Geoscience Reference
In-Depth Information
2.3.6.
Stratification, mixed layer, thermocline
The exchanges of surface fluxes determine the value of the
surface's properties of temperature and salinity, and therefore the
volumic mass. An initial approach is to consider the deep ocean as
stable and to presume that only the surface ocean varies locally with
the surface fluxes. In most parts of the oceans, the thermal fluxes are
crucial for defining the stratification. At mid-latitudes, these fluxes
have a marked seasonal cycle. They start to heat the ocean in spring,
creating a fine and hot surface layer called the mixed layer. This layer
becomes hotter with the advance of the summer season and deeper
with the nocturnal mixing, when the surface heat budget becomes
negative, and with the gusts of wind that create vertical turbulence.
The region of significant thermal difference between the surface ocean
and the deep ocean is called thermocline. It is close to the surface,
between 50 and 200 m, and very sharp in tropical regions; it becomes
deeper and wider toward the poles.
Diurnal and seasonal mixed layers form with the advance of the
seasons, a positive surface heat budget contributing to stratify the
surface layers, whereas the negative heat budget causes vertical
mixing, which deepens the mixed layer. The main thermocline is
defined as the most extreme position reached by the vertical mixing at
the end of winter. This notion is important since the mean thermocline
characterizes the limit of the ocean that will be in interaction with the
atmosphere, whereas the ocean beneath remains isolated and only
evolves in relation to the surrounding waters.
In mid- and tropical latitudes, the stratification is principally
dominated by gradients in temperature, the salinity only becoming
involved in regions of strong precipitation where episodic barrier
layers form (the warm pool in the west Pacific, the Bay of Bengal or
the mouth of Amazon). The notion of a “barrier layer” results from the
fact that the supply of fresh water at the surface isolates the lower part
of the mixed layer from the atmosphere, and thus creates a barrier
effect for exchanges between the ocean and the atmosphere. In polar
regions, the tough conditions at the surface tend to homogenize the
temperature profile, and the variations in salinity are therefore
responsible for the stratification.
