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 H0 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 (>H0) is the undisturbed upper layer depth and is bounded inshore by a free streamline along which the upper layer depth is a constant hc (<H). The steady state downstream configuration of a separation branch and a deflected main stream along the step is connected to the upstream approach current through the consideration of the constancy of the Bernoulli function and the conservation of potential vorticity and mass. In the immediate neighborhood of the incidence, a complicated flow region is anticipated in which recirculation is featured and an anticyclonic gyre is expected further onto the step. The volume‐integrated balance of along‐step momentum of the constant potential‐vorticity flow allows the determination of the angle the branch current makes with the step as a function of the angle of incidence and two depth ratios, hc/H and H0/H. The bifurcation quotient, defined as the ratio of the branch current transport to that of the approach current, is found to be also such a function, except it is independent of the incidence angle. These functions yield results that are qualitatively in agreement with the observed bifurcation of the Kuroshio west of Kyushu that gives rise to the Tsushima Current.