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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 90~1600E 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).
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