The effect of large thermohaline steps on acoustic propagation near the northeast coast of South America

Abstract

The acoustical impact on transmission loss due to a thermohaline step profile has been studied using a typical thermohaline step profile taken off the northeast coast of South America in July 1983. Superimposed on the smooth canonical temperature and salinity profiles of the ocean are vertical “fine structure” (1–100 m) perturbations which can affect the acoustic transmission. One mechanism which can cause this “fine structure” is double diffusion (multi-component diffusion) in which each component has a different molecular diffusivity. Under the proper conditions, very dramatic thermohaline steps can occur in which very well mixed regions of 5–200 m thickness are separated by sharp, several meter thick, high gradient interfaces. The impact on the sound velocity profile (SVP) is to create adjacent regions of near isovelocity connected by a high gradient small transition region. Only since the advent of continuous profiling temperature and salinity sensors have these steps been detected; discrete water samples (“bottle casts”) usually did not reveal them. It is seen that transmission loss can be significantly affected even at low frequencies (50 Hz) by failing to include the detailed step structure in the SVP. Furthermore, SVP thinning algorithms using tolerances which seem to preserve the step structure in the SVP can also give erroneous results in the transmission loss.