As stated earlier, ocean currents are induced by the combined effect of a number of factors. However, of all the factors, the following two factors are the most important: (i) the internal field of force of the mass structure, (ii) and the external field of force due to the winds.
Other forces such as the supply of fresh water by precipitation or the removal of sea-water by evaporation are less effective than the wind force. Besides, there are many factors which enter into the mechanism of the generation of ocean currents.
There factors act in a very complex combination. It cannot be said of any one current that it is the result of only one cause; density differences, for example, combine with wind effects in producing and sustaining a current.
Ocean currents can be divided into the following three categories according to their genetic origin.
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(i) Currents induced by thermo-haline convection mainly due to cooling of surface water in higher latitudes.
(ii) Currents that are caused directly by the stress of the wind on the sea surface.
(iii) Currents that are caused by the excess of precipitation over evaporation, or vice versa occurring in a particular oceanic region.
It is worth remembering that each of these types of currents has its own physical behaviour and acquires its own form because of the influence of other less important factors.
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Early in the eighteenth century it was the general belief that winds provide the motive force for the major ocean currents. Such a view was widely accepted.
However, by about-1870, a fierce controversy arose as to whether ocean currents are caused by the stress which the wind exerts on the sea surface or by differences of specific gravity (density) between the waters of the equatorial and Polar Regions.
It is hoped that the steps that have been taken towards a mathematical treatment of the issue might settle the matter conclusively.
Otto Pettersson was of the opinion that the primary effect of prevailing winds is the generation of surface currents of great intensity. He thought that the intensity of motion must decrease with depth.
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He considered that the major part of any ocean current system must be due to the thermodynamic cycle of latent heat consisting of the formation of ice in the Polar Regions and melting of ice in sea-water at lower latitudes.
Redfield (1834) considered that density contrasts between water masses provide some of the motive force behind major ocean currents. As a matter of fact, density of ocean waters affects circulation because differences in density cause gravitational displacement of the water.
Since all the three types of currents are present at the same time, it is very difficult to ascertain their causes on an entirely empirical basis. For this purpose it would be essential to take measurements from anchored ships at numerous places for a long period of time.
Besides, measurements must be conducted at different depths. However, the currents caused by the density differences can be computed from the observation of temperatures and salinities.
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Wind currents, on the other hand, can be examined theoretically. To this end in view, one must be familiar with the application of the laws of hydrodynamics to oceanography.
While considering the ocean currents the following major forces acting on them must be taken into account: (i) gravitational forces, (ii) pressure gradients, (iii) Coriolis force, and (iv) frictional forces.