Role of monsoon gyre in the interannual variation of tropical cyclone formation over the western North Pacific

The south-southeast periphery of a monsoon gyre in the western North Pacific (WNP) is a favorable region for tropical cyclone/tropical depression (TC/TD) genesis. The TC genesis frequency is interannually modulated by the WNP monsoon circulation in response to a change in tropical Pacific sea surface temperature (SST). These findings from previous studies lead to the hypothesis that the effect of tropical Pacific SST changes on the WNP TC/TD genesis frequency is accomplished through a modulation of the monsoon gyre activity by WNP monsoon circulation variations. The 6-h TC/TD track records and NCEP-NCAR reanalysis data for the period of 1979-2002 were analyzed to test this hypothesis. Results show that roughly 70% of WNP TC/TD geneses are linked to monsoon gyres. The interannual variation of these geneses is highly correlated (with a correlation coefficient of 0.89) with that of monsoon gyre activity, which is out-of-phase with the interannual variation of SST over the NOAA Niño-3 region.