Research on the cluster of tropical cyclogenesis in the South China Sea-western North Pacific monsoon trough Ⅰ. Major features about the cluster of tropical cyclogenesis

By using the western North Pacific tropical cyclone(TC)data and the NCEP/DOE AMIP-Ⅱ reanalysis daily data during 1979-2005, the features of TCs in the South China Sea(SCS)-western North Pacific monsoon trough (MTTC) and the cluster of tropical cyclogensis (CTC) are analysed. The main results are shown as follows: (1)The average frequency of the CTC is 3~4 times per year. In August, the number of the CTC is maximum, occupying 30%, followed by September, and it is minimum in May, only accounting for 4%. (2) The MTTCs genesis area is smaller (bigger), located more south-(north-) and west-(eastward) than normal in inactive (active) years.(3)CTC mainly formed in four monsoon trough (MT) patterns: eastward extend MT, reverse-oriented MT, the three streams MT and western Pacific MT, of which the first pattern is the chief, in which 60.0% of MTTC form, the last one is the least, only accounting for 4.8%. (4) Anomalies in the intensity and pattern of monsoon trough lead to the abnormal CTCs which result in the abnormal frequency of MTTCs.