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Islands (120øE -150øE, 5øN-20øN) and that the location of their generation shifts toward the east during El Nifio years. This results in inactive convection in the NWEP during El Nifio years due to an increase in sea level pressure over ocean areas.

The variability of tropical cyclone (TC) frequency over the western North Pacific (WNP) was investigated using a high-resolution atmosphere-ocean Coupled General Circulation Model (CGCM) of the National Research Institute for Earth Science and Disaster Prevention. A 50-year CGCM integration simulated highly frequent occurrence periods (HFOPs), comprising periods with a greater number of TCs than the long-term mean number, and lowly frequent occurrence periods (LFOPs), comprising periods with a lower number than the mean, as the interdecadal variability of TC frequency. The variability corresponded to the interdecadal variability of the actual TC frequency over the WNP during the past 50 years, 1951-2000. This CGCM integration also showed that the oceanic and atmospheric conditions (sea surface temperature, horizontal wind and relative vorticity at the 850 hPa level, equivalent potential temperature, and precipitation rate) favorable for TC formation during the HFOPs were the same as the observed oceanic and atmospheric conditions during the observed HFOPs. In the HFOPs, the sea surface temperatures over the tropical WNP are higher than in the LFOPs. The differences in atmospheric conditions between HFOPs and LFOPs are characterized by an anomalous westerly wind and positive relative vorticity at the 850 hPa level and an increase in the convective available potential energy. These differences in the environmental conditions simulated by the CGCM are consistent with those found in the observations.The CGCM was able to simulate the interdecadal variability of model TC frequency and the oceanic and atmospheric conditions related to the variability, which were similar to the observations. However, we need to improve the CGCM to more realistically simulate TC activity and oceanic and atmospheric conditions hereafter, because the CGCM has several deficiencies.