1. Wind and frictional drag:
The prevailing winds play the most effective role in setting the currents in motion. Wind-driven horizontal circulation in the surface waters of the oceans is induced as a result of the stress which the winds exert on ocean water.
The word "stress" represents a force that is produced by the friction of the lowermost layer of the wind with the sea-surface. The frictional force is directly proportional to the wind velocity.
At low wind velocities, the frictional force is considerably lower. Because the winds exert a frictional drag on the water surface, they produce an upslope in the direction of flow.
The trade winds blowing from north-east and south-east in the northern and southern hemispheres respectively pile up water that is warm as well as light, on the western side of the oceans in the trade-wind belts.
Their effect, according to C.A.M. King, extends to 150 meters in the Atlantic and 300 meters in the Pacific Ocean. The trade winds may be considered as the main force in providing the backbone of the system of surface currents in the oceans.
As a result of the stress exerted by the trade winds, north and south equatorial currents are produced. These currents are present in the Atlantic, the Pacific and the Indian Oceans.
Under the impact of the Coriolis force, these currents move in a direction parallel to the equator, and they are deflected into clockwise gyres in the northern hemisphere and anticlockwise gyres in the southern hemisphere.
A gyre is a circular spiral form and it refers to the circular motion of water in all the major oceans. It is centered in the subtropical high pressure regions.
In the equatorial regions of all the oceans, the Equatorial Counter Current develops between the north and south equatorial currents to make up the water deficiency at the surface. These counter currents flow eastwards.
The best example of the effect of wind on the ocean currents can be seen in the Indian Ocean. During the northern summer, the southeast trade winds cross the equator and start blowing as the southwest monsoon.
During the northern winter, the wind direction is reversed. During the summer months the equatorial currents flow westwards following the summer monsoons. On the contrary, during the winter months the equatorial currents in the north Indian Ocean start flowing towards the east.
If we compare the distribution maps of the prevailing winds with those of the ocean currents, it becomes amply clear that there is a very close relationship between the two.
As a matter of fact, the necessary energy needed to maintain the circulation system of the oceans comes from the planetary wind system.
It may be pointed out that even though the direct effect of winds on the ocean surface is limited to only about 100 m depth, and the subsurface circulation mainly results from density differences, it is true that winds exercise indirect control on the oceanic circulation at various depths.
On the basis of observations made by Nansen during the voyage of the Fram, it was found that the currents flow at an angle of 20 to, 40° from the direction of the wind.
To summarise, the energy needed by ocean currents is supplied exclusively by winds, which owing to tangential stress at the sea surface, initiate and maintain ocean currents.
Ekman, a physicist, developed the mathematical relationship associated with the observations made by Fridtj Nansen regarding the deflection of ocean currents relative to the wind direction.
Ekman developed the theory called the Ekman Spiral. The theory assumes a homogeneous water column that is set in motion by wind blowing across its surface. In the northern hemisphere the surface current moves in a direction 45° to the right of the wind.
Further, this surface layer of water moving as a sheet sets another layer beneath it in motion. The energy of the wind passes through the water column from the surface downwards.
Thus, each successive layer of water is set into motion, but their velocity goes on decreasing with increasing depth. The direction of each successive layer is to the right of the direction of the just upper layer.
However, at a certain depth, the energy passed by the wind to the moving sheet of water is lost, and the motion stops as a result of the wind stress at the surface of the ocean water.
According to Ekman, this depth occurs at about 100 meters from the surface. At this depth the water actually moves in the opposite direction of the wind that induced the movement of the surface water. The spiral movement of water.
The length of each arrow in the figure is directly proportional to the velocity of the individual sheet of water, and the direction of each arrow shows the direction of movement of the sheet.
Under such theoretical conditions, the surface current should flow at an angle of 45° to the direction of the wind. The net water movement should be at right angles to the direction of the wind.
But in reality, no such conditions are found in the ocean. The movement actually taking place because of wind stress on the surface of the ocean is slightly different from the ideal.
Generally the surface current will move at an angle of less than 45° to the direction of the wind, and the net transport will be at angles less than 90° to the direction of the wind.
This is true in shallow coastal waters, where all the movement may be very nearly in the direction of the wind and the deflection with increasing depth is at a very slow speed. The net water movement is at right angles to the wind direction.
In view of the Ekman Spiral, in a closed gyre in the northern hemisphere in the North Atlantic, a clockwise circulation will produce a piling up of water in the centre of that gyre.
But it is seen that within all such gyres hills of water rising up to 2 meters above the water level are found at their margins. The piling up of water on these hills continues until the gravitational force acting on each and every water particle balances the Coriolis force, and the water particles begin to roll down the slopes of the hill.
Due to the Coriolis force, the particles are deflected to the right and move very slowly down the slope. The particles mostly move in a direction parallel to the side of the hill.
