Typhoon-induced strong surface flows in the Taiwan strait and pacific

Surface Velocity Program drifters drogued at 15 m depth were deployed in the Taiwan Strait (TS) and Luzon Strait in 2005 and 2006. Several drifters in the TS and the Pacific were fortuitously overrun by the typhoon Hai-Tang (July 2005) and Shan-Shan (September 2006), respectively. The drifter and QuikSCAT wind data clearly demonstrate that the surface current over the TS and the Pacific can change dramatically for a period of about two days due to the strong winds of a typhoon during its passage. Our results show that the area of storm-affected surface currents is considerably smaller for a weaker typhoon (category 2 Shan-Shan), about 300~400 km in radius, than for a stronger typhoon (category 5 Hai-Tang), about 800 km in radius. The maximum observed current speed in the TS was 1.7 ms^-1 (or 2.2 ms^-1 in net speed change) under the influence of Hai-Tang, and 2 ms^-1 in the Pacific under the influence of Shan-Shan. Drifter observations revealed the unusual phenomenon of flow reversal in the surface layer of TS and the Kuroshio induced by the typhoon passage. The effect of a typhoon on surface flows is amplified by the long, narrow geometry of the TS. Surface currents generated by wind forcing along the passage of a traveling typhoon can be explained by the Ekman drift.