!----------------------------------------------------------------------- &nameos ! ocean physical parameters !----------------------------------------------------------------------- nn_eos = -1 ! type of equation of state and Brunt-Vaisala frequency ! =-1, TEOS-10 ! = 0, EOS-80 ! = 1, S-EOS (simplified eos) ln_useCT = .true. ! use of Conservative Temp. ==> surface CT converted in Pot. Temp. in sbcssm ! ! ! ! S-EOS coefficients : ! ! rd(T,S,Z)*rau0 = -a0*(1+.5*lambda*dT+mu*Z+nu*dS)*dT+b0*dS rn_a0 = 1.6550e-1 ! thermal expension coefficient (nn_eos= 1) rn_b0 = 7.6554e-1 ! saline expension coefficient (nn_eos= 1) rn_lambda1 = 5.9520e-2 ! cabbeling coeff in T^2 (=0 for linear eos) rn_lambda2 = 7.4914e-4 ! cabbeling coeff in S^2 (=0 for linear eos) rn_mu1 = 1.4970e-4 ! thermobaric coeff. in T (=0 for linear eos) rn_mu2 = 1.1090e-5 ! thermobaric coeff. in S (=0 for linear eos) rn_nu = 2.4341e-3 ! cabbeling coeff in T*S (=0 for linear eos) /
The Equation Of Seawater (EOS) is an empirical nonlinear thermodynamic relationship linking seawater density, , to a number of state variables, most typically temperature, salinity and pressure. Because density gradients control the pressure gradient force through the hydrostatic balance, the equation of state provides a fundamental bridge between the distribution of active tracers and the fluid dynamics. Nonlinearities of the EOS are of major importance, in particular influencing the circulation through determination of the static stability below the mixed layer, thus controlling rates of exchange between the atmosphere and the ocean interior [Roquet et al., 2015a]. Therefore an accurate EOS based on either the 1980 equation of state (EOS-80, UNESCO ) or TEOS-10 [IOC et al., 2010] standards should be used anytime a simulation of the real ocean circulation is attempted [Roquet et al., 2015a]. The use of TEOS-10 is highly recommended because (i) it is the new official EOS, (ii) it is more accurate, being based on an updated database of laboratory measurements, and (iii) it uses Conservative Temperature and Absolute Salinity (instead of potential temperature and practical salinity for EOS-980, both variables being more suitable for use as model variables [Graham and McDougall, 2013, IOC et al., 2010]. EOS-80 is an obsolescent feature of the NEMO system, kept only for backward compatibility. For process studies, it is often convenient to use an approximation of the EOS. To that purposed, a simplified EOS (S-EOS) inspired by Vallis  is also available.
In the computer code, a density anomaly, , is computed, with a reference density. Called rau0 in the code, is set in phycst.F90 to a value of . This is a sensible choice for the reference density used in a Boussinesq ocean climate model, as, with the exception of only a small percentage of the ocean, density in the World Ocean varies by no more than 2 from that value [Gill, 1982].
Options are defined through the nameos namelist variables, and in particular nn_eos which controls the EOS used (=-1 for TEOS10 ; =0 for EOS-80 ; =1 for S-EOS).
Choosing polyTEOS10-bsq implies that the state variables used by the model are and . In particular, the initial state deined by the user have to be given as Conservative Temperature and Absolute Salinity. In addition, setting ln_useCT to true convert the Conservative SST to potential SST prior to either computing the air-sea and ice-sea fluxes (forced mode) or sending the SST field to the atmosphere (coupled mode).
An accurate computation of the ocean stability (i.e. of , the brunt-Väisälä frequency) is of paramount importance as determine the ocean stratification and is used in several ocean parameterisations (namely TKE, GLS, Richardson number dependent vertical diffusion, enhanced vertical diffusion, non-penetrative convection, tidal mixing parameterisation, iso-neutral diffusion). In particular, has to be computed at the local pressure (pressure in decibar being approximated by the depth in meters). The expression for is given by:
The freezing point of seawater is a function of salinity and pressure [UNESCO, 1983]:
(5.27) is only used to compute the potential freezing point of sea water ( referenced to the surface ), thus the pressure dependent terms in (5.27) (last term) have been dropped. The freezing point is computed through eos_fzp, a FORTRAN function that can be found in eosbn2.F90.
Gurvan Madec and the NEMO Team
NEMO European Consortium2017-02-17