Isopycnal

An isopycnal is a surface of constant density, in practice constant potential density referenced to a chosen pressure (or neutral density for the full water column). Because interior stirring is far stronger along density surfaces than across them, water masses and tracers spread mainly along isopycnals while diapycnal mixing across them is weak, with diffusivity near 10^-5 square meters per second. Isopycnal analysis underlies water-mass tracing, subduction of mode waters, and the isopycnal coordinate used in many ocean models. The tilt of isopycnals stores the available potential energy that baroclinic instability releases.