Shelf-break tidally induced environmental influences on acoustic propagation

Abstract

Continuous wave propagation in the 100–500 Hz band in littoral regions depends upon both time-dependent oceanography and bathymetry. The environmental influences interact nonlinearly in the acoustical time variation, especially since the diurnal tide surface height changes creates time-dependent total water depth. A submesoscale hydrodynamic model developed by Shen and Evans is used with tidal forcing and a simple shelf-break bathymetry to produce surface height variation and internal wave activity due to internal tide in a stratified ocean environment. A three-dimensional parabolic equation acoustic model is used to acoustically probe this environment at various bearings relative to the shelf break and the resulting internal tidal dynamics. In particular, the acoustical results are examined for three-dimensional effects such as horizontal refraction. First, the influence of bathymetry alone is shown, and then compared to the full environment due to hydrodynamic action. The relative influences will then be compared by various measures such as modal decomposition, acoustic energy summed over depth, and signal gain degradation. [This research is sponsored by the ONR.]