intrusion events are identified. Background wind conditions influence the occurrence of intrusion events, with higher frequency of events in a background with stronger upper level westerly wind. The intrusion events are most frequent during La Nina years and least frequent during El Nino years, further confirming their dependence on background wind. The upper tropospheric intrusions are found to induce low-level vortex initiation to the southwest of the PV intrusion with high efficiency (80%). These events are associated with a mid-level cyclonic vortex which weakens as the low-level vortex intensifies. The intrusion of cold and dry extratropical stratospheric air over warm and moist tropical air reduces the static stability, conditioning the environment for convection. Deep convection intensifies the low-level vortex through vortex stretching and generation of low-level PV by diabatic heating. Additionally, intrusion events are found to become more frequent over the WNP, although no significant trend is found in the number of low-level vortices induced by upper level PV intrusions.