On the branching of the Kuroshio west of Kyushu

Abstract

Trajectories of satellite-tracked surface drifters released in the continental margin area west of Kyushu indicate a separation of the inshore side of the Kuroshio that appears to form the beginning of the Tsushima Current. The trajectories also indicate an anticyclonic sense of circulation in the area between the separated branch and the eastward directed Kuroshio main stream, consistent with long-term geomagnetic electrokinetograph measurements of surface currents, which show a broad southward flow along the west Kyushu coast. A theory is put forth that ties together these observed flow features. In the theory, the incidence is considered of an inertial current upon a step rise in topography (characterized by a step depth of H-0 to simulate the west Kyushu coastal bathymetry) in an f plane, two-layered, inviscid ocean with a quiescent lower layer. The inertial current is of a constant potential vorticity f/H, where H (>H-0) is the undisturbed upper layer depth and is bounded inshore by a free streamline along which the upper layer depth is a constant h(c) (