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Abstract A differentiable function whose contours are orthogonal to potential density ( σ ) contours does not exist. However, such a function, called potential spicity ( π ), can be defined in the least square sense; these two functions form a practically orthogonal coordinate system in potential temperature‐salinity ( θ ‐S) space. Thus, in addition to the classical potential temperature‐salinity ( θ ‐S) diagram, seawater properties can be studied in the potential density‐potential spicity ( σ − π ) diagram.
Huang et al. (Mon,) studied this question.