Large Scale Circulation Patterns Favorable to Tropical Cyclogenesis over the Western North Pacific

It is important to study on the large scale patterns favorable to tropical cyclogenesis over the western North Pacific (WNP) for forecasting a tropical cyclone formation. The NCEP/NCAR reanalysis data and the JTWC best track data in nearly 20 years are used to examine the large scale patterns associated with tropical cyclones (TC) formation over the WNP. The result shows that there are five large scale patters favorable for TC genesis over the WNP according to the specific characteristic of low level circulation during TC formation period. They are monsoon shear line, monsoon confluence region, reverse-oriented monsoon trough, monsoon gyre, trade easterlies. Especially, the first three patterns are important for TC formation, about 80% or more of TC formed in these circulation patterns over the WNP. This is due to these three large scale patterns can provide favorable environment of relative vorticity,low-level convergence and small vertical wind shear of horizontal wind to TC genesis. Moreover, barotropic energy conversion processes of TC genesis in these five large scale circulation patterns are discussed respectively. It is concluded that there are differences in the barotropic energy transiting procedure of these five patterns. For the monsoon shear line and the reverse-oriented monsoon trough patterns, zonal wind shear is important favorable condition for TC genesis. And zonal wind convergence is significant favorable condition for TC genesis in the monsoon confluence region. When a tropical cyclone formed in a monsoon gyre, zonal wind shear and convergence is still important, however, the effect of meridional wind shear cannot be ignored for the TC formation. Besides, for those TC formed in trade easterlies, zonal wind convergence also plays a major role in the genesis process, but it is weaker than other four patterns.