Isopycnal

An isopycnal is a surface within a fluid on which the density is constant. In oceanography, seawater density depends on temperature, salinity, and pressure. Because pressure increases with depth, density varies with depth even at fixed temperature and salinity. To compare densities at different depths, oceanographers commonly use potential density, θ, which is the density a water parcel would have if moved adiabatically to a reference pressure. An isopycnal surface is thus a surface of constant θ (or constant ρ).

Isopycnals are used as a vertical coordinate in some ocean models, forming isopycnal coordinates. They align with the natural stratification of the water column and help describe how properties such as heat, salt, or nutrients are redistributed within layers. In many formulations, mixing is modeled as more effective along isopycnal surfaces than across them, which helps represent layered transport and reduces spurious vertical diffusion in strongly stratified regions.

The geometry of isopycnals reflects the ocean’s temperature–salinity structure and varies with geography and season. In regions with a strong density gradient—the pycnocline—isos lines tilt steeply with depth; where density changes little, isopycnals are relatively horizontal. This curvature provides a compact way to summarize stratification and vertical structure in the water column.

Related concepts include isoneutral (neutral) surfaces, which are associated with small displacements that do not change density to first order. Isopycnals and neutral surfaces are both fundamental tools in physical oceanography and geophysical fluid dynamics.