TEOS-10

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TEOS-10 (Thermodynamic Equation of Seawater - 2010) is the international standard for the use and calculation of the thermodynamic properties of seawater, humid air and ice. It supersedes the former standard EOS-80 (Equation of State of Seawater 1980).[1] TEOS-10 is used by oceanographers and climate scientists to calculate and model properties of the oceans in an internationally comparable way.

History[]

TEOS-10 was developed by the SCOR(Scientific Committee on Oceanic Research)/IAPSO(International Association for the Physical Sciences of the Oceans) Working Group 127 [2] which was chaired by Trevor McDougall. It has been approved as the official description of the thermodynamic properties of seawater, humid air and ice in 2009 by the Intergovernmental Oceanographic Commission (IOC)[3] and in 2011 by the International Union of Geodesy and Geophysics (IUGG).[4]

Physical basis[]

TEOS-10 is based on thermodynamic potentials. Fluids like humid air and liquid water in TEOS-10 are therefore described by the Helmholtz energy F(m,T,V)=F(m,T,m/ρ) or the specific Helmholtz-energy f(T,ρ)=F(m,T,m/ρ)/m. The Helmholtz energy has a unique value across phase boundaries.[5] For the calculation of the thermodynamic properties of seawater and ice, TEOS-10 uses the specific Gibbs potential g(T,P)=G/m, G=F+pV, because the pressure is a more easily measurable property than density in a geophysical context. Gibbs energies are multivalued around phase boundaries and need to be defined for each phase separately.[6]

The thermodynamic potential functions are determined by a set of adjustable parameters which are tuned to fit experimental data and theoretical laws of physics like the ideal gas equation. Since absolute energy and entropy cannot be directly measured, arbitrary reference states for liquid water, seawater and dry air in TEOS-10 are defined in a way that

  • internal energy and entropy of liquid water at the solid-liquid-gas triple point are zero,
  • entropy and enthalpy of seawater are zero at SA (Absolute Salinity) = 35.16504 g/kg, T (Temperature) = 273.15 K, p (pressure) = 101325 Pa,
  • entropy and enthalpy of dry air are zero at T (Temperature) = 273.15 K, p (pressure) = 101325 Pa.[6]

Included thermodynamic properties[]

Distribution of the Absolute Salinity Anomaly at 2500dbar (approx 2500m depth), created with the GSW Oceanographic Toolbox of TEOS-10

TEOS-10 covers all thermodynamic properties of liquid water, seawater, ice, water vapour and humid air within their particular ranges of validity as well as their mutual equilibrium composites such as sea ice or cloudy (wet and icy) air.

Additionally, TEOS-10 covers derived properties, for example the potential temperature and Conservative Temperature, the buoyancy frequency, the planetary vorticity and the Montgomery and Cunningham geostrophic streamfunctions. A complete list of featured properties can be found in the TEOS-10 Manual.

The handling of salinity was one of the novelties in TEOS-10. It defines the relationship between Reference Salinity and Practical Salinity, Chlorinity or Absolute Salinity and accounts for the different chemical compositions by adding a regionally variable