Pycnocline

A pycnocline is the cline or layer where the density gradient (∂ρ/∂z) is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of celestial bodies. In addition, the physical properties in a pycnocline driven by density gradients also affect the flows and vertical profiles in the ocean. These changes can be connected to the transport of heat, salt, and nutrients through the ocean, and the pycnocline diffusion controls upwelling.^[1]

Below the mixed layer, a stable density gradient (or pycnocline) separates the upper and lower water, hindering vertical transport.^[2] This separation has important biological effects on the ocean and the marine living organisms. However, vertical mixing across a pycnocline is a regular phenomenon in oceans, and occurs through shear-produced turbulence.^[3] Such mixing plays a key role in the transport of nutrients.^[4]