Recent weakening of the relationship between El Niño–Southern Oscillation and western North Pacific tropical cyclone season onset date

This study investigates the climate drivers of western North Pacific (WNP) tropical cyclone season onset date (TCSOD). We find that the interannual relationship between WNP TCSOD and El Niño–Southern Oscillation (ENSO) varies considerably on interdecadal time scales. The association between TCSOD and ENSO, as measured by the Niño3.4 index, is significant during the first subperiod from 1979 to 1998 (r = .51) but becomes insignificant (r = .01) during the second subperiod from 1999 to 2019. This relationship degradation is mainly due to shifting ENSO conditions with more frequent occurrences of tropical central Pacific ENSO events since the late 1990s. Tropical eastern Pacific ENSO events cause changes in the low-level large-scale circulation over the tropical western WNP, where early season WNP TC genesis typically occurs, via a Gill-type Rossby wave response. Tropical central Pacific ENSO events have a limited impact over this region. These relationships are confirmed by a stable and strong association between tropical eastern Pacific ENSO and TCSOD for the whole period (r = .63) and for the two subperiods (r = .72/r = .56), while the correlation between central Pacific ENSO events and TCSOD is weak for the whole period (r = −.01) and for the two subperiods (r = −.03/r = −.13). We further find that the Pacific meridional mode is a robust indicator of TCSOD over the WNP regardless of shifting ENSO conditions. Results of this study may provide useful information for decadal climate prediction of WNP TCSOD.