STRONG VARIATIONS OF THE ACOUSTIC NOISE SPECTRA ACCOMPANYING INTERNAL WAVE SOLITONS

This paper investigates the nature of rapid, strong (20–40 dB), broadband variations of the acoustic noise intensity that were observed with moored hydrophones in the Shallow Water 2006 experiment. The experiment took place on the Atlantic shelf off New Jersey (USA) in an area with high levels of internal gravity wave activity. In particular, approximately twice a day trains of tidally generated nonlinear internal gravity waves (NLIW) propagated in the general upslope direction through the area, where a number of single hydrophone units (SHRUs), an L-shaped hydrophone array SHARK, and a number of thermistor chains were moored. Each NLIW train consisted of up to ten separate soliton-like features with peak isothermal displacement of 10–15 m. The thermistors provided 4-D sampling of the ocean variability, which allowed us to estimate the wavefronts, shape, and evolution of propagating NLIW trains and model NLIW activity at the hydrophone locations. For a number of NLIW events and for various hydrophones, we find a strong correlation between noise intensity spikes recorded by a hydrophone and NLIW-induced thermocline displacement at the hydrophone position. Noise intensity spikes tend to occur when the near-bottom current velocity is at maximum. Various possible physical mechanisms of the noise intermittency have been analyzed. Sediment saltation is identified as the most likely cause of the observed variations in the noise spectra.