Sound Speed Perturbations Due to Internal Tides

Abstract

Internal tides are perhaps the most prevalent, and largest amplitude, internal gravity waves in the ocean. They are commonly generated by the scattering of surface tidal energy into internal modes when the surface tides propagate on to continental shelves. A portion of the internal tidal energy propagates into the deep ocean. One of the effects of internal tides on underwater acoustic propagation is to perturb the temperature and salinity versus depth profiles, and, thus, the sound speed versus depth profile. Starting with temperature and salinity versus depth profiles from near Eleuthera Island, the internal tide eigenfunctions, as well as the sound speed profile, have been computed for a water depth of 4500 m. Assuming a value for the vertical displacement amplitude of a first vertical mode internal tide, the perturbed temperature and salinity profiles have been computed. From these perturbed profiles, the consequent perturbed sound speed profiles have been computed for cases of maximum up and down internal tide displacement. The first mode internal tide has no vertical displacement at the sea surface and bottom and a maximum near the base of the main thermocline (depth of about 1200 m). Since the SOFAR axis is also near the base of the main thermocline, and since the pressure effects on sound speed are predominant below the SOFAR axis, the sound speed perturbation due to the first mode internal tide is greatest at a depth (about 550 m) above the SOFAR axis. An internal tide with a maximum vertical displacement of 10 m produces a maximum sound speed perturbation of ±0.7 m/sec.