Trough‐Scale Slope Countercurrent Over the East China Sea Continental Slope Driven by Upwelling Divergence.

Observations have revealed the existence of persistent slope countercurrents (SCCs) that flow southwestward beneath the Kuroshio Current at several locations over the East China Sea (ECS) continental slope. It was not clear whether these flows are localized circulation features or segments of a trough‐scale circulation system in the Okinawa Trough (OT). We demonstrate that there indeed exists a potentially continuous trough‐scale SCC along the ECS slope that is associated with an OT‐wide cyclonic circulation using high‐resolution model simulations and physical interpretations. The detailed features of the deep OT circulation are illustrated by the trajectories of the Lagrangian drifters and the time‐varying distributions of passive tracers. The SCC in the ECS is characterized by its weak yet persistent nature, typically located in narrow sloping regions at the isopycnal layer of 26.6–27.3 kg m−3. It exhibits a characteristic speed of approximately O‐(1) cm s−1. Analyses and experiments suggest that the divergence of upwelling in the SCC layer (26.6–27.3 σθ surface) gives rise to lateral potential vorticity transport, ultimately driving the deep cyclonic circulation. Furthermore, the SCC also displays a substantial connection with the onshore intrusion of the Kuroshio Current, particularly to the northeast of Taiwan Island. The SCC may potentially play a crucial role in the transport of heat and nutrients, as well as in regulating sediment distributions within the deep OT. This mechanism offers fresh insights into explaining the presence of undercurrents in semi‐enclosed marginal seas. Plain Language Summary: The Okinawa Trough (OT) is a key link between the Pacific Ocean and the East China Sea (ECS) continental shelf. It was observed by previous studies that persistent southwestward slope countercurrents (SCCs) exist beneath the northeastward Kuroshio Current at several locations over the ECS continental slope. These countercurrents have been attributed to a variety of mechanisms based on analyses and interpretations of observations made at different locations. It is yet not clear whether these flows are localized circulation features or segments of an OT‐wide circulation system. In this study, we find that there indeed exists a trough‐scale SCC along the ECS slope that is associated with an anti‐clockwise circulation. Analyses indicate the OT experiences different vertical volume exchanges at the deep layer, which induces lateral exchanges of potential vorticity (a dynamically conserved quantity in an ideal and rotating fluid) and eventually drives the SCC. Numerical experiments are performed to show the validity of this process. The SCC may be of vital importance in transporting heat and nutrients and regulating the sediment distributions in the OT. This mechanism is potentially applicable to explain the undercurrent in similar semi‐enclosed marginal seas. Key Points: Over the entire East China Sea continental slope, the slope countercurrent beneath the surface Kuroshio Current is potentially continuousThe slope countercurrent is the western part of a deep cyclonic circulation in the Okinawa TroughThe upwelling divergence along the East China Sea continental slope is the main forcing mechanism of the slope countercurrent [ABSTRACT FROM AUTHOR]