mom_eos_UNESCO module reference¶

Detailed Description¶

UNESCO (Jackett & McDougall) equation of state¶

The UNESCO (1981) equation of state is an internationally defined standard fit valid over the range of pressures up to 10000 dbar, temperatures between the freezing point and 40 degC, and salinities between 0 and 42 PSU. Unfortunately, these expressions used in situ temperatures, whereas ocean models (including MOM6) effectively use potential temperatures as their state variables. To avoid needing multiple conversions, Jackett and McDougall (1995) refit the UNESCO equation of state to take potential temperature as a state variable, over the same valid range and functional form as the original UNESCO expressions. It is this refit from Jackett and McDougall (1995) that is coded up in this module.

The functional form of the equation of state includes terms proportional to salinity to the 3/2 power. This introduces a singularity in the second derivative of density with salinity at a salinity of 0, but this has been addressed here by setting a floor of 1e-8 PSU on the salinity that is used in the denominator of these second derivative expressions. This value was chosen to imply a contribution that is smaller than numerical roundoff in the expression for density, which is the field for which the UNESCO equation of state was originally derived.

Originally coded in 1999 by J. Stephens, revised in 2023 to unambiguously specify the order of arithmetic with parenthesis in every real sum of three or more terms.

References¶

Gill, A. E., 1982: Atmosphere-Ocean Dynamics. Academic Press, 662 pp.

Jackett, D. and T. McDougall, 1995: Minimal adjustment of hydrographic profiles to achieve static stability. J. Atmos. Ocean. Tech., 12, 381-389.

UNESCO, 1981: Tenth report of the joint panel on oceanographic tables and standards. UNESCO Technical Papers in Marine Sci. No. 36, UNESCO, Paris.