Mixed layer
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The oceanic or limnological mixed layer is the top zone in the ocean or a lake, having variable depth depending on how far the energy from the wind has penetrated into the water. The atmospheric mixed layer is a zone having nearly constant potential temperature and 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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[edit] Oceanic mixed layer formation
Within the mixed layer, wind energy has mixed up the water. It may be actively mixing, like an aquarium with a bubbler in it, or have been recently mixed, like oil and vinegar salad dressing that's still trying to reform layers. The mixed layer is characterized by having nearly uniform physical properties throughout the 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 it maybe 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.
[edit] Oceanic mixed layer depth determination
The depth of the mixed layer is 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.
The mixed layer depth is in fact greater in winter than summer in each hemisphere. This is because during summer, increased solar heating of the surface water leads to more stable density stratification, which reduces mixing.
[edit] 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 4 °C (degrees Celsius). The study of limnology encompasses all inland water bodies, including bodies of water with salt in them. In saline lakes and seas, mixed layer formation generally behaves similarly to the ocean. In fresh water lakes, seasonal overturn when the surface waters cool to 4 °C is an important process that contributes to mixing in the lake.
[edit] Atmospheric mixed layer formation
The atmospheric mixed layer results from convective air motions, typically seen towards the middle of the day when rising warm air (warmed by the sun) has reached the top of the atmospheric boundary layer. As plumes of air rise, they lose moisture and clouds form. Dry cold air comes down, to replace the rising air. The zone that contains the rising and falling air masses is called the entrainment zone. If enough air is moving around, it generates turbulent mixing, which can generate an atmospheric mixed layer from the earth's surface to a height of over a kilometer.
[edit] Links
See Lake effect snow for a link to a NASA image from the SeaWiFS satellite showing clouds in the atmospheric mixed layer.
[edit] References
- Levitus, Sydney (1982), Climatological Atlas of the World Ocean, NOAA Professional Paper 13, U.S. Department of Commerce.
- Wallace, J. M., and P.V. Hobbs (1977), "Atmospheric Science: An Introductory Survey", Academic Press, San Diego.
