Anticyclone Eddies Favor the Genesis of Off-Season Tropical Cyclone in the Western North Pacific

Mesoscale ocean eddies, which typically exhibit different heat and atmospheric conditions to their surroundings, significantly affect the path and intensity of tropical cyclones (TCs). By analyzing 27 years (1993-2019) of eddy and TC data, here we show that mesoscale eddies also play non-negligible roles in the genesis of TCs (TCG). During the TC off-season months of December to May, TCs generating over anticyclone eddies (AEs) clearly outnumber those over cyclone eddies (CEs) (14 vs. 6) in the western North Pacific, and featuring a further north location. Composite analysis also exhibits a higher genesis potential index (GPI(ocean)) in AEs than that in CEs, indicative of a higher TCG probability over AEs especially under a relatively cooler oceanic background, which is mainly attributed to the eddy-associated warming in the upper ocean. These results motivate future studies on better forecasting regional TC activities by considering the impacts of ocean eddies in air-sea models.