During the 1997--1998 El Ni¿o event, the average sea surface temperature (SST) in the Taiwan Strait (TWS) in the winter was ~1.4¿C higher than that of the winter climatological mean. The areal ratio of the warm water (≥2¿C above the regional mean) to the cold water (≥2¿C below the regional mean) in the TWS increased by 25% while the area of the eutrophic water (chlorophyll a >1 mg m-3) was halved. Field observations also indicate that the mixed layer in the TWS became more nutrient-poor during this winter. These observations are consistent with a diminished advection of the cold and eutrophic Zhe-Min Coastal Water, and, concomitantly, an expansive intrusion of the warm and oligotrophic South China Sea Warm Current/Kuroshio Branch Water to the TWS as the northeast monsoon was weakened. Thus, El Ni¿o events potentially can have significant ecological impacts on the TWS.