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ΣΑ = Δρ/0Σ– A Summary for Policy Makers2010ΣΑThe InternationalThermodynamicEquation of SeawaterPresent Practice for Data Storing Stays the SameData stored in national and international databasesshould, as a matter of principle, bemeasured values rather than derived quantities.Consistent with this, the working group recommendscontinuing to store the measured (in situ)temperature rather than the derived quantity,potential temperature. Similarly they recommendthat Practical Salinity (S P ) continue to bethe salinity variable that is stored in such databasessince S P is closely related to the measuredvalues of conductivity. This recommendation hasthe very important advantage that there is nochange to the present practice and so there is lesschance of transitional errors occurring in nationaland international databases because of the adoptionof Absolute Salinity (S A ) in oceanography(for more see page 9: Data Representations: Metadata requirements of TEOS-10).In 2010, the algorithm for calculating salinityincorporated for the first time more informationthan is contained in measurements of seawaterconductivity. The more accurate way of identifyingAbsolute Salinity everywhere in the oceanwas devised and incorporated into TEOS-10.The new equation incorporated the location ofthe conductivity measurements with chemicalanalysis from those regions into a new AbsoluteSalinity calculation. The working group alsoredefined how the properties of seawater arecalculated using this new Absolute Salinitymethod and combining it with the principlesbehind thermodynamics to form a single newthermodynamic equation for seawater.The re-evaluation of the 1980s equationsprovided seawater with a ‘Gibbs function’. Theprevious mathematical equations for determiningthe properties of seawater had not accountedfor water’s ability to transfer heat from warmerto cooler currents. Nor did the old equationsset a standard for comparing how difficult sucha transfer of energy might be, based on thewater’s inherent pressure and volume. The thermodynamicequation of seawater replaces theold equations with a recipe of computer algorithmsthat modellers crave.On 24 June 2009, the 25th Assembly ofUNESCO’s Intergovernmental OceanographicCommission (IOC) recommended that theoceanographic community adopt TEOS-10.The TEOS-10 method has already become anindustrial standard, approved by IAPWS. Forexample, any company interested in providingdrinking water for desert cities near the coast,is required to use the new method of calculationin building seawater desalination plants.The thermodynamic equation uses IAPWS-08and IAPWS-06 as the official descriptions forseawater and ice.Under TEOS-10 it is recognized that the compositionof seawater varies around the world andthat the thermodynamic properties of seawaterare more accurately represented as functionsof Absolute Salinity than of Practical Salinity.These new international standards were adoptedwhile recognizing that the techniques forestimating Absolute Salinity will likely improveover the coming decades. The algorithm forevaluating Absolute Salinity in terms of PracticalSalinity, latitude, longitude and pressure willbe updated on the website www.TEOS-10.orgfollowing peer-reviewed publications. Users ofthis software should always state in their publishedwork which version of the software wasused to calculate Absolute Salinity.4© CSIRO
.75179 γMotivation for an Updated ThermodynamicDescription of SeawaterThe Practical Salinity Scale and EOS-80, whichexpresses the density of seawater as a functionof Practical Salinity, temperature, and pressure,have served the oceanographic community for30 years.In recent years, however, the following aspectsof the thermodynamics of seawater, ice, andmoist air became apparent and suggested thatit was time to redefine the properties of thesesubstances. tionof State of Seawater (EOS-80) were notconsistent. The thermodynamic approacheliminates this problem. Properties of Water and Steam developed amore accurate and more broadly applicablethermodynamic description of pure water.Also higher accuracy measurements havebeen made of several properties of seawatersuch as (i) heat capacity, (ii) sound speed and(iii) the temperature of maximum density.These have been incorporated into the newthermodynamic description of seawater. standingof how variation in the compositionof seawater can change seawater density inthe different ocean basins. In order to furtherprogress this aspect of seawater, a standardmodel of seawater composition is needed toserve as a generally recognised reference fortheoretical and chemical investigations. being an integral part of the global heatengine points to the need for accurate expressionsfor the entropy, enthalpy and internalenergy of seawater so that heat fluxes can bemore accurately determined in the ocean andacross the interfaces between the ocean andthe atmosphere and ice (entropy, enthalpyand internal energy were not available fromEOS-80). tentdescription of the interactions betweenseawater, ice and moist air; in particular, theneed for accurate expressions for the latentheats of evaporation and freezing, both atthe sea surface and in the atmosphere. 1990 and TEOS-10 incorporates this revisionas well as the latest update from the InternationalUnion of Pure and Applied Chemistryfor the atomic weights of the elements.5
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