Hsu, Tai-Wen (National Taiwan Ocean University) | Chou, Meng-Hsien (National Cheng-Kung University) | Chao, Wei-Ting (National Taiwan Ocean University) | Liang, Shin-Jye (National Taiwan Ocean University)
Green Island locates in the typhoon active southeastern Taiwan coastal water. A high resolution (250–2250 m) shallow-water flow model is used to investigate the effect of typhoon Soulik on the hydrodynamics of Kuroshio and Green Island wakes. Simulation results indicate salient characteristics of Kuroshio and meandering downstream island wakes seems less affected by typhoon Soulik. Moreover, Kuroshio currents increase when flow is in the same direction as the counterclockwise rotation of typhoon, and vice versa. This finding is in favorable agreements with the TOROS (Taiwan Ocean Radar Observing System) observed data.
According to Zdravkovich (2003) flow past a bluff obstacle with the Reynolds number Re = u∞L/vt in the range of 50 and 800, a well-organized downstream wakes occur, where u∞ is the characteristic flow velocity, L the characteristic length, and vt the eddy viscosity of fluid. Flow becomes periodic with detachment of the free shear layers, and consequently alternate shedding of vortices. The periodic phenomenon is referred to as the vortex shedding, where the anti-symmetric clockwise and counterclockwise wakes pattern is called the von Karman vortex streets (Zdravkovich, 2003).
Vortex streets occur frequently in the atmosphere and oceans (Nunalee and Basu, 2014; Chelton et al., 2011). The phenomenon of vortex shedding behind bluff bodies has long been of interest to the fluid dynamics community and has been intensively studied by many researchers (Roshko, 1955; Tritton, 1959; Williamson, 1996; Zdravkovich, 2003). This kind of phenomenon is often captured by satellite imagery (Hubert and Krueger, 1962; Thomson et al., 1977; Li et al., 2000; Young and Zawislak, 2006; Zheng et al., 2008), field measurement (Barkley, 1972), and numerical modeling (Ruscher and Deardorff, 1982; Wolanski et al., 1984; Heywood et al., 1996; Dong et al., 2007).
The Kuroshio, a western boundary current of the sub-tropic North Pacific Ocean, originates from the North Equatorial Current and flows northward to the eastern coast of Taiwan. The passage of the Kuroshio mainstream parallels to the eastern shoreline of Taiwan. Green Island is located at (121°28'E, 22°35'N) and is 40 km off the eastern coast of Taiwan. The climatology of the Kuroshio velocity revealed from in-situ measurements shows that Green Island is approximately located in the mainstream of the energetic Kuroshio passage and acts as an obstacle to the stream. Hence downstream island wakes are prone to occur. Characteristics of the vortex streets downstream Green Island can be found from the MODIS (moderate resolution imaging spectroradiometer) satellite images and in-situ measurements (Chang et al., 2013). Seasonal variation of mainstream, current patterns, and transport of the Kuroshio in the east of Taiwan has been numerically investigated (Hsin et al., 2008). Spatial and temporal scales of downstream Green Island wakes due to passing of the Kuroshio have been numerically studied (Liang et al., 2013).