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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
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.