Variations in the seasonal mean (July-October) genesis positions of tropical cyclones (TCs) in the western North Pacific associated with variations in the large-scale atmospheric circulation are investigated. Analysis shows considerable interannual variability in the seasonal TC mean genesis positions (MGPs) during the 1979-99 period. The variability is shown to be related to the 200-850-hPa vertical wind shear, the west Pacific sea surface temperature (SST), the position and strength of the monsoon trough, and the position and strength of the western Pacific subtropical high (WPSH). Each of these circulation features as well as the SST is, in turn, related to the El Nino-Southern Oscillation (ENSO). However, while this study suggests that ENSO is a major factor in determining seasonal MGP, the relatively short satellite observational period also suggests that ENSO is not the sole determinant, the La Nina year of 1988 being one example. The study further suggests that the role of ENSO is complicated by the differences in the timing and evolution of individual ENSOs with respect to the peak in the mean annual cycle of the TC genesis.
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