Tropical Cyclone-Induced Downwelling: Observational Evidence for an Overlooked Mechanism

Abstract Tropical cyclones (TCs) cause intense natural hazards and drive upper ocean cooling via vertical mixing and upwelling through a series of processes involving air-sea interaction. Among these processes, TC-induced upper ocean warming and the underlying mechanisms have rarely been noted. Here, we analyzed upper ocean responses to 32 TC events observed by two buoys in the western North Pacific from 2016 to 2021. Supplemented with numerical experiments, we suggest that downwelling, a long-neglected response, frequently occurs around the radius of the 34-knot wind speed during and after TC passage. Downwelling was identified via pronounced warming anomalies under a shallow mixed layer depth. We further found that a negative wind stress curl and current-induced convergence are two primary mechanisms of downwelling. These findings highlight downwelling existence, which is crucial for understanding the relationship between TCs and biogeochemical processes in the upper ocean.