Rethinking the Characteristic Parameters of Typhoon Activity Frequency in the Western North Pacific

Typhoons are among the most dangerous weather systems on earth and can cause devastating disasters. Although great improvements have been made in the prediction of typhoon tracks during the past decades, typhoon frequency remains a major challenge in operational forecasting and climate prediction. Typhoon genesis frequency focuses on the frequency of formation of a single typhoon. However, the accumulation of single typhoons cannot effectively represent the anomalous characteristics of the corresponding circulation because of considerable differences among the typhoons, such as life history, track, and intensity. The occurrence of multiple typhoons or a single typhoon lasting for a long time over a particular region is usually closely related to the atmospheric circulation over that region. Therefore, in this study, the frequency of typhoons during a certain period and region is used to measure typhoon activity in the western North Pacific (WNP). It is found that the variation of typhoon existence frequency is different from that of typhoon genesis frequency. Typhoon existence frequency greatly improves the connection between typhoon activity and large-scale air–sea interactions (such as the El Niño–Southern Oscillation) and can better indicate the difference in weather and climate corresponding to typhoon activity anomalies. Further investigation reveals that typhoon existence frequency in the WNP during typhoon season is closely related to the North Pacific tripole pattern of sea surface temperature (SST) anomaly (NPSST) and the SST anomaly along the eastern coast of Australia in the South Pacific (SPSST) in the spring (April). A regression model using the NPSST and SPSST indices in April as predictors proved to have good skill in predicting the existence frequency of typhoons in the WNP during typhoon season.