The interannual variation in tropical cyclone genesis frequency over the western North Pacific was examined for the active tropical cyclone (including summer and fall) during 1979-94. An emphasis was put on the possible effect of the interannual variation of atmospheric circulation and monsoon trough on tropical cyclone occurrence. The major findings of this study are the following. 1) A distinct increase (decrease) of tropical cyclone genesis frequency occurs north of the climatological location of the monsoon trough in the Philippine Sea during summers (June-August) with anomalous cold (warm) sea surface temperature (SST) over the NINO3 region. The interannual variation of tropical cyclone genesis in this region results from the appearance of an anomalous cyclonic (anticyclonic) cell situated in a summer teleconnection wave train emanating from the western tropical Pacific and progressing along the rim of the North Pacific. In addition to the north-south interannual variation, there is also a longitudinal interannual variation in the summer tropical cyclone genesis frequency over this region. The contrast of tropical cyclone genesis between the regions west and east of 150ЊE is reduced (enhanced) when the monsoon trough extends (retreats) eastward (westward) across this longitude during warm (cold) summers. 2) For fall (September-November), there is no clear relationship between the north-south interannual variation in the tropical cyclone genesis over the western North Pacific and SST (NINO3). However, there is a perceptible tendency of the longitudinal interannual variation in tropical cyclone genesis frequency to follow the eastward extension/westward retreat of the monsoon trough in a way such as it does during the summer season.

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