Articles | Volume 12, issue 1
Research article
18 Feb 2016
Research article |  | 18 Feb 2016

The sound speed anomaly of Baltic seawater

C. von Rohden, S. Weinreben, and F. Fehres

Abstract. The effect of the anomalous chemical composition of Baltic seawater on the speed of sound relative to seawater with quasi-standard composition was quantified at atmospheric pressure and temperatures of 1 to 46 °C. Three modern oceanographic time-of-flight sensors were applied in a laboratory setup for measuring the speed-of-sound difference δw in a pure water diluted sample of North Atlantic seawater and a sample of Baltic seawater of the same conductivity, i.e., the same practical salinity (SP = 7.766). The average δw amounts to 0.069 ± 0.014 m s−1, which is significantly larger than the resolution and reproducibility of the sensors and independent of temperature. This magnitude for the anomaly effect was verified with offshore measurements conducted at different sites in the Baltic Sea using one of the sensors. The results from both measurements show values up to 1 order of magnitude smaller than existing predictions based on chemical models.

Short summary
The variability of the relative salt composition in seawater may have significant influence on physical properties. Based on off-shore and laboratory measurements, this effect is quantified for the first time for speed of sound in Baltic seawater. Time-of-flight sensors with sufficient resolution were applied to resolve the small sound speed anomaly to values in the range of several cm s−1. The results suggest that the effect can be reasonably predicted by the recent equation of state (TEOS-10).