The effects of the surface heat flux and wind forcing on the East Sea (Japan Sea) circulation and the branching of the Tsushima Warm Current were investigated using a three-dimensional circulation model (RIAMOM). For the numerical experiments with surface heat flux, regardless of wind forcing, the Tsushima Warm Current, the Nearshore Branch, and the East Korea Warm Current were well simulated in the southern part of the East Sea. Furthermore, the cold water mass of 1–2 °C was formed at a depth of 200–250 m because of the convection of the upper layer and ventilation that actively occurs in the northern part of the East Sea. However, in the experiments without surface heat flux, the East Korea Warm Current was not simulated. The Tsushima Warm Current through the Korea/Tsushima Strait flows mostly northeastward along the Japanese coast as the Nearshore Branch. Moreover, the cold water mass was not formed at the northern part of the East Sea, thus the 1–2 °C cold water was distributed below 300–400 m depth with weak ventilation. The upper layer in the southern part of the East Sea, ranging from the surface to the bottom of the pycnocline, was thicker than the case of the surface heat flux forcing. Since the Tsushima Warm Current flowing through the Korea/Tsushima Strait is dominantly affected by the continental slope of the Japanese coast, the current does not lead to formation of the East Korea Warm Current as a western boundary current. If the effect of the continental slope of the Japanese coast is not applied, the Tsushima Warm Current flows only along the Korean coast as a western boundary current. The numerical experiments reveal that the surface heat flux serves as a critical external forcing which controls the topographic and planetary beta effects on the upper layer circulation in the southern part of the East Sea. Therefore, if the surface heat flux is changed due to global warming in the future, the distribution of surface currents is also expected to change in the southern part of the East Sea.
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