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, 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.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 10 °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.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.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 without gaining or losing heat. As 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, provided it is free of clouds. Such a layer may have gradients in the humidity, though. As is the case with the ocean mixed layer, velocities will not be constant throughout the atmospheric mixed layer.