the mixed-layer is the layer between the ocean surface and a depth usually ranging between 25 and 200m, where the density is about the same as at the surface. The mixed-layer owes its existence to the mixing initiated by waves and turbulence caused by the wind stress on the sea surface. An effect of mixing is to make both properties of water, temperature and salinity, thus density, more uniform. The penetration of mixing to a certain depth (the mixed-layer depth) mostly depends on the stability of the sea water and on the incoming energy from the wind. The more stable is the surface water, the less mixing occurs, and the shallower is the mixed-layer. Sea water stability in the near-surface is determined by the atmospheric fluxes through the ocean surface (wind stress, heat and fresh water exchange). A typical unstable configuration is when water is denser (“heavier”) at the surface than below. The mixing that ensues, for example with some impulse from waves or turbulence, renders the density more uniform and deepens the mixed-layer. In certain conditions occurring only in a few areas of the high-latitude seas (e.g. Labrador Sea in North Atlantic, Weddell Sea in the Antarctic waters), instability is so strong that denser surface water literally sinks and mixes over large depths reaching more than 1000m.
In many situations, the mixed-layer can be identified with the layer of mixed temperature, when the salinity does not vary much with increasing depth in general. However, this becomes untrue as soon as for instance fresh water is exchanged between the ocean and the air above (evaporation or rain), which may create large salinity contrast.
The mixed-layer is the oceanic surface zone that responds the most quickly and directly to atmospheric fluxes, and it is through the mixed-layer that such influence is transmitted to the whole ocean in the long term. Conversely, the mixed-layer is the part of the ocean through which the ocean influences directly the atmosphere. Many important processes occur within the mixed-layer, whether physical (e.g. direct wind-forcing of the ocean circulation), chemical (e.g. dissolution of incoming CO2 from the atmosphere), or biological (e.g. phytoplankton production).
Who cares? physical oceanographers, marine biologists, climatologists…, therefore any specialist dealing with the upper ocean will need to consider the surface mixed-layer. In a practical way, this oceanic feature is very familiar for instance to fishermen and scuba divers.