Geoscience Reference
In-Depth Information
A second deep circulation cell is initiated by the formation of Antarctic
Bottom Water. Its origin is to a large extent in the North Atlantic. In summer the
ocean around Antarctica is covered by a layer of warm, low saline surface water
generated by solar radiation, precipitation and melted sea ice. In winter the water
cools to freezing temperatures, sea ice forms and the salinity increases again and cold
and saline Winter Water is formed. Below the Winter Water, there is a layer of warm
and salty water which intrudes from the north, the Circumpolar Deep Water. Due to
its high salinity this warm layer is stable below the cold, less saline Winter Water.
South of and below the Circumpolar Deep Water colder deep waters are found.
This bottom water originates from a
flow of dense water from the shelves along
the continental slope into the deep sea forming the deep circulation cell by
transforming the ascending Circumpolar Deep Water in a sinking water mass
which ventilates the deep ocean.
Frontal systems
The changes from one type of water mass to another do not occur
steadily along a constant gradient, but in sharp steps across areas of strong
gradients called fronts which are transition zones between the water masses
(
Figure 5.9
).
Fronts were
first discovered in the atmosphere as sharp separation lines
between air masses of different temperatures (warm or cold fronts) and the same
phenomenon was observed by oceanographers in the sea.
Fronts are of particular interest for several reasons. They are relatively simple
visible indicators for the water mass distribution as the gradient is often visible at the
sea surface as a change in water colour. The fronts are not only surface structures but
they also separate deep water masses.
These frontal gradients in temperature and salinity produce density gradients
in the ocean, which in turn induce pressure gradients that drive ocean currents.
In consequence, ocean fronts are often related to enhanced ocean currents.
In addition to the
flow along the front, there is motion perpendicular to it.
Converging water masses produce a sinking motion, and diverging water masses
produce an ascending motion, often called an upwelling. These cross-frontal
motions concentrate material and plankton in the fronts and it is because of this that
biological production can be so high in these areas. Deeper waters are normally
higher in nutrients and frontal upwelling transports nutrients into the near surface
waters. There, in the light, the higher nutrients meet concentrations of
phytoplankton and biological production is enhanced. The fronts often generate
stable layers, which keep the phytoplankton up near the surface and enhance
primary production. This biomass feeds the zooplankton, which in turn attracts the
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