The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer in which this turbulence is generated by winds, surface heat fluxes, or processes such as evaporation or sea ice formation which result in an increase in salinity. The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height. The depth of the atmospheric mixed layer is known as the mixing height. Turbulence typically plays a role in the formation of fluid mixed layers.
Oceanic mixed layer
Importance of the mixed layer The mixed layer plays an important role in the physical climate. Because the specific heat of ocean water is much larger than that of air, the top 2.5 m of the ocean holds as much heat as the entire atmosphere above it. Thus the heat required to change a mixed layer of 2.5 m by 1 °C would be sufficient to raise the temperature of the atmosphere by 1 °C. The depth of the mixed layer is thus very important for determining the temperature range in oceanic and coastal regions. In addition, the heat stored within the oceanic mixed layer provides a source for heat that drives global variability such as El Niño. The mixed layer is also important as its depth determines the average level of light seen by marine organisms. In very deep mixed layers, the tiny marine organisms known as phytoplankton are unable to get enough light to maintain their metabolism. The deepening of the mixed layer in the wintertime in the North Atlantic is therefore associated with a strong decrease in surface chlorophyll a. However, this deep mixing also replenishes near-surface nutrient stocks. Thus when the mixed layer becomes shallow in the spring, and light levels increase, there is often a concomitant increase of phytoplankton biomass, known as the "spring bloom".
Oceanic mixed layer formation There are three primary sources of energy for driving turbulent mixing within the open-ocean mixed layer. The first is the ocean waves, which act in two ways. The first is the generation of turbulence near the ocean surface, which acts to stir light water downwards. Although this process injects a great deal of energy into the upper few meters, most of it dissipates relatively rapidly. If ocean currents vary with depth, waves can interact with them to drive the process known as Langmuir circulation, large eddies that stir down to depths of tens of meters. The second is wind-driven currents, which create layers in which there are velocity shears. When these shears reach sufficient magnitude, they can eat into stratified fluid. This process is often described and modelled as an example of Kelvin-Helmholtz instability, though other processes may play a role as well. Finally, if cooling, addition of brine from freezing sea ice, or evaporation at the surface causes the surface density to increase, convection will occur. The deepest mixed layers (exceeding 2000 m in regions such as the Labrador Sea) are formed through this final process, which is a form of Rayleigh–Taylor instability. Early models of the mixed layer such as those of Mellor and Durbin included the final two processes. In coastal zones, large velocities due to tides may also play an important role in establishing the mixed layer. The mixed layer is characterized by being nearly uniform in properties such as temperature and salinity throughout the layer. Velocities, however, may exhibit significant shears within the mixed layer. The bottom of the mixed layer is characterized by a gradient, where the water properties change. Oceanographers use various definitions of the number to use as the mixed layer depth at any given time, based on making measurements of physical properties of the water. Often, an abrupt temperature change called a thermocline occurs to mark the bottom of the mixed layer; sometimes there may be an abrupt salinity change called a halocline that occurs as well. The combined influence of temperature and salinity changes results in an abrupt density change, or pycnocline. Additionally, sharp gradients in nutrients (nutricline) and oxygen (oxycline) and a maximum in chlorophyll concentration are often co-located with the base of the seasonal mixed layer.
Oceanic mixed layer depth determination
The depth of the mixed layer is often determined by hydrography—making measurements of water properties. Two criteria often used to determine the mixed layer depth are temperature and sigma-t (density) change from a reference value (usually the surface measurement). The temperature criterion used in Levitus (1982) defines the mixed layer as the depth at which the temperature change from the surface temperature is 0.5 °C. However, work done by Kara et. al. (2000) suggest that the temperature difference is closer to .8 °C. The sigma-t (density) criterion used in Levitus uses the depth at which a change from the surface sigma-t of 0.125 has occurred. Neither criterion implies that active mixing is occurring to the mixed layer depth at all times. Rather, the mixed layer depth estimated from hydrography is a measure of the depth to which mixing occurs over the course of a few weeks.
Barrier layer thickness
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