A numerical study of the Kuroshio-induced circulation in Tosa Bay, off the southern coast of Japan

We performed a numerical experiment for the circulation in Tosa Bay using a triply-nested 1/50-degree model. This study revisits the climatological features of the Kuroshio-induced circulation in Tosa Bay by comparison with previous studies based on hydrographic observations. The model well reproduces the climatological features previously reported and reveals the following new findings: long-term mean circulation in Tosa Bay is characterized by an anticlockwise circulation (AC) spreading over the bay. The AC exhibits a cold-dome structure with deep baroclinic velocity shears. The velocity shears can be accounted for mainly by geostrophic shear. In this regard, however, the gradient wind balance is maintained because an ageostrophic velocity attributed to the momentum advection is caused by the state where the AC is intensified. The primary condition for controlling the development and decay of the AC is the mesoscale onshore–offshore movement of the Kuroshio axis; the secondary condition is a high-frequency submesoscale variation such as a Kuroshio frontal wave, which can be regarded as noise for the primary condition. Sequential development and decay of the AC is frequently simulated when the Kuroshio takes a nearshore path, and the frontal waves propagate eastward and interact with coastal water in Tosa Bay. That is, the horizontal advection of relative vorticity due to the frontal wave propagation plays an important role in controlling the sequential response.