Observations of cnoidal internal waves and their effect on acoustic propagation in shallow water

Abstract

Distinctive packets of periodic internal waves were observed during an experiment in the Gulf of Mexico. There was a 65-m-deep mixed layer overlying a thin strong density interface. A layer of weaker density stratification extended below the interface to the bottom, at a depth of 185 m. The waves had 2-10-m amplitudes, narrow frequency bandwidths with central frequencies of 8.5 cph, and they propagated in the upslope direction. The wave packets were observed on three consecutive days. They lasted about 3 h and were always observed at the same time of day, clearly in response to tidal forcing. A model of the time/space structure of the waves was tuned to match that of the observations, showing that the data are consistent with a cnoidal wave hypothesis. Observations of low-frequency acoustic propagation along two baselines show fluctuations that we hypothesize are due to interactions with the cnoidal waves. The fluctuations have spatial correlation scales (in the slantwise direction) on the order of 76 m. We simulate these effects using a time-step PE approach. We find that a mode-coupling resonance with the internal wave field results in elevated acoustic variability along a set of discrete spokes, emanating from the acoustic source. While acoustic variability tends to increase with range and with internal wave amplitude, tangential and radial correlation scales do not show a systematic dependence. The patterns in tangential and radial correlation scales show strong anisotropic patterns in azimuth, but little systematic trend in range.