The flow system in the Japan Sea caused by a sea level difference through shallow straits

Abstract

We investigate how the current system through the Japan Sea is driven and what determines the volume transport. We suppose that a part of the difference in geopotential anomaly between the subtropical and subpolar gyre is converted into a barotropic sea level difference across the three shallow straits which connect the Japan Sea with the Pacific and that this difference is the primary driving force of the current system. Then, we examine the flow under the condition that there is a constant sea level difference between two oceans connected through a shallow strait. We found that the strait acts as a source of arrested shelf waves or steady coastal flows for the timescale beyond the inertial period; in the northern hemisphere, steady flows are established along the shelves with the coast to the right (left) in the ocean of low (high) sea level. We apply this notion to the current system through the Japan Sea. The Tsushima nearshore branch, the Tsugaru coastal mode, and the Soya Current can all be interpreted as a coastally trapped flow whose source is the upstream strait. Further, a series of northeastward flows along the South and East China Seas shelves should be interpreted as coastally trapped flows whose source is the downstream Tsushima Strait. Numerical model experiments incorporating the realistic topography also simulate the observed flow fields. The volume flux through each strait being limited geostrophically, relations between the sea level difference and volume transport can be represented by simple formulas.