Tropical cyclogenesis associated with four types of summer monsoon circulation over the South China Sea

During the summer monsoon period from 1958 to 2013, 80% of tropical cyclone (TC) genesis over the South China Sea (SCS) occurs in four types of East Asian summer monsoon (EASM) circulation which are named the monsoon trough (MT), the monsoon gyre (MG), the southwesterly (SW), and the monsoon shear (MS), respectively. The first three types account for about 90% of TC geneses. These circulation types provide favorable environmental fields for TC genesis. In the northern-central SCS, lower-level positive relative vorticity anomalies are accompanied by mid-troposphere ascent and upper-level positive divergence anomalies, which match with positive relative humidity anomalies in the mid-low troposphere. Corresponding to these environmental fields, the eddy kinetic energy (EKE) growth from the barotropic energy conversion in four circulation types is obvious, which can help evolve incipient disturbances to tropical cyclones. In the transformation from the mean kinetic energy (MKE) to the EKE, the zonal wind shear and the meridional wind convergence make a crucial contribution to the EKE growth.