Revised equation for sound speed in seawater consistent with long-range acoustic travel times.

Measurements of long-range acoustic travel times have suggested that the Del Grosso [(1974). J. Acoust. Soc. Am. 56, 1084-1091] (DG1974) and the internationally accepted Thermodynamic Equation of Seawater 2010 (TEOS-10, which is largely based on DG1974) sound-speed equations (SSE) in seawater give values at high pressure and low temperature that are too high. The revised SSE obtained here updates DG1974 and yields sound-speed estimates below 1000 m that are approximately 0.10 m/s slower. The SSE is derived by performing a Bayesian estimate that includes: (1) original laboratory observations, (2) sound-speeds derived from exceptionally precise travel-time measurements made during the 2016-2017 Canada Basin Acoustic Propagation Experiment (CANAPE) (± 0.03 m/s), (3) the uncertainty in the data, and (4) a heuristic prior parameter covariance matrix. The residuals indicate that the DG1974 and TEOS-10 SSEs do not fit the original laboratory observations within their reported uncertainties, and there is inconsistency between the laboratory observations and the CANAPE sound-speed estimates. The equation is validated by estimating sound-speed profiles from conductivity-temperature-depth casts obtained in connection with previous long-range travel-time measurements. The proposed equation fits the laboratory observations as well as the DG1974 SSE (root mean square difference of 0.06 m/s).