PD does not agree completely with sigma_0 derived from SALT and THETA

[cspencerjones]

I've been trying to compare PD ("Potential Density Ref to Surface") with the potential density referenced to p=0 calculated using https://github.com/xgcm/fastjmd95 from SALT and THETA. These match relatively well, but not perfectly (and not enough for me to feel confident just using them). The size of the difference is correlated with depth, suggesting that maybe I am using a different equation of state.

(color is depth in the final plot and each point is a grid cell)

As far as I know, CESM uses the JMD95 equation of state. Does anyone have suggestions about what might be going wrong here?

[dcherian]

i think pop2 uses MDJWF 2003 though (https://github.com/NCAR/pop-tools/blob/master/pop_tools/eos.py)

[cspencerjones]

Thanks @dcherian , this sounded very promising, but sadly in the end it didn't help! The error has the same size and structure even if I use the pop-tools eos.

[dcherian]

cc @mnlevy1981