Mixed layer

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 where this turbulence is generated by winds, cooling, 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.

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Oceanic mixed layer formation

There are three primary sources of energy for driving turbulent mixing within the open-ocean mixed layer. The first is breaking of surface waves, which injects a great deal of energy into the upper few meters, where most of it dissipates. 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 2000m 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.

Defining what constitutes a mixed layer is difficult, as the surface layer may be actively mixing, like an aquarium with a bubbler in it, or have been recently mixed, like oil and vinegar salad dressing that is still trying to reform layers. Because there are cycles of heating and cooling on daily and seasonal time scales, oceanographers sometimes distinguish the diurnal mixed layer (over which mixing varies on daily time scales) from the seasonal mixed layer (which is mixed at least once per year).

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 degrees Celsius. The sigma-t (density) criterion used in Levitus (1982) uses the depth at which a change from the surface sigma-t of 0.125 has occurred. It should be noted that 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.

The mixed layer depth is in fact greater in winter than summer in each hemisphere. During the summer increased solar heating of the surface water leads to more stable density stratification, reducing the penetration of wind-driven mixing. Because seawater is most dense just before it freezes, wintertime cooling over the ocean always reduces stable stratification, allowing a deeper penetration of wind-driven turbulence but also generating turbulence that can penetrate to great depths.

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 2.5m of the ocean holds as much heat as the entire atmosphere above it. Thus the heat required to change a mixed layer of 25m by 1°C would be sufficient to raise the temperature of the atmosphere by 10°C. The depth of the mixed layer is thus very important for determining the temperature range in oceanic and coastal regions.

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 plants known as phytoplankton are unable to get enough light to maintain their metabolism. The shallowing of the mixed layers in the springtime in the North Atlantic is therefore associated with a strong spring bloom of plankton.

Limnological mixed layer formation

Formation of a mixed layer in a lake is similar to that in the ocean, but mixing is more likely to occur in lakes solely due to the molecular properties of water. Water changes density as it changes temperature. In lakes, temperature structure is complicated by the fact that fresh water is heaviest at 3.98 °C (degrees Celsius). Thus in lakes where the surface gets very cold, the mixed layer briefly extends all the way to the bottom in the spring, as surface warms as well as in the fall, as the surface cools. This overturning is often important for maintaining the oxygenation of very deep lakes.

It should be noted that the study of limnology encompasses all inland water bodies, including bodies of water with salt in them. In saline lakes and seas (such as the Caspian Sea), mixed layer formation generally behaves similarly to the ocean.

Atmospheric mixed layer formation

The atmospheric mixed layer results from convective air motions, typically seen towards the middle of the day when air at the surface is warmed and rises. It is thus mixed by Rayleigh-Taylor instability. The standard procedure for determining the mixed layer depth is to examine the profile of potential temperature, the temperature which the air would have if it were brought to the pressure found at the surface. As this such an increase of pressure involves compressing the air, the potential temperature is higher than the in-situ temperature, with the difference increasing as one goes higher in the atmosphere. The atmospheric mixed layer is defined as a layer of (approximately) constant potential temperature, or a layer in which the temperature falls at a rate of approximately 10°C/km. Such a layer may have gradients in the humidity, but is generally free of clouds. As is the case with the ocean mixed layer, velocities will not be constant throughout the atmospheric mixed layer.

Links

See Lake effect snow for a link to a NASA image from the SeaWiFS satellite showing clouds in the atmospheric mixed layer.

References

• Levitus, Sydney (1982), Climatological Atlas of the World Ocean, NOAA Professional Paper 13, U.S. Department of Commerce.
• Mellor, G. L., and P. A. Durbin, 1975: The structure and dynamics of the ocean surface mixed layer. Journal of Physical Oceanography, 5, 718-728.
• Wallace, J. M., and P.V. Hobbs (1977), "Atmospheric Science: An Introductory Survey", Academic Press, San Diego.