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.
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.
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
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
3. Regions where deep water is formed
2. Ascent of waters
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.