Evolution of Kuroshio-shed anticyclonic eddies in the South China Sea

The South China Sea (SCS) is a large semi-enclosed marginal sea between East Asian continent and the West Pacific Ocean. Originating from the North Equatorial Current, the Kuroshio is the strongest western boundary current in the North Pacific. When it flows northward along the east Philippine coast, a branch of the Kuroshio intrudes northwestward into the SCS through Luzon Strait, significantly affecting the temperature, salinity, circulation, and eddy generation in the SCS. Previous studies have shown that Kuroshio intrusions are frequently accompanied by anticyclonic eddy shedding in the northern SCS. Based on satellite altimeter data and cruise observations, we explored the statistical characteristics and evolution of Kuroshio-shed eddies in the SCS. The Kuroshio eddy shedding events occur nearly annually, especially in boreal fall and winter. The shedding eddies propagated southwestward along the continental slope and generally dissipated around the Xisha Islands in spring, while some of them re-intensified east of Xisha Islands and survived until autumn. Further analysis found that eddy merging provides energy for re-intensification of long-lived Kuroshio-shed eddies in the SCS. Combined reanalysis data, we further investigated the evolution of eddy vertical structure. The Kuroshio anticyclonic eddies transited from surface-intensified eddies to subsurface-intensified eddies in the SCS with seasonal changes, exhibiting a unique surface cold-core and vertical lens-shaped structure. Changes of sea surface heat flux and eddy-induced Ekman pumping are conducive to the formation of lens-shaped structure.