There is a great contrast between the relative distributions of potential density referred too decibars and of potential density referred to 4000 decibars in the deep water of the South Atlantic Ocean. Both parameters are useful in a descriptive analysis of deep water; the former is an identifier of water origin, the latter graphically displays stability in the deep ocean and can be used to infer deep-ocean geostrophic flow.
Lynn, Ronald J.. 1971. "On potential density in the deep South Atlantic Ocean." Journal of Marine Research 29, (2). https://elischolar.library.yale.edu/journal_of_marine_research/1210