What is a pycnocline and how does it limit mixing in the ocean?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

A pycnocline is a boundary layer in the ocean that separates two liquid masses of differing densities. It is formed by variations in salinity or temperature and is characterized by a zone where density increases rapidly with depth.12

Because the pycnocline is an extremely stable layer, it acts as a physical barrier that limits vertical mixing between surface waters and the deep ocean. By inhibiting the transfer of water and energy, the pycnocline effectively isolates the deep ocean from surface processes, meaning that fluctuations in temperature or salinity are significantly more pronounced in surface waters than in the depths below. This barrier is generally present throughout the world's oceans, with the notable exception of polar regions where the pycnocline is absent.12