Equation of State of Seawater

The equation of state of seawater is a function from which the thermodynamic properties of seawater can be derived mathematically.  A Gibbs function for this has been developed by Working Group 127 “Thermodynamic and Equation of State of Seawater” as a collaboration of the Scientific Committee on Oceanic Research (SCOR) and International Association of the Physical Sciences of the Oceans (IAPSO).  The particular Gibbs function, or Gibbs potential, is a function of absolute salinity, temperature, and pressure. This equation is presented in the following work together with statements of basic thermodynamic properties and such derived properties as potential temperature, potential enthalpy, conservative temperature, potential density, among others.

IOC, SCOR and IAPSO, “The international thermodynamic equation of seawater – 2010: Calculation and use of thermodynamic properties,” Intergovernmental Oceanographic Commission, Manuals and Guides No. 56, UNESCO (English), 196 pp.

This is available for free download at http://www.teos-10.org/pubs/TEOS-10_Manual.pdf (last viewed 29 July 2013).

Two associated documents of potential interest are:

“Getting started with TEOS-10 and the Gibbs Seawater (GSW) Oceanographic Toolbox,” Version 3.0, May 2011, available at http://www.teos-10.org/pubs/Getting_Started.pdf (last viewed 29 July 2013).

A primer: R. Pawlowicz, “What every oceanographer needs to know about TEOS-10 (The TEOS-10 Primer)” is available at http://www.teos-10.org/pubs/TEOS-10_Primer.pdf (last viewed 29 July 2013).

The equation was adopted by the Intergovernmental Oceanographic Commission in June 2009, replacing a series of algorithm dating from 1978.  These historical references include the Practical Salinity Scale (PSS-78) and the International Equation of State of Seawater (UNESCO 1981) as well as other algorithms for the specific heat capacity of seawater at constant pressure, sound speed of seawater, and freezing-point temperature of seawater.  Details are given in the cited manual.

Of special note is the use of Absolute Salinity, expressed in SI units, in the Gibbs function.  This quantity is derived from the measurements underlying Practical Salinity in combination with other measurements and correlations.  It is recommended by IOC that Practical Salinity remain the standard quantity for storing in national databases.