A MIxED Layer is a quasi-homogeneous turbu-lized layer in the world's ocean. Its upper boundary directly contacts with the atmosphere boundary layer, while the mixed layer bottom contacts with underlying thermocline. That is why mixed layer is also called the upper mixed layer. Vertical gradients of temperature and salinity within the mixed layer usually do not exceed 0.01 Kelvin degrees and 0.01 promille per meter. Mixed layer is formed as a result of turbulent mixing by thermohaline convection, vertical shear of oceanic currents, and surface waves (periodically breaking when their slope reaches some critical level).
The process of mixed layer deepening is accompanied by entrainment of more cold water of thermocline into the upper turbulized layer. This leads to negative mixed layer heat fluxes at its bottom. That is why mixed layer deepening is accompanied by its cooling. As a result of entrainment, the sharp interface between turbulized mixed layer and untur-bulized thermocline is formed. If surface sources of turbulence are too weak to mix stably stratified ther-mocline, the mixed layer turbulence is decaying and the depth of mixed layer is decreasing. The mixed-layer shoaling causes its isolation from thermocline, because the new turbulized upper mixed layer is joined with the underlying old (relic) mixed layer with decaying turbulence. As a result, the heat fluxes at the bottom of new upper mixed layer is close to zero, and this does not prevent the upper mixed layer from the warming as, for example, from a generalized analysis of this processes discussed by Eric Kraus.
The mixed layer occupies almost the whole world's ocean. Its thickness is at a maximum in the sub-Arctic and sub-Antarctic regions in winter, when north Atlantic deep water and Antarctic bottom water are sinking up to 1.2-3.7 mi. (2-6 km.). In the tropics and subtrop-ics, the typical mixed layer depth is of about 32-328 ft. (10-100 m.) throughout the entire year. It is controlled not only by intensity of surface turbulent mixing, but also by regular vertical motion, because of the divergence or convergence of oceanic currents. Therefore, in the vicinity of oceanic jets (such as the Gulf Stream or Equatorial Undercurrents/Countercurrents), the mixed layer thickness is changed significantly, and horizontal in-homogeneity should be taken into account for simulation of the mixed layer evolution.
In middle and high latitudes of the ocean interior, there is a strong seasonal cycle of mixed-layer parameters depending on heat fluxes at the surface. In winter, mixed layer is cooling and deepening as a result of net heat lost at the ocean surface and mixed-layer bottom. In that time, intense thermal convection is a principal cause of the mixed-layer deepening. A maximum mixed-layer thickness in the end of winter is, in fact, the depth of penetration of seasonal variations of temperature, salinity, and density. In spring and summer, the typical mixed-layer thickness is at a minimum as a result of surface heating. In that time, mixed-layer is turbulized by dynamic processes (that is, the vertical shear of oceanic currents and surface waves) and its thickness does not exceed a few to tens of meters.
Buoyancy forcing due to thermal convection is a principal cause of winter mixing of mixed layer elsewhere in subtropics, middle, and high latitudes of the world's oceans. However, haline convection also may be important, especially in the subtropical ocean bounded by large desert (such as the north subtropical Atlantic and Sahara), where very dry conditions lead to intense evaporation and sinking of salty waters to the depth. In these specific regions, buoyancy mixed-layer forcing by haline effects prevails. The same is true for the some close or semi-close seas situated in the arid zone (such as in the Red Sea and the Dead Sea).
sEE also: Atmospheric Boundary Layer; Thermocline; Thermohaline Circulation.
BIBLIOGRAPHY. Robert Dinwiddie, Ocean (DK, 2006); Eric Kraus, ed., Modelling and Prediction of the Upper Layers of the Ocean (Pergamon Press, 1977); Alexander Soloviev and Roger Lukas, The Near-Surface Layer of the Ocean: Structure, Dynamics and Applications (Springer, 2006).
Alexander Boris Polonsky Marine Hydrophysical Institute, Sebastopol
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