Research on the cluster of tropical cyclogenesis in the South China Sea-western north Pacific monsoon trough Ⅱ. Mechanism of the influence

By using the western north Pacific tropical cyclone(TC)data, OLR data and the NCEP/DOE AMIP-Ⅱ reanalysis daily data during 1979-2005, the possible mechanism of the cluster of TCs in the South China Sea (SCS)-western north Pacific monsoon trough (CTC) are discussed, the main results are as follows: (1)During May-October, the intensity and pattern of monsoon trough are closely related to the strength of Somalia’s cross-equatorial flow, the location of western north Pacific subtropical high (WNPSH) and the intensity of the Southern Hemisphere Australian winter monsoon. Enhanced intensity of monsoon trough in different regions may lead to CTC. (2) CTC and the low-frequency oscillations of monsoon trough are closely connected.. the CTC often occurs when the monsoon trough is in the active phase both of 10~20 and 30~60 d oscillations . (3) CTC is affected by intensity and pattern of monsoon trough which are related to the location of WNPSH, the intra-seasonal oscillation phases of cross-equatorial flows over the Somalia and western Pacific. It supplies favorable conditions to CTC when WNPSH retreats to eastward or is in the north of mean and the cross-equatorial flows enhance either over Somalia or the western Pacific, as well as enhance in both of them. When WNPSH is west and south to the normal, the cross-equatorial flows enhance over Somalia area or the western Pacific, the monsoon trough is located southward leading to less CTC. (4) Typical cases analysis shows that CTC exhibits inter-annual variation due to the different convection and moisture conditions in the main TC generating areas which are caused by the different intensity and pattern of monsoon trough.