Strong long-lived Kuroshio-shed anticyclonic eddies and their re-intensification in the northern South China Sea

The Kuroshio anticyclonic eddy shedding event occurs nearly every winter in the northeastern South China Sea (SCS). Between 1993 and 2023, 27 prominent Kuroshio-shed anticyclonic eddies were identified based on satellite altimetry data. These eddies propagated southwestward along the continental slope and typically dissipated near the Xisha Islands in spring, with an average lifespan of approximately 89 days. Notably, three exceptionally strong and long-lived eddies were observed in 2010, 2017, and 2021, each persisting for more than 180 days. Unlike the regular eddies that dissipated upon encountering the Xisha Islands, these long-lived eddies slightly moved eastward along the topography and re-intensified during summer. Eddy-current interactions and eddy mergers were identified as the primary mechanisms driving their re-intensification. Hydrographic observations revealed that the long-lived Kuroshio eddies evolved seasonally from surface-intensified to subsurface-intensified eddies, ultimately exhibiting a lens-shaped structure in the upper 300 m. The intense lens core was enclosed by a strong potential vorticity (PV) gradient, acting as a PV barrier that prevents water exchange between the eddy interior and its surroundings. Despite undergoing interactions that re-intensified the eddy, the eddy core maintained its high-salinity and low-PV characteristics. Such intrinsic eddies may play an important role in local air-sea interactions, heat-salt balances, and biogeochemical processes.