An upper ocean numerical model of the North Pacific is used to investigate the influence on western boundary current separation of different wind stress climatologies, variation of net surface buoyancy forcing, and flow exchange with the Sea of Japan. It was found previously (Cherniawsky and Holloway, 1991) that the modeled Kuroshio splits into two branches near 35¿N. The stronger (Kuroshio-like) flow separates east, while a weaker (East Japan) coastal current continues to about 42¿N. The latter is opposite to observed flow. The present study explores possible reasons for this discrepancy. When we use the Harrison (1989) instead of the Hellerman and Rosenstein (1983) monthly wind stress data, the model Oyashio front moves south by about 200 km. This frontal shift is not sufficient to reverse the direction of the East Japan current. Increasing buoyancy loss at high latitudes also enhances a southward intrusion of the Oyashio front, which is consistent with thermocline theories. However, this effect is inhibited by a small subpolar gyre and stability of upper layers in the North Pacific. Separation latitude of the model western boundary current was found to be sensitive to the opening of the Sea of Japan. The latter results in a Tsugaru Strait outflow driving a southward buoyant jet along the east coast of Japan and Kuroshio separation near 36¿N, in better agreement with observations. ¿ American Geophysical Union 1993