Influences of ENSO and intraseasonal oscillations on distinct tropical cyclone clusters over the western North Pacific

Although the influences of El Niño–Southern Oscillation (ENSO) and boreal summer intraseasonal oscillation (ISO) on basin-wide tropical cyclone (TC) activity over the western North Pacific (WNP) have been widely recognized, how the seasonal and subseasonal anomalies of sea surface temperature and atmospheric ISO variations modulate different types of WNP TCs needed further examination, as addressed in this study. Using a fuzzy c-means clustering method, we objectively classified the WNP TCs into seven distinct clusters with different genesis locations and trajectories. The genesis numbers of all seven TC clusters revealed significant spectral variance at the intraseasonal timescale in the bands of 10–30 and 30–90 days. Based on the diagnosis of scale-decomposed genesis potential index, we found that the increase in ISO-related mid-tropospheric moistening plays the most important role in TC genesis for all seven clusters, while anomalous circulations (low-level vorticity and mid-level vertical motion) are secondary. The trajectories associated with straight-moving and recurving TC clusters are modulated by ISO-related steering flows. These modulations of TC activities by ISO vary with the phase of ENSO. The modulations of ISO are significantly greater for TCs generated in the southeast quadrant of the WNP in El Niño years than in La Niña years, while ISO imposes a larger impact on landfalling TCs occurring in La Niña years, which are changed by the low-level winds associated with ENSO conditions. The compound effects of ENSO and ISO on TC clusters provide useful sources of subseasonal TC predictability. Our statistical model using the information of ENSO and ISO shows skillful predictions of WNP TC genesis numbers and track distributions at the lead time up to 30 days.