[MITgcm-support] Nonlinear equations of state

[Martin Losch]

Hi Dave,

JMD95Z (based on Jackett and McDougall 1995) is basically a “re-tuned” UNESCO formula (UNESCO for in-situ temperature and JMD95Z for potential temperature), both use p = -rhoConst*gravity*z as an approximation for pressure.
JMD95Z uses the actual hydrostatic pressure (lagged by one timestep), and hence requires a little more computation
MDJWF implements McDougall, et al (2003). The EOS is based on a different fromula and principles and contains fewer terms, so that the authors (?) claim that there fewer computations involved, hence it should be faster. This EOS also uses the hydrostatic pressure of the previous time step

In practice, the EOS is not the bottleneck of any simulation, so that I would not expect any significant speed differences. For potential temperature I wouild use MDJWF, just because it is “newer”. 

There’s also TEOS10, which requires that “THETA” and “SALT” are interpreted as conservative temperature and absolute salinity. This implementation is not quite complete yet, see PR #812: https://github.com/MITgcm/MITgcm/pull/812

Martin

> On 11. Mar 2024, at 18:35, Dave Munday - BAS <danday at bas.ac.uk> wrote:
> 
> Dear MITgcm-ers,
> 
> I’ve just been trying to make a decision about which particular nonlinear EOS to use for a very long-running two basin sector model with biogeochemistry. In reading the docs the MDJWF EOS is described as “more accurate and less expensive”, which is appealing when running models for 10-20 000 years. A lot of the verification experiments are set to use JMD95Z or JMD95P. Is there a reason to prefer them over MDJWF?
> 
> Many thanks,
> 
> Dave
> 
> 
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