Branching Mechanism of the Tsushima Current in the Korea Strait

Abstract

Hydrographic studies show the seasonal variation of the East Korean Warm Current (EKWC), which is a branch of the Tsushima Current along the Korean coast. To understand the dynamics of the branching mechanism of the TC in the Korea Strait, a hydraulic model with two active layers was investigated in a rectangular strait with varying depth. When the lower cold water flows southward in a shallow meridional channel from the deep northern basin, it separates from the eastern boundary because of the sloping bottom to conserve potential vorticity. After separation, the lower layer hugs the western boundary as the channel becomes shallow. In a region where the lower layer is absent due to separation, the northward flow in the upper layer has a positive relative vorticity to conserve potential vorticity because the bottom topography becomes deeper from south to north. The northward velocity has its maximum on the eastern boundary. This mechanism may explain the formation of the branch along the Japanese coast. The upper layer along the western boundary experiences shrinking of its water column because of the presence of the lower layer, and negative relative vorticities are induced to conserve potential vorticity. The negative relative vorticity intensifies the northward flow of the upper layer near the western boundary. This is believed to be the causal mechanism of the EKWC. If the top of the lower layer in the basin is deep, such as it is in winter, the lower layer cannot reach the strait since the Bernoulli potential of the lower layer is small. This may explain why the EKWC is absent in winter.