Seasonal and interannual variations of surface current in the southern Taiwan Strait to the west of Taiwan Shoals

Seasonal and interannual variations of surface currents in the southern Taiwan Strait to the west of Taiwan Shoals were investigated by using measurements of high frequency(HF)ground wave radars from January 2006 to April 2009.The results demonstrate that surface currents in the water channel to the west of Taiwan Shoals are composed of a significant,seasonally fluctuating component and a relatively stable northward component.Forced by the East Asia monsoon,the annual variation of the surface longshore current is linearly correlated with the longshore wind.Behind the seasonal signal is a stable northward flow with speeds of O(10 cm/s).The observations also show that surface currents in the area are subject to distinct interannual variation,and the southward surface flow was more profound in winter 2007/2008 than in other winters.Observations from bottom-mounted ADCP also indicated that,in that winter,longshore currents in the west side of the Taiwan Strait are very different from the previous winter.The northward flow appeared much weaker,the currents were southward in most layers,and the interannual differences were observed throughout the water column.Hydrographic observations in the same period suggest that the large-scale southward intrusion of the Zhe-Min coastal water is the direct cause of the interannual difference.The enhancement of the East Asian monsoon during the 2007/2008 La Ni a event is considered as the main reason for the winter current anomalies in the study area.