The International thermodynamic equation of ... - unesdoc - Unesco

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