Ocean turbulence with application to acoustic coherence

Abstract

The coherence of long range underwater acoustic signals is strongly influenced by the complex transmission medium, in particular, by the power in the fluctuating temperature field. A statistical description of horizontal thermal microstructure in anisotropic turbulence is developed for application with an existing acoustic propagation theory. A two-term power law representation for buoyancy and convective forces is applicable for wavenumbers greater than those of the internal-wavefield, but less than those in the dissipation range. The model predicts a temperature power density spectrum which decays as −53 and −3 with wavenumber in the convective and buoyancy ranges, respectively. The power in the two ranges (i.e., the strength of the turbulence) is a function of depth and depends on the total rate of energy dissipation, the total rate of dissipation of temperature variance, and the Brunt-Väisälä frequency. Companion experimental data from 54 long horizontal tows at depths between 97.5 and 1454 m at two widely separated stations in the Bermuda area of the North Atlantic verify the theoretical predictions. The environmental effects on the theoretical acoustic correlation length are examined for limiting values of measured strengths of turbulence.