Influence of different La Niña decay types on tropical cyclone genesis over the western North Pacific

This study investigates changes in tropical cyclone (TC) genesis over the western North Pacific (WNP) during two different types of La Niña decaying events: rapidly-decaying La Niña and slowly-decaying La Niña. These events are selected based on the evolution of Niño-3.4 sea surface temperature anomalies. TC genesis changes during these two La Niña decaying types exhibit both similarities and differences. For both types, TC genesis significantly increases over the northwestern quadrant of the WNP (15°-30°N, 125°-145°E), while it significantly decreases over the southeastern corner of the WNP (5°-20°N, 165°E-180°). Over the region spanning 15°-30°N and 145°-165°E, TC genesis is suppressed during rapidly-decaying La Niña events, primarily due to anomalous upper-level convergent flow. Over this same region in slowly-decaying La Niña events, TC genesis is enhanced due to both increased lower-level vorticity and upper-level divergence. Over the region spanning 5°-15°N and 140°-165°E, enhanced TC genesis in rapidly-decaying La Niña events is linked to reduced vertical wind shear, while suppressed TC genesis in slowly-decaying La Niña events is mainly the result of reduced lower-level vorticity. • Opposite changes in tropical cyclone genesis occur for two sub-regions in the western North Pacific between the two La Niña decay types. • East–west and south-north shifts of mean tropical cyclone genesis location are observed for rapidly-decaying and slowly-decaying La Niña, respectively. • Large-scale dynamical conditions primarily drive the observed tropical cyclone genesis differences between the different La Niña decay types.