Change in relationship between western North Pacific tropical cyclone frequency and the tropical Pacific SST

We examine the relationship between the number of tropical cyclones (TCs) in the western North Pacific and the tropical Pacific sea surface temperature (SST) during the main TC season (from July to November). Results show that there are periods when TC frequency and the tropical Pacific SST are well correlated and periods when the relationship breaks down. Therefore, decadal variation is readily apparent in the relationship between the TC frequency and the SST variations in the tropical Pacific. We further examine the oceanic and atmospheric states in the two periods when marked contrast in the correlation between the TC frequency and the tropical Pacific SST is observed. The analysis indicates that before 1990 oceanic conditions largely influenced anomalous TC frequency, whereas atmospheric conditions had little impact. After 1990, there the reverse appears to be the case. A role of atmosphere and ocean in relation to the TC development in the western North Pacific changes, which is consistent with change of the correlations between the TC frequency and the tropical Pacific SST.