[MITgcm-support] Calculating buoyancy gradients

[Liam Brannigan]

Dear mitgcmers


I've been going through various old threads to understand how buoyancy gradients should be calculated from the potential temperature and salinity outputs.  However, I'm still a bit confused on some points and thought I'd seek some help.


Firstly, with a linear equation of state it seems that: buoyancy b = g*tAlpha*(theta(i,j,k,bi,bj)-tRef(k)) (and similarly for salinity).  Buoyancy gradients can be calculated from that in a straightforward way.


With the non-linear equation of state (in z-coordinates), I'm less sure.  One approach is to use jmd95.py (or equivalently densjmd95.m) to calculate sigma.  This takes S, T and p as inputs.  The salinity and temperature are just the model output salinity and potential temperatures.  Should pressure be calculated simply as p = -g/rho_{0}z?  Do latitudinal corrections have to be made, as in sw_pres.m?  The jmd95.py script gives "the density of seawater"  - but it looks to me like it is the potential density.  Are the horizontal buoyancy gradients then horizontal gradients of the output of the jmd95.py script while stratification (N2) is the vertical gradient of the output (once it has been multiplied by -g/rho_{0})?


Thanks


Liam

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