A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August-October 1990. The wind direction reversed in mid-September, indicating the onset of northeast monsoons. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in the response to the wind change. ¿ American Geophysical Union 1992