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

In boreal winter during the period 1958-2013, more than two third of tropical cyclone (TC) genesis over the South China Sea (SCS) is found to occur in specific atmospheric environmental fields associated with four types of East Asian winter monsoon circulation which are named the monsoon gyre (MG), the easterly, the reverse-oriented monsoon trough (RMT), and the monsoon confluence (MC), respectively. The first two types account for about 80% of TC geneses. Before TC formation over the SCS, lower-level positive relative vorticity and humidity anomalies are accompanied by mid-troposphere ascent and upper-level divergence anomalies, which are favorable for TC genesis. These anomalies are the most significant in the MG type. Moreover, the eddy kinetic energy (EKE) growth from the barotropic energy conversion contributes beneficially to the evolving of incipient disturbances to a TC over the SCS. In all four types, the meridional wind convergence and the zonal wind shear play an important role in the EKE growth.