The Initial Mesoscale Vortexes Leading to the Formation of Tropical Cyclones in the Western North Pacific

A statistical analysis of the initial vortexes leading to tropical cyclone(TC)formation in the western North Pacific(WNP)is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018.It is found that TCs in the WNP basically originate from three kinds of vortexes,i.e.,a mid-level vortex(MV),a low-level vortex(LV),and a relatively deep vortex with notable vorticity in both the lower and middle troposphere(DV).Among them,LV and DV account for 47.9%and 24.2%of tropical cyclogenesis events,respectively,while only 27.9%of TCs develop from the MV,which is much lower than that which occurs in the North Atlantic and eastern Pacific.Such a difference might be ascribed to the active monsoon systems in the WNP all year round.Due to the nearly upright structure of mid-level convergence in the early pre-genesis stage,TC genesis efficiency is the highest in DV.Compared with MV,LV generally takes a shorter time to intensify to a TC because of the higher humidity and the stronger low-level cyclonic circulation,which is related to air-sea interaction and boundary-layer convergence.Further examination of the relationship between tropical cyclogenesis and large-scale flow patterns indicate that the TC genesis events associated with LV are primarily related to the monsoon shear line,monsoon confluence region,and monsoon gyre,while those associated with MV are frequently connected with easterly waves and wave energy dispersion of preexisting TC.Compared with other flow patterns,tropical cyclones usually form and intensify faster in the monsoon confluence region.