Observations of High-Frequency Internal Waves in the Southern Taiwan Strait

Based on in situ hydrographic measurements at a station at about 60 m depth near the local shelf break in July 2011 and a satellite image of the sea-surface roughness, we present the evidence of existence and characteristics of high-frequency internal waves (HIWs) in the southern Taiwan Strait. Variations of the thermohaline structure at the observational site were revealed by repeating conductivity-temperature-depth (CTD) profiling measurements of every 2 hours, while fluctuations of the isothermals due to the passage of HIWs were recorded by continuous CTD measurements with a probe positioned approximately in the middle of the pycnocline. A fast sampling frequency of 8 Hz allows the structure of the HIWs to be captured in great detail. The waves were depression waves, with a period of about 6.2 mm and an amplitude of about 25 m. The propagation speed of the waves is estimated to be 0.56 m/s by solving the KdV equation with the observed background stratification. The frequent occurrence of HIWs in the Taiwan Strait is evidenced by the analysis of a MODIS true color image of the sea-surface roughness. The possible generation mechanism of the HIWs is discussed based on a MODIS image and the perturbed KdV equation.