Branching of the Tsushima Current in the Japan Sea

Abstract

Three branches of the Tsushima Current are reproduced in a numerical model, and their formation mechanisms are studied. Two types of a two-layer, inflow-outflow model with a bottom slope along the Japanese coast are used. One has a bottom slope only in the lower layer (Model A), and the other has bottom slopes in both layers (Model B). Model B represents the typical situation in the Japan Sea, i.e., the main pycnocline intersects the bottom slope. The onshore side of the line where the pycnocline intersects the bottom slope has only one layer in Model B. Seasonal variation of inflow in the upper layer of the western half in the entrance section (the Tsushima Strait) is incorporated into the model. Three branches are formed in Model B and not in Model A. The first branch is the bottom-controlled steady current due to the topographicβ-effect on the upper-layer slope which exists in the one-layer region along the Japanese coast. The second branch is a temporal current which is formed along the offshore edge of the coastal one-layer region in association with the variation of inflow. The third branch is the steady western boundary current due to the planetaryβ-effect. These results compare favorably with observations in Part I of this study. The mechanism of formation of the second branch is examined in detail. This branch is caused by the propagation of the lowest two modes of the upper shelf wave caused by the topographicβ-effect on the upper-layer slope which are generated by the significant increase in inflow from June to August.