Dynamics of Kuroshio separation and associated eddy formation and instability off southern Japan

The Kuroshio is well-known for its variability south of Japan, where it often meanders and forms stable eddies near the coast. In this study, we investigate the relevant process and physical mechanism using a well-validated high-resolution China Sea Multiscale Ocean Modeling System (CMOMS, https://odmp.hkust.edu.hk/cmoms/) and a process-oriented modeling. We found that the variability is chiefly governed by the dynamics of Kuroshio separation. The analysis shows that Kuroshio separates from the coast and forms eddy by inverse pressure gradient force due to both high nonlinearity of Kuroshio and the inverse wind-driven Ekman transport. Typically, the separation occurs near protruding capes where bottom pressure torque (BPT) injects inverse vorticity to the separated Kuroshio. Accompanying the separation, an accumulation of shear vorticity from shoreside/seaside of the separated Kuroshio forms a cyclonic/anticyclonic eddy dipole. During the course of active dynamics adjustment, increased barotropic and baroclinic instability strengthen the eddy which, in turn, stabilize and persist the separation. The study provides a new insight into the variability dynamics of western boundary current.