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

ΣΑ = Δρ/0The InternationalThermodynamicEquation of Seawater– 2010ΣASummary for Policy Makers0In order to improve the determination of AbsoluteSalinity oceanographers need to begin collectingand storing values of the salinity anomalyδS A =S A −S R based on measured values of density(such as can be measured with a vibratingtube densimeter). The 4-letter GF3 code DENSis currently defined for in situ measurements orcomputed values from EOS-80. It is recommendedthat the density measurements made witha vibrating beam densimeter be reported withUsing the TEOS-10:Algorithms and Programmesthe GF3 code DENS along with the laboratorytemperature (TLAB in °C) and laboratory pressure(PLAB, the sea pressure in the laboratory,usually 0 dbar). From this information and thePractical Salinity of the seawater sample, the absolutesalinity anomaly δS A =S A −S R can be calculatedusing an inversion of the TEOS-10 equationfor density to determine S A . For completeness,it is advisable to also report δS A under the newGF3 code DELS.The computer software to calculate the variousthermodynamic quantities is available from twoseparate libraries, the Seawater-Ice-Air (SIA) libraryand the Gibbs-SeaWater (GSW) library. Thefunctions in the SIA library are generally availablein basic-SI units, both for their input parametersand for the outputs of the algorithms. Some additionalroutines are included in the SIA libraryin terms of other commonly used units for theconvenience of users. The SIA library takes significantlymore computer time to evaluate mostquantities (approximately a factor of 65 morecomputer time for many quantities, comparingoptimized code in both cases) and provides significantlymore properties than does the GSW library.The SIA library uses the world-wide standardfor the thermodynamic description of pure watersubstance (IAPWS-95). Since this is defined overextended ranges of temperature and pressure,the algorithms are long and their evaluation time-consuming.The GSW library uses the Gibbsfunction of Feistel (2003) (IAPWS-09) to evaluatethe properties of pure water, and since this is validonly over the restricted ranges of temperatureand pressure appropriate for the ocean, the algorithmsare shorter and their execution is faster.The GSW library is not as comprehensive as theSIA library; for example, the properties of moistair are only available in the SIA library. In addition,computationally efficient expressions for densityρ in terms of Conservative Temperature (ratherthan in terms of in situ temperature) involvingjust 25 coefficients are also available.The input and output parameters of the GSWlibrary are in units which oceanographers willfind more familiar than basic SI units. We expectthat oceanographers will mostly use thisGSW library because of its greater simplicityand computational efficiency, and because ofthe more familiar units compared with the SIAlibrary. The library name GSW (Gibbs-SeaWater)has been chosen to be similar to, but differentfrom the existing “sw” (Sea Water) library whichis already in wide circulation. Both the SIA andGSW libraries, together with the TEOS-10 Manualand this summary are available from thewebsite www.TEOS-10.org. Initially the SIA libraryis being made available in Visual Basic andFORTRAN while the GSW library is available inMATLAB.10