Interannual variation of tropical cyclone activity over the Central North Pacific

The annual mean number of tropical cyclones in the Central North Pacific (CNP) is approximately three. Although this number is low in comparison to other basins, the interannual variability of tropical cyclones, which includes tropical storms and hurricanes, is high. For the period 1966-1997, the annual number ranges from 0 to 10 tropical cyclones. There is a large and positive correlation between CNP tropical cyclone counts and El Nino 3.4 region SST anomalies with a 95% significance level. In the El Nino Hurricane Season (ENHS), a greater number of cyclones formed in the CNP and more cyclones propagated into this area from the east. The monsoon trough, low level relative vorticity, and tropospheric vertical wind shear in the CNP undergo pronounced changes during warm and cold phases of ENSO. For instance, the 1000 hPa relative vorticity values within the CNP in an El Nino autumn composite are double the values in a corresponding La Nina composite. The El Nino autumn composite of tropospheric vertical wind shear shows a two to three times reduction equatorward of 16degreesN-17degreesN when compared to the La Nina autumn composite. The increased values of the dynamic potential term in Gray's (1977) seasonal genesis parameter correspond well with the increased cyclone frequency in the CNP for an ENHS composite. Furthermore, a majority of initial detection points of named storms is found within a band of relatively large values of dynamic potential. This suggests that this term can be used to diagnose favorable areas for tropical cyclogenesis on a seasonal time scale.