In-depth studies of the structure and ocean dynamics
has resulted in the discovery of ring-shaped motion (eddies) on the edges
of large currents (Gulf Stream, Kuroshio etc.) as well as on the open Ocean,
and these are called seasonal eddies. They are mainly formed as a result
of the dynamic instability of currents and produce high-intensity cyclones
and anticyclones. The life of some of these eddies ranges from several
days to 3 and even 5 years. Their diameter can reach hundreds of kilometres,
their depth distribution up to a thousand or more meters, and their speed
of movement up to a few kilometres per day. The temperature contrast between
eddies and the ambient waters can reach 10?
C and more. In the open Ocean it is lower than on the edges of currents.
The discovery of synoptic eddies has an important
value. They can complicate surface and submarine navigation, acoustic transmission
calculations and change the density distribution of water. The differences
in the sound speed between the centre of an eddy and its edge occasionally
reaches several tens of meters per second.
Satellites, aircraft, drifting platforms and long-duration
buoys are used for detection and the investigation of eddies on the Ocean.
Synoptic eddies
During the 1970s, synoptic eddy observations were
often carried out by scientists in multi-national projects. In particular,
extensive research under the POLYMODE (1974-1979) program was carried out
by the USSR and the USA during experiments executed in the North-western
Atlantic Ocean. The resultant information was published in "THE POLYMODE
ATLAS", issued in 1986. The data are also used for modelling currents,
studies of synoptic eddies, etc.
Synoptic eddies are shown on the chart, as mapped
by Soviet oceanographers in the Gulf Stream at the end of 1976.
Counter-currents are usually associated with surface
currents, i.e., currents flowing an direction opposite to the main surface
currents. These are known to exist widely, and can be found at the surface
in Equatorial counter currents, and on the edges of the Gulf Stream and
Kuroshio currents.
In the 1950s, subsurface and deep counter-currents
were mapped. These were located in Equatorial zones of the Pacific, Atlantic
and Indian Oceans They were named after Cromwell, Lomonosov and Tareev.
Subsurface counter-currents flow from west to east. The entire system of
counter-currents covers about 26,000 km, and moves up to 80 million m3
of water every second. It consists of three jets: the central, most powerful
one is found on the Equator and two symmetrical jets, one in the Northern
Hemisphere and one in the Southern Hemisphere. The Equatorial jet covers
an ocean layer of 50-300 m and has a velocity of up to 1.5 m/s.
Deep counter-currents are also found under such currents
as the Gulf Stream, Kuroshio and others. The upper boundary of counter-currents
is at depths of 1,000-2,000 m. The speeds generally do not exceed 0.2-0.3
m/s. In 1967, speeds of counter-currents found under the Gulf Stream flow
were measured at 0.01-0.18 m/s.
Equatorial subsurface counter-currents
|
Counter-currents:
1.Lomonosov
2. Tareev
3. Cromwell |
CIRCULATION
OF DEEP WATERS
The main factors determining formation of deep water,
are temperature and salinity.
The surface water is cooled in sub-polar regions
of the World Ocean. Upon freezing, the salts are removed and this process
increases the salinity of the adjacent water. As a result, the water becomes
denser and descends. The areas where these deep waters are generated are
in the Greenland Sea in the north and the Weddell and Ross Seas adjacent
to Antarctica in the south.
From these sub-polar regions, deep waters are dispersed
into the Ocean. Their speed is very slow. For example, Antarctic Bottom
Water (AABW) travelling into the Pacific Ocean from the south requires
ten years to arrive.
The distribution of deep waters is controlled by
sea floor topography. For example, it has been found that North Atlantic
deep waters, following the bottom relief, are partially derived from the
Westwind Drift, which follows bottom topography from Antarctica.
Deep waters in motion
|
Speed
in m/s
1. <0.05
2. 0.05-0.15
3. Regions where deep water is formed |
Coastal upwelling (Northern Hemisphere)
|
1.
Wind
2. Ascent of waters |
Upwelling, or the ascent of deep waters to the surface,
is observed in coastal and in open regions of oceans. It is characterised
by a reduction of the surface water temperature and sometimes by a difference
in colour. The difference of water temperatures between a zone of upwelling
and an adjacent region can reach several degrees.
Coastal upwelling is caused by wind-driven surface
waters and currents controlled by coastlines. Such currents can be generated
by sustained winds blowing parallel to a coast, driving surface currents
(governed by the Coriolis effect) while forcing them to deviate away from
the shore to the right in the Northern Hemisphere and to the left in Southern
Hemisphere.
Regions of upwelling
The greatest upwelling has been observed on the coasts
of California, Peru, Morocco, South Africa, Somalia and Western Australia.
The extent of coastal upwelling zones can reach hundreds of km and their
widths, tens of km. The speed of upwelling is basically insignificant.
As an example, on the coast of California, it moves about 20 metres in
a month, and the waters originate at depths of less than 200 m.
In the open Ocean, upwelling is most often associated
with regions of diverging currents. The waters which have come to the surface
carry a large number of different biota, which promotes primary production
of basic elements of the food chain in the surface layer and makes zones
of upwelling major fishing regions of the World Ocean.
In regions where convergent currents and strong winds
act on coastal waters, downwelling occurs. Surface waters descend, thereby
providing oxygen to deeper layers.