Nonlinear Modulation of Western North Pacific Tropical Cyclone Season Start by El Niño‐Southern Oscillation

Abstract This study investigates changes in tropical cyclone (TC) season start over the western North Pacific (WNP) as modulated by El Niño‐Southern Oscillation (ENSO). In 1979–2024, there is a significant nonlinear relationship between the WNP TC season start and the preceding November–January ENSO. Relative to ENSO neutral years, there is a significant delay in the TC season start in both El Niño and La Niña decaying years, corresponding to a significant reduction in basinwide TC frequency during January–May. In El Niño decaying years, TC genesis is suppressed over almost the entire WNP. In La Niña decaying years, the region with significant TC genesis decreases over the eastern WNP is of a larger magnitude and covers a larger area than that with significant increases over the western WNP. These TC genesis changes can be well described by changes in the dynamic genesis potential index, which is predominantly controlled by changes in 500‐hPa vertical velocity following El Niño and La Niña. Following El Niño, basinwide descending motion results from the El Niño‐driven warming of the Indian Ocean through the Indo‐western Pacific Ocean capacitor mechanism. By comparison, following La Niña, descending anomalies over the eastern WNP and ascending anomalies over the western WNP are driven by an anomalous meridional circulation cell over the equatorial Pacific and the equatorial Indian Ocean.