Mixed Layer

The mixed layer (ML) is a quasi-homogeneous, turbulized layer in the world ocean. Its upper boundary directly contacts with the atmosphere boundary layer, while the ML bottom contacts with the underlying thermocline. That is why ML is also called the upper mixed layer. Vertical gradients of temperature and salinity within the ML usually don’t exceed 0.01 Kelvin degree and 0.01 promille per m. ML is formed as a result of turbulent mixing because of thermohaline convection, vertical shear of oceanic currents and surface waves (periodically breaking when their slope reaches some critical level). Sometimes, the most turbulized lower part of the atmospheric boundary layer is also called ML. It is a result of strong mixing because of thermal convection and/ or dynamic shear turbulence. Convection prevails when surface heating occurs, while dynamic shear turbulence is generated by strong winds. Its typical thickness is a few hundred meters.

The process of oceanic ML deepening is accompanied by the entrainment of more cold water of the thermocline into the upper turbulized layer. This leads to negative ML heat fluxes at its bottom, which is why the ML deepening is accompanied by its cooling. As a result of entrainment, the sharp interface between turbulized ML and unturbulized thermocline is formed. If surface sources of turbulence are too weak to mix a stably stratified thermocline, the ML turbulence is decaying and the depth of ML is decreasing. The ML shoaling causes its isolation from the thermocline because the new, turbulized, upper mixed layer is joined with underlying old (relict) ML with decaying turbulence. As a result, the heat fluxes at the bottom of the new upper mixed layer is close to zero, which doesn’t prevent the upper mixed layer from the warming. This follows, for example, from a generalized analysis of these processes discussed by Eric Kraus.

The ML occupies almost the whole world’s oceans. 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 subtropics, the typical ML depth is of about 33–330 ft. (10–100 m) throughout the year. It is controlled not only by intensity of surface turbulent mixing, but also by regular vertical motion because of divergence or convergence of oceanic currents. Therefore, in the vicinity of oceanic jets (such as the Gulf Stream or Equatorial Undercurrents/Countercurrents), the ML thickness is changed significantly, and horizontal inhomogeneity should be taken into account for simulation of the ML evolution. In middle and high latitudes of the ocean interior, there is a strong seasonal cycle of ML parameters depending on heat fluxes at the surface. In winter, ML is cooling and deepening as a result of net heat lost at the ocean surface and the mixed layer bottom. In that time, intense thermal convection is a principal cause of the ML deepening.

A maximum ML 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 ML thickness is at a minimum as a result of surface heating. In that time, ML is turbulized by dynamic processes (that is, vertical shear of oceanic currents and surface waves) and is no thicker than a few meters to tens of meters.

Buoyancy forcing because of thermal convection is a principal cause of winter mixing of ML elsewhere in the subtropics and the middle and high latitudes of the world’s oceans. However, haline convection may also be important, especially in the subtropical ocean bounded by a 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 depths. In these specific regions, buoyancy ML forcing because of haline effects prevails. The same is true for some close or semiclose seas situated in an arid zone (such as the Red Sea and the Dead Sea).