The Annual Cycle of the Japan Sea Throughflow

The mechanism responsible for the annual cycle of the flows through the straits of Japan Sea is investigated using a two-layer model. Japan Sea is one of the marginal sea located in the western North Pacific that is separated from the Pacific by the islands of Japan. Three narrow and shallow straits, the Tsushima, Tsugaru, and Soya Straits, connect this sea towards the Pacific Ocean and Okhotsk Sea and observations show that the flow through these three straits vary annually with a maximum transport in summer-fall and a minimum transport in winter. The variability is large for Soya (north) and Tsushima (south) Straits but weak for the Tsugaru Strait (middle). We find the subpolar winds located to the north of Soya Strait to be the primary forcing agent of this annual cycle rather than the subtropical winds located to the east of Japan. The subpolar winds generate baroclinic Kelvin waves that perturb the sea surface height at the Soya Strait, cause barotropic adjustment to occur within the Japan Sea, and change the flow at the other straits. The shallow topography at the straits plays an important role. This mechanism explains why the annual cycle at the three straits occur almost synchronously. We also find the around-island integral constraint a useful tool for explaining how the magnitude of the annual cycle at the three straits are controlled. The theorem show the magnitude and direction of the flow controlled largely by the ratio of the meridional length of the two islands that is bounded by the three straits..