Dynamics of the Kuroshio Separation, Instability, and Associated Eddy Formation off Southern Japan

Abstract

The Kuroshio is well-known for its variability south of Japan, where its complex dynamics lead to the formation of meandering jet and eddies along the stream. In this study, we investigated the relevant processes and physical mechanism that remain ambiguous, using a well-validated high-resolution China Sea Multiscale Ocean Modeling System and a process-oriented numerical modeling. We found that this variability along Kuroshio is chiefly governed by the dynamics of the boundary jet separation. Our analysis shows that the variability follows a sequential dynamical pathway. First, the Kuroshio separation is induced by an inverse along-stream pressure-gradient force associated with the highly nonlinear jet, which decelerates the nearshore flow and induces a coastal countercurrent. In wind-favorable events, Ekman transport can further reinforce this countercurrent, thereby promoting separation. Near protruding coastal capes, bottom pressure torque and topographic vorticity adjustment help anchor the separation point. An accumulation of shear vorticity on the shoreside/seaward side of the separated Kuroshio forms a cyclonic/anticyclonic eddy dipole. Through active dynamic adjustment along the jet, the increasing barotropic and baroclinic instabilities strengthen the meandering jet and eddy, thereby maintaining the separation. This study provides new insight into how boundary-current separation, topographic constraint, wind-favorable countercurrent formation, and jet instability jointly regulate western boundary current variability.