The Penetration of Kuroshio Water into the Sea of Japan

Abstract

The nonlinear leakage of Kuroshio water (via the Tsushima Strait) into the Sea of Japan is examined using a “reduced gravity” analytical model. The flow in our conceptual model results from the zonal sea level difference between the Pacific Ocean and the Sea of Japan. The landmasses separating the Pacific from the adjacent Sea of Japan are represented by thin walls and the Tsushima Strait, whose width is larger than the Rossby radius, is represented by a gap. The gap is initially closed and the problem is treated as an adjustment process resulting from an abrupt opening of the gap. Steady analytical solutions for the final state associated with the adjustment are constructed using the integrated momentum equation on a β plane, conservation of energy, and potential vorticity. Also, a perturbation expansion in ε≡βL/f0 (the ratio of the change in the Coriolis parameter, as one moves from the gap to the boundary current separation latitude, to the Coriolis parameter at the center of the gap) is used. It is found that the leakage associated with the Kuroshio's penetration is only a function of the latitude of the gap, the separation latitude of the Kuroshio, and the stratification in the Pacific (i.e., the undisturbed depth of the Pacific upper layer, H, and the “reduced gravity,” gΔρ/ρ). As other studies for flows through broad gaps have demonstrated, the transport is controlled by the geostrophic flows upstream and downstream. However, in addition to the so-called “geostrophic control,” the flow is also controlled by β and, therefore, we term it “beta control.” Specifically, the penetration flux is given by which for values typical for the Kuroshio and the latitude of the Tsushima Strait, gives 3–4 (× 106 m3 s−1). This computed flux is much greater than the linear transport, In addition, it is approximately equal to the observed transport; the location of the observed penetrated current (immediately to the west of Japan) also agrees with the theoretical aspect.