Analysis of interannual features of tropical cyclones originating from the monsoon trough in the western North Pacific

Based on 1979-2005 typhoon data and NCEP/DOE AMIP-II reanalysis, a study is performed of behaviors of tropical cyclones generated in the monsoon trough (MTTC hereinafter) in the western North Pacific as well as effects of the monsoon trough strength on their production. Evidence suggests that (1) during this period the MTTC yearly number experiences stages as follows: normal (1979~87), more MTTCs (1988~94), and fewer MTTCs (1995~2005); MTTC variation is marked by quasi-4 and -2 yearly periods, with 1994 as the change from more to fewer MTTCs in annual number; (2) in the years of anomalous MTTC number there are great difference in the onset/ending day and genesis position. In the years of fewer (more) MTTCs in comparison to mean, MTCC starts its activity later (earlier), terminating on an earlier (later) day, its genesis area is smaller (bigger), located south- (north-) and/or west- (eastward) of mean; 3) the ITCZ intensity affects the MTTC genesis position and yearly number. When the lower-level western North Pacific subtropical high is positioned south- (north-) of normal, cross-equatorial flows at Somali and 90~160 0 E are weaker (stronger), the monsoon trough is weaker (stronger) with its position south- and/or westward (north- and/or eastward) with respect to normal. At that time, in the tropopause, the south-Asian high is east- (westward) of mean and the oceanic upper-air trough is south- and/or westward (north- and/or eastward). And the distribution in the high and lower troposphere allows the small-value band of vertical wind shear to decrease (increase) for a smaller (bigger) domain for MTTC genesis, and convection is suppressed (intensified), leading to positive (negative) OLR anomalies over waters east of the Philippine so that MTTC is generated south- and/or westward (north- and/or eastward) relative to normal and MTTC annual number is anomalously smaller (greater).