Kuroshio-induced wake in the lee of Green Island off Taiwan

摘要

The wake of Green Island was investigated in the alongshore flowing Kuroshio east of Taiwan, where current speeds range from 1 to 1.5 ms~(-1). Vertical profiling with shipboard lowered acoustic Doppler current profiler and conductivity-temperature-depth profiler revealed sizable anomalies in flow and water characteristics in the lee of Green Island. Two different stages of wake evolution were observed from two shipboard surveys. In the first stage, a recirculation developed leeward of the island, followed by a wavy (wave-like) tail that resembled a weak vortex street. In the second stage, a cold eddy probably originating from the leeward side of the island showed up 14 km downstream of the island. The wake water was colder, saltier, higher in chlorophyll-a concentration, and produced isopycnal doming up to 60 m. In the recirculation area, the relative vorticity, either positive or negative, was 10 times of planetary vorticity, and the horizontal divergence or convergence was O (10~(-4) s~(-1)) on average. Flow divergence and convergence in the wake were expected to form upwelling and downwelling zones, producing a vertical circulation that vertically displaces isotherms. High inverse Richardson number, produced by strong vertical shear of horizontal currents, was associated with intense overturning events in the wake. High vertical shear of horizontal currents drove the mixing. The dissipation rate of turbulent kinetic energy in the overturn regions is O (10~(-7)-10~(-5)) W Kg~(-1); the corresponding eddy diffusivity is O (10~(-3)-10~(-1)) m2 s~(-1). The wake water properties are vertically diffused via upwelling and turbulence and can be delivered downstream through eddy shedding or advection. The extent of downstream influence remains to be investigated.