Abstract
Dynamics of ocean current-induced island wake has been an important issue in global oceanography. Green Island, a small island located off southeast of Taiwan on the Kuroshio path was selected as the study area to more understand the spatial structure and temporal variation of well-organized vortices formed by the interaction between the Kuroshio and the island. Sea surface temperature (SST) and chlorophyll-a (Chl-a) concentration data derived from the Himawari-8 satellite and the second generation global imager (SGLI) of global change observation mission (GCOM-C) were used in this study. The spatial SST and Chl-a variations in designed observation lines and the cooling zone transitions on the left and right sides of the vortices were investigated using 250 m spatial resolution GCOM-C data. The Massachusetts Institute of Technology general circulation model (MITgcm) simulation confirmed that the positive and negative vortices were sequentially detached from each other in a few hours. In addition, totals of 101 vortexes from July 2015 to December 2019 were calculated from the 1-h temporal resolution Himawari-8 imagery. The average vortex propagation speed was 0.95 m/s. Totals of 38 cases of two continuous vortices suggested that the average vortex shedding period is 14.8 h with 1.15 m/s of the average incoming surface current speed of Green Island, and the results agreed to the ideal Strouhal-Reynolds number fitting curve relation. Combined with the satellite observation and numerical model simulation, this study demonstrates the structure of the wake area could change quickly, and the water may mix in different vorticity states for each observation station.
Highlights
The increased surface Chl-a concentration and decreased sea surface temperature (SST) are induced by upwelling from deep-layer waters in the island wakes [5,9]
The maximum cross-correlation (MCC) method was applied to hourly Himawari-8 SST images to calculate the incoming surface current velocity for Green Island (22.5◦–22.7◦N, 122.3◦–122.5◦E)
The principle of the MCC method is to find the maximum correlation between a template image of SST patterns and the selected search image in the time interval
Summary
Kuroshio passes Green Island and causes a change in flow fields, such as the Von Kármán vortex street [5,6,7,8,9]. In the lee of the island, a high chlorophyll-a (Chl-a) concentration and low sea surface temperature (SST) wake region can be formed [8]. This recirculation area contained a cyclonic/anticyclonic vorticity pair accompanied by a density overturn and water upwelling [9]. The increased surface Chl-a concentration and decreased SST are induced by upwelling from deep-layer waters in the island wakes [5,9]. Like a supply depot for marine life, the SST fronts between the wake area and the Kuroshio could provide
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