Effects of sound-speed equations on critical depths in the oceans

Abstract

Critical depths (and consequently depth excesses required for convergence zone propagation) are generally computed from depth, temperature, and salinity data. In 1972, V. A. Del Grosso published a 19-term formula that differs, up to 1.5 m/sec at depth, from the widely accepted 23-term equation of W. D. Wilson (1960). A simple equation to determine sound speed from temperature, salinity, and depth, instead of pressure, was developed for computations, employing a pocket calculator [Joint Oceanographic Assembly, Edinburgh, Sept. 1976]. To generate a practical equation, 15 worldwide deep stations of high quality were selected, and depths corresponding to a number of pressures were computed for each profile. A polynomial of only nine terms fits Del Grosso sound speeds for all stations to depths of 8000 m with a standard deviation of only 0.07 m/sec. The simplified equation facilitates comparisons with Wilson, since depth, temperature, salinity, and Wilson sound speed appear in practically every profile tabulation. Results will be presented for a number of realistic situations.