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Improving the uncertainty of seawater density - absolute salinity relationships
by Dr. Marc Le Menn


The density of seawater rho is the most important thermodynamic property for modelling ocean circulation or water level anomalies. Its measurement is also a means of determining the absolute salinity SA of seawater, which is the quantity chosen by the authors of TEOS-10 (Thermodynamic Equation Of Seawater of 2010) to the detriment of the practical salinity SP, which remains the only quantity (without unit) that can be measured in situ. In order to be able to follow the variations linked to climate change, oceanographers have a very low tolerance on the measurement of rho: 0.0015 kg/m3 or 1.5 ppm. This tolerance is very close to the best measurements made in metrology laboratories at atmospheric pressure. The current uncertainties in the relationships between SA, temperature t and pressure p range from 0.007% to 0.012% at best, and do not meet the needs of oceanographers. They are also an obstacle to the development and calibration of instruments for in situ measurements. The TEOS-10 equation of state for calculating the specific volume (inverse of rho) is based on a function containing 72 coefficients, and the uncertainty in the resulting values is poorly established. Improved databases to establish the relationship rho = f(SA, t, p) would provide a better estimate of the uncertainties in the TEOS-10 calculation relationships. The most advanced in situ measurement instruments for SA are based on measurements of the refractive index of seawater n. Similarly, while the empirical relationship currently used to calibrate refractometers has an uncertainty of 0.4 ppm for distilled water at atmospheric pressure, it is only 80 ppm for seawater at elevated pressure. In order to meet the needs of oceanographers, much progress is therefore needed to improve the uncertainty in the fundamental relationships between rho, SA, p and n, when p is different from atmospheric pressure.


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Topic : Theme 1: Oceans and Hydrology.
Reference : T1-B17

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