ON THE BRANCHING OF THE KUROSHIO WEST OF KYUSHU

Trajectories of satellite-tracked surface drifters released in the con tinental margin area west of Kyushu indicate a separation of the insho re side of the Kuroshio that appears to form the beginning of the Tsus hima 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 e lectrokinetograph measurements of surface currents, which show a broad southward flow along the west Kyushu coast. A theory is put forth tha t ties together these observed flow features. In the theory, the incid ence is considered of an inertial current upon a step rise in topograp hy (characterized by a step depth of H-0 to simulate the west Kyushu c oastal bathymetry) in an f plane, two-layered, inviscid ocean with a q uiescent 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 d epth is a constant h(c) (<H). The steady state downstream configuratio n 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 pot ential vorticity and mass. In the immediate neighborhood of the incide nce, a complicated flow region is anticipated in which recirculation i s 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 bra nch current makes with the step as a function of the angle of incidenc e and two depth ratios, h(c)/H and H-0/H. The bifurcation quotient, de fined as the ratio of the branch current transport to that of the appr oach current, is found to be also such a function, except it is indepe ndent of the incidence angle. These functions yield results that are q ualitatively in agreement with the observed bifurcation of the Kuroshi o west of Kyushu that gives rise to the Tsushima Current.