Amplification of coastal-trapped waves resonantly generated by wind around Sado Island, Japan

Abstract

Current observations were made from 4 July 2009 to 6 March 2011 with an acoustic Doppler current profiler mounted on the seafloor near the northern tip of Sado Island, Japan, and an electromagnetic current meter off the western coast of the island to investigate wind-induced coastal-trapped waves (CTW). Oscillating alongshore flows with a period of about 2 days occurred intermittently in association with atmospheric disturbances throughout the observation period. The period of the current fluctuation changed from 48 to 60 h in both summer and winter, and the fluctuations were coherent with northward wind velocity fluctuations with a period of about 2 days around the island, even though northward wind fluctuations with periods of about 3, 4, and 6 days were predominant. Thus, the 2-day-period current fluctuations were selectively enhanced by the wind. Because the wavelength of internal Kelvin waves with a 2-day period was comparable to the distance around Sado Island (210 km), we inferred that the wind-induced CTWs around the island were resonant with the 2-day-period northward wind fluctuations. This inference was supported by the results of a numerical experiment performed with a two-layer model using realistic topography, which showed that resonance effectively occurred along the western coast of the island, where the water is deep and the continental shelf is narrow, and the resonance period was controlled by the ocean stratification strength and the shape of the island.