Potential Density

Potential density is the density a water parcel would have if brought adiabatically to a chosen reference pressure, removing the compressibility effect so that buoyancy can be compared across depths. Because seawater’s compressibility depends on temperature, a single reference pressure can misrank deep water, so oceanographers use neutral density or pressure-referenced sigma surfaces (sigma-0, sigma-2, sigma-4). Surfaces of constant potential density are isopycnals along which water masses spread.