With consideration of the comprehensive effects of runoff from the Yangtze River, East China Sea background circulation, and tidal currents,a regional circulation model, including the Yangtze Estuary, Hangzhou Bay, and adjacent sea areas was established. The 2002/2003 El Ni~no event was chosen for study of the anomalies of circulation outside the Yangtze Estuary. The coastal and ocean current systems of the East China Sea and the current structure outside the Yangtze Estuary were accurately described. The results of vertical circulation analysis basically coincide with those of horizontal circulation analysis, showing that the circulation outside the Yangtze Estuary is mainly affected by the Taiwan warm current, runoff from the Yangtze Estuary, and the tide-induced residual current. The El Ni~no event weakens Yangtze Estuary circulation to a certain degree, and the impacts are more significant in summer than in winter. During the 2002/2003 El Ni~no event, the flux of the Taiwan warm current decreases, and the northward component of the Taiwan warm current decreases by half in February 2003 compared to the previous year. However, its path is relatively stable. With consideration of the comprehensive effects of runoff from the Yangtze River, East China Sea background circulation, and tidal currents, a regional circulation model, including the Yangtze Estuary, Hangzhou Bay, and adjacent sea areas was established. The 2002/2003 El Ni ~ no event was chosen for study of the anomalies of circulation an the Yangtze Estuary. The coastal and ocean current systems of the East China Sea and the current structure outside the Yangtze Estuary were exactly described. horizontal circulation, showing that the circulation is outside the Yangtze Estuary is mainly affected by the Taiwan warm current, runoff from the Yangtze Estuary, and the tide-induced residual current. The El Ni ~ no event weakens Yangtze Estuary circulation to a certain degree, and the impacts are more significant in summer than in winter. During the 2002/2003 El Ni ~ no event, the flux of the Taiwan warm cu rrent decreases, and the northward component of the Taiwan warm current decreases by half in February 2003 compared to the previous year. However, its path is relatively stable.