Kuroshio path variation south of Japan: 1. Barotropic inflow‐outflow model

Abstract

We examined the dynamics of the variation in the path of the Kuroshio current along the south coast of Japan by use of a barotropic inflow‐outflow ocean model. The model basin has an inclined coastline and includes a part of the East China Sea so that we could investigate the effect of Kyushu on the Kuroshio. We obtained three regimes of the Kuroshio's path variation according to the magnitude of its inlet velocity Vmax, similar to the previous study. The Kuroshio takes a straight path along the south coast of Japan in regime I with a low Vmax, a meandering path in regime II with a high Vmax, and both of these paths (multiple‐equilibrium state) in regime III with an intermediate Vmax. The transition from the straight state to the meandering state occurs with an increase in Vmax from regime I (III) to II, preceded by an eastward progression of a small meander which is produced as a separated vortex by the nonlinear effect of the flow and the effect of Kyushu. It takes 120 days, consistent with observations. The nonlinearity of the flow has two effects in the transition. One causes a downstream shift of the small meander, and the other enlarges it by pulling out positive vorticity from the no‐slip coast. With an increase in the meander scale due to the latter effect, the former effect becomes small to be balanced with the beta effect, causing an upstream shift of the meander. Thus a large‐meander path is formed. A small meander does not progress eastward to develop into a large meander until Vmax exceeds a critical value. This behavior causes a multiple‐equilibrium state in Regime III and only the meandering path in Regime II.