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.
