isopycnals

Isopycnals are surfaces within a fluid along which density is the same. In the ocean, density ρ depends on temperature, salinity, and pressure, so isopycnals are curved surfaces that slope with depth and geography. Because density generally increases with depth due to cooling and/or freshwater variation, isopycnals typically tilt, forming stratified layers. They serve as density coordinate surfaces used to describe ocean structure; common density variables include sigma (σ) densities, such as sigma-theta (potential density anomaly) and sigma-t, which help remove some compressibility effects.

In fluid dynamics, buoyancy forces drive vertical motions, and flows tend to advect along isopycnals, while mixing across isopycnals is called diapycnal mixing. The region with rapid density change is the pycnocline, a strong gradient surface that aligns with several contiguous isopycnals. Observationally, isopycnals are mapped by measuring temperature and salinity with CTD casts, computing density, and then interpolating to constant density surfaces or using potential density to account for pressure changes.

In numerical modeling and oceanography, isopycnal coordinates or layers are used to represent vertical structure and to minimize numerical diffusion across density surfaces. The concept also appears in atmospheric science, where isopycnals refer to surfaces of constant air density, though the term is more commonly used in oceanography. Isopycnal analysis remains fundamental for understanding buoyancy, stratification, and the pathways of ocean circulation.