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ΣΑ = Δρ/0The InternationalThermodynamicEquation of Seawater– 2010ΣASummary for Policy Makers0Practical Salinity vs. Absolute Salinity Part II:How the Relationship Works TogetherThe salinity input to the TEOS-10 Gibbs functionrequires knowledge of the Absolute Salinity ofseawater (S A ), which is based upon the ReferenceSalinity of seawater (S R ). The ReferenceSalinity is our best estimate of the AbsoluteSalinity of the seawater that was used to developthe Practical Salinity Scale (S P ), the equation ofstate, and the other thermodynamic propertiesof seawater. Reference Salinity is related toPractical Salinity byS R = S P –1and Absolute Salinity is related to ReferenceSalinity byδS A = S R + δS Awhere δS A is due to the added solutes in seawaterin deep waters resulting from the dissolutionof CaCO 3 (soluble) and SiO 2 (soluble), CO 2 , andnutrients like NO 3 and PO 4 from the oxidation ofplant material. The δS A values due to the addedsolutes are estimated from the differencesbetween the measured densities of seawatersamples compared with the densities calculatedfrom the TEOS-10 equation of state at the sameReference Salinity, temperature, and pressure.The values of δS A in the ocean can be estimatedfor waters at given longitude, latitude, and depthusing correlations of δS A and the concentrationof Si(OH) 4 in the waters. Other methods of estimatingδS A are also available in cases wherethe composition changes are measured or canbe modelled. The δS A values can then be usedto calculate all the thermodynamic propertiesof seawater in the major ocean basins using thenew TEOS-10.Improving the accuracyof climate modelsThe new thermodynamic equation for seawater also allows climatemodels to better account for changes in density and heat transferin the ocean. Early tests of the use of the new equation show anestimated 1% change in how the ocean circulates heat from theequator to the poles. The change in the West to East temperaturedifference in the equatorial Pacific is about 0.1°C, and this is anotheraspect which is expected to be a valuable improvement in climatemodelling.The fundamental properties of seawater — salinity,temperature and pressure, along with the freezingand boiling points, heat capacity, speed of soundand density — are intricately tied together. Beingable to measure salinity is important, as salinitylevels are indicators of climate change. They indicatehow much freshwater is evaporating from theoceans. For instance, parts of the Atlantic Oceanappear to be getting saltier. A possible explanationcould be that trapped heat from higher atmosphericconcentrations of CO 2 is causing more seawater toevaporate than before, leaving the salt behind.Salinity levels affect water density.Density especially determineswhether a current risestowards the surface or sinks towardsthe seafloor, as the denserthe seawater, the deeper itwill sink. Density depends ontemperature, pressure and theamount of dissolved materialin the water. Knowing the density of seawateris crucial to monitoring the Earth’s climate. Theocean transports heat via currents collectivelycalled the ocean conveyor belt in a processknown as thermohaline circulation. In the Arcticand Southern oceans, cool and salty waters sinkto form deep water currents. Over thousandsof years, these currents travel around the worlduntil they reach areas of upwelling which bringthem to the surface. Once at the surface, thesun-warmed, rain-freshened currents head back6
.75179 γ© Chris Linder, WHOIto the poles, where the formation of ice allowsthe cycle to continue.Several factors influence ocean circulation patterns:wind, rain, seafloor topography, the propertiesof the surrounding water, as well as theposition and distance of the moon and the rotationof the Earth. Ocean circulation models includeall of these factors and the computer algorithmsthat generate the models take weeks torun. Climate change models, which incorporatethe ocean’s ability to transport heat, take evenlonger. To see what model works best, what fitswith the Earth’s climate record from the pastthen run the model forward a century or twocan take the best part of a year, on the world’sfastest supercomputers.The Search for Salinity‘The exact chemical composition of seawater isunknown at the present time,’ says Frank Milleroof the Rosenstiel School of Marine and AtmosphericScience at the University of Miami in Working Group 127. It is not for want of trying.Marine scientists have been searching for the‘magic formula’ for measuring salinity for more Georg Forchhammer found 27 differentsubstances in seawater he sampled fromdifferent regions of the ocean. ‘Next to chlorine,oxygen and hydrogen, sodium is the most abundantelement in seawater,’ he wrote. Other majorsubstances he found included sulphuric acid,soda, potash, lime and magnesia. ‘Those whichoccur in less but still determinable quantity aresilica, phosphoric acid, carbonic acid and oxideof iron,’ he concluded. His tables were useduntil 1902 when Danish oceanographer MartinKnudsen filtered and distilled North Atlanticwater as a seawater standard that all marinescientists could use to calibrate their instrumentseasily and compare their samples fromaround the world with a control.In the 1930s, the introduction of instrumentsthat could measure seawater’s electricalconductivity set oceanographers scramblingto determine whether chemical analysis or thenew physical analysis worked better to determinesalinity. Conductivity won and by themid-1970s, deploying a rosette of samplingtubes equipped with conductivity, temperatureand depth recorders (CTDs) was becominga routine part of oceanographic cruises. Tomaintain consistency, a change to the internationalstandard for seawater was made in the1970s that allowed oceanographers to compareconductivity to a Practical Salinity Scale.Unlike the Practical Salinity Scale, whichaccounts only for ions, the new AbsoluteSalinity will incorporate non-electrolytes usingtables that account for how these additionalsubstances vary region by region. Once again,the latitude and longitude at which the seawatersamples are taken will play an important role incalculating salinity.7
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