 This is true even if the density of the fluid decreases upward.In stable conditions (potential density decreasing upward) motion along surfaces of constant potential density (isopycnals) is energetically favored over flow across these surfaces (diapycnal flow), so most of the motion within a 3-D geophysical fluid takes place along these 2-D surfaces.In oceanography, the symbol formula_3 is used to denote "potential density", with the reference pressure formula_4 taken to be the pressure at the ocean surface.