How does the onset time of El Niño events affect tropical cyclone genesis and intensity over the western North Pacific?

Previous studies have shown that the equatorial Pacific Ocean warming can be separated into spring (April~June; SP) and summer (July~October; SU) modes based on the onset time of El Niño events. Different onset times are caused by different atmospheric and oceanic conditions, which, in turn, should influence tropical cyclone (TC) activity. While the location of TC formation in the western North Pacific (WNP) shifts significantly eastward and equatorward in the SP type relative to climatology (1951–2017), only an eastward shift occurs in the SU type. Only intense TCs (category 1 + 2 + 3 and 4 + 5) have an obvious eastward and equatorward shift in the SP type, with no such shift in any TC category in the SU type. The number of category 4 + 5 TCs in the SP type increases significantly, but fewer category 1 + 2 + 3 TCs in the SU type. In the SP type, TCs are generally stronger, with more in category 4 + 5, and longer life spans occurring across all categories. However, no obvious change in TC intensity can be identified except for category 1 + 2 + 3 in the SU type. The eastward and equatorward shift of the mean genesis position in the developing phases of SP can be attributed to a southeastward shift of favourable atmospheric and oceanic conditions, which include weaker zonal vertical wind shear, stronger relative vorticity, higher mid-level relative humidity, and enhanced upper-ocean temperature (5–100 m). All these conditions are mainly related to the onset time of the westerly anomalies near the central equatorial Pacific. These results suggest that the onset time of El Niño events could serve as a good forecasting indicator for TC genesis position and intensity over the WNP.