Abstract
Large, transient increases in underwater noise intensity (noise bursts) were repeatedly observed during the ASIAEX experiment on a 48-hydrophone array consisting of vertical and horizontal line arrays. The arrays were located in the South China Sea in the area known for exceptionally strong nonlinear internal waves (NIWs). The ASIAEX experiment was intended to study sound propagation in the presence of NIWs and featured extensive, concurrent acoustic and oceanographic observations. The NIWs propagating past the acoustic array site were characterized using water temperature measurements with a nearby thermistor chain. Remarkable correlation is found between noise intensity increases on the vertical array hydrophones and the NIW presence. Intensity of the noise bursts strongly depends on the hydrophone depth. The low acoustic frequencies below 30–40 Hz are primarily responsible for the noise enhancement during the NIW passage. Analysis of the spectral properties and the depth dependence of the noise intensity suggests the flow noise due to NIW-induced currents as the physical mechanism of the noise bursts in the South China Sea. [Work was supported, part, by ISF award 946/20.]
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