Mixed layers (oceanography)

Article By:

Sarkar, Nandita Environmental Research Division, National Oceanic and Atmospheric Administration, Pacific Grove, California.

Last reviewed:2009

DOI:https://doi.org/10.1036/1097-8542.YB090117

The oceanic surface mixed layer is the layer of almost uniform density resulting from the interaction between stratifying and destratifying processes. The mixed layer extends from the surface of the ocean to the top of the pycnocline (see illustration). The pycnocline is the region where seawater density changes rapidly with depth. In the ocean, density is not directly measured but calculated using temperature, salinity, and pressure measurements from a variety of data collection platforms, such as ship-board surveys, moored instruments, and profiling floats. At shallow depths, as in the vicinity of the surface mixed layer, the pressure can be assumed to be constant, and density depends on temperature (inversely) and salinity (directly) through a very complex, nonlinear relationship which is quantified using the equation of state. Stratification results from an increase in temperature, a decrease in salinity, or both. Conversely, destratification results from a decrease in temperature, an increase in salinity, or both. Common processes that cause stratification include surface heating by the Sun and freshwater addition by precipitation (rain or snow) or by ice melting. Destratifying processes include surface cooling, increase of salinity due to evaporation or brine rejection during formation of ice, and mechanical mixing by winds, tides, and currents. Flow of water over a rough bottom can increase mixing.

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