Combined Effects of Annual Cycle and ENSO on Tropical Cyclone Genesis over the Western North Pacific

Abstract The El Niño–Southern Oscillation (ENSO) is important to the interannual variability of tropical cyclone (TC) genesis in the Western North Pacific (WNP). However, the mechanisms for the TC seasonal changes in ENSO developing phase remain unclear. This study addresses the critical influences of the background sea surface temperature (SST) annual cycle and ENSO related sea surface temperature anomalies (SSTA) in regulating TC genesis seasonal changes. The results show that during eastern Pacific (EP) El Niño years, TC genesis increases in the southeastern WNP and decreases in the northwest in summer, while in autumn, TC genesis strengthens in the east and weakens in the west. For central Pacific (CP) El Niño events, basin-wide enhancement occurs in summer, with a southeast–northwest contrast in fall, whereas La Niña developing years exhibit nearly opposite patterns. These variations arise from anomalous convection generated by the combination of warm background SST annual cycle and ENSO SSTA, which modify large-scale conditions for TC genesis. Background SST can also strengthen ENSO evolution via the Bjerknes feedback, amplifying circulation impacts. This work provides a new perspective on the critical role of background SST annual cycle in shaping the ENSO–TC relationship, which is potentially helpful for improving seasonal TC prediction over the WNP.