Contrast of tropical cyclone frequency in the western North Pacific between two types of La Niña events

According to the different pattern of sea surface temperature anomaly (SSTA) in the previous year of La Niña events, we categorized La Niña events into two types to investigate the different characteristics of tropical cyclone (TC) activity over the western North Pacific (WNP) in TC peak season of two types La Niña events. One type is following the previous El Niño event (La Niña I); the other is following the previous neutral phase or developing La Niña event (La Niña II). Results show that TC genesis frequency in the WNP during TC peak season of La Niña I is less than normal year, whereas it has no differences from normal year during La Niña II. The main reason is attributed to the different amplitude SSTA in the East Indian Ocean (EIO) and the western Pacific Ocean (WPO). Similar to the capacitor effect, strongly positive SSTA in the EIO-WPO during La Niña I triggers an equatorial baroclinic Kelvin wave, which intensifies the easterly in the lower troposphere and weakens the East Asian summer monsoon, and thus the TC frequency decreased during La Niña I. However, the easterly anomaly shows a weak response to the SSTA in the EIO-WPO during La Niña II, and there is no significant change in the environmental pattern over the WNP; so is TC frequency. The modulation of strong EIO-WPO SSTA on large-scale circulation over the WNP reduces the environmental barotropic energy conversion into synoptic-scale disturbances during La Niña I, and also suppresses TC disturbances. The understanding of two different types of La Niña events could help improve the seasonal prediction of TC activity in the WNP during La Niña.