Review of acoustical and optical techniques to measure absolute salinity of seawater

The salinity of seawater is of fundamental importance in climate studies, and the measurement of the variable requires high accuracy and precision in order to be able to resolve its typically small variations in the oceans with depth and over long-time scales. This is currently only possible through the measurement of conductivity, which has led to the definition of a Practical Salinity scale. However, seawater is also composed of a large number of non-conducting substances that constitute salinity anomalies. Differences of the ratios of the constituents of sea salt from the Reference Composition may also change salinity anomalies. The establishment of formulae for calculating the thermodynamic properties of seawater has led to the definition of the concept of Absolute Salinity (S-A), which includes such anomalies and is similar in approach to the notion of density. Although the routine in situ measurement of S-A is still a huge challenge, numerous developments based on acoustic techniques, but above all, refractometry, interferometry or complex fiber optic assemblies, have been tested for this purpose. The development of monolithic components has also been initiated. The measurement of the refractive index by these techniques has the advantage of taking into account all the dissolved substances in seawater. This paper reviews the difficulties encountered in establishing theoretical or empirical relations between S-A and the sound velocity, the refractive index or the density, and discusses the latest and most promising developments in S-A measurement with a particular focus on in situ applications.

Automated summary

The salinity of seawater is crucial for climate studies, and accurately measuring this variable is challenging due to small variations over long time scales. Currently, conductivity measurement is used to define practical salinity, but non-conductive substances and differences in sea salt composition can cause salinity anomalies. The concept of absolute salinity (S-A) has been introduced to account for these anomalies. Recent developments in acoustic techniques, refractometry, and fiber optic assemblies have shown promise in in situ absolute salinity measurement.