A Numerical Study of the Kuroshio South of Japan

Abstract

A numerical model is used to study the interaction of the Kuroshio with boundary constraints south of Japan. The model consists of two layers with the lower layer inert, and is both nonlinear and quasi-geostrophic. The boundary constraints are highly idealized. The Kyushu Peninsula and its adjacent continental shelf have the form of a V-shaped wedge on an otherwise rectilinear coastline; the Izu Ridge is represented as a square island. Solutions almost always form a meander downstream from Kyushu. The dynamics of meander formation involve the interaction of a Rossby lee wave generated by the Kyushu Peninsula and a westward disturbance forced by the Izu Ridge. Three different quasi-steady paths have been identified. The first path has a meander between Kyushu and the Izu Ridge, passes to the north of the ridge, and is associated with a current of low-volume transport. The second path has a large meander between Kyushu and the Izu Ridge, passes to the south of the Izu Ridge, and always occurs when the volume transport is sufficiently large. The third path primarily meanders downstream of the Izu Ridge, and occurs for intermediate values of transport. Deceleration of the current decreases the wavelength of the meander and markedly increases its amplitude; conversely, acceleration increases. the meander wavelength and decreases its amplitude. The model does not possess a steady-state path without any meander; such a path only occurs during periods of intense acceleration. For intermediate values of transport the quasi-steady path selected by the model does not depend on transport alone, but depends on the nature of the spin-up of the solution as well. For example, when the current is turned on abruptly, the second path develops; when the current is turned on gradually, the first path develops. This behavior indicates that the model possesses multiple states of equilibrium.