Thus, under the combined effect of the gravitational force pulling the water d6wnwards and the Coriolis force deflecting it around the hill, geostrophic currents are produced.
These currents flow nearly parallel to the contour of the hill and represent equilibrium between the Coriolis force pushing water up the slope and the gravitational force acting to move the water down the slope.
Therefore due to the displacement of the apex of the hill to the west, the ocean current moves more rapidly along the western margin than along the more gently sloping eastern margin.
Another effect of the Coriolis force or the deflecting force produced by the rotation of the earth from west to east is the westward intensification of ocean currents.
Examples may be cited from the North Atlantic and North Pacific Oceans where the currents flowing on the western side of each ocean are much stronger and narrower in cross section than their counter parts on the eastern side. It is this phenomenon that is called the western intensification of currents.
The Gulf Stream or the Kuroshio currents are far stronger, but are narrower than the Canary or Californian Current, even though these currents, warm as well as cold, transport about the equal amount of water so as to maintain continuity of flow.
2. Density of ocean water:
The spatial variation of density causes movement of ocean water. Density differences are produced by variations in the physical properties of sea-water like temperature, salinity, amount of precipitation, supply of fresh water by rivers and melting ice, the amount of materials held in suspension, atmospheric pressure and mixing of different currents.
It would be proper to point out that density differences are the second most important factor causing the generation of ocean currents. The density difference in ocean water leads to the establishment of pressure gradient in it, and this causes vertical movement.
Denser water tends to move downwards, and less dense water being lighter tends to move upwards. As we have already seen, high temperature leads to lower density and lower temperature results in higher density. Salinity also affects density. Higher salinity means higher density, whereas lower salinity results in lower density of sea water.
Density is controlled by the rate of evaporation also. If the rate of evaporation is high, as in the case of subtropical regions, the density is bound to increase. On the other hand, the larger amount of precipitation and the supply of fresh water by the inflowing rivers and by the melt- water tend to lower the density of sea water.
Whatever be the reason, the density contrast in the surface water causes the movement of ocean water in the form of ocean currents. The exchange of water between the open ocean and its partially enclosed seas in the form of surface and sub-surface currents is the result of density differences in them.
The North Atlantic Ocean and the Mediterranean Sea offer the best example of this type of water movement. It is undoubtedly true that next to the influence of the prevailing winds, the density exercises the dominant influence on the initiation of surface currents in the oceans.
3. Coriolis force:
The earth rotates on its axis from west to east. Because of the diurnal rotation of the earth, besides centrifugal force, another deflecting force is produced which is called the Coriolis force, after the French physicist G.G. Coriolis, who discovered it in 1885.
In fact, the so-called Coriolis force is the resultant of the centripetal as well as the centrifugal force. It always acts towards the right in the northern hemisphere, and left in the southern hemisphere. The Coriolis force is a function of the latitude.
It is maximum at the poles and minimum at the equator. Thus, under the influence of this force, the ocean currents in the northern hemisphere tend to turn to their right, and in the southern hemisphere to their left.
It is, therefore, obvious that both the rotation of the earth as well as the Coridlis or the deflective force have dominant control on the movement of ocean currents.
4. Gravitational force:
The force of gravity is the resultant of the mass attraction of the earth and the centrifugal force of the earth's rotation. Gravity depends on the latitude.
It is maximum at the poles, because of the polar flattening, and is minimum at the equator because of the rotation and the oblate configuration of the earth.
Besides, gravity depends on the distance of the sea surface from the centre of the earth and on regional in homogeneities of the earth's crust. Every water particle is attracted towards the centre of the earth by the gravitational force acting on it.
The surface water of oceans is subjected to smaller gravitational force than the deeper parts. Thus, it is clear that the gravitational force acting on very particle of ocean currents depends on the latitude and the depth of the ocean.
The gravitational force, as a matter of fact, controls the Coriolis force produced due to the rotation of the earth. Tidal currents are largely determined by the force of gravitation of the sun and moon.
But its influence on ocean currents is negligible. However, the effect of this factor is that the movement of water is directed towards the centre of the earth. That is why denser water always tends to sink under lighter water.
5. Atmospheric pressure:
Atmospheric pressure also exercises some influence on the initiation of ocean currents. Horizontal pressure differences determine the ocean currents. Different regions of the oceanic areas respond differently to the impact of atmospheric pressure.
Because of higher pressure, there is reduction in the volume of surface water which results in lowering of the sea-level. On the contrary, lower pressure on the sea surface results in raising the water level.
The result of the variation in the atmospheric pressure is that the water movement starts from the higher water level towards the lower level. The currents produced due to difference in the water level are called gradient currents.
A typical example of this type of currents is offered by the ocean currents flowing from the Baltic Sea towards the North Sea. Remember that due to low atmospheric pressure the water level in the Baltic Sea is relatively higher, whereas in the North Sea the water level is lower under the impact of higher pressure.
However, the velocity of these currents is directly proportional to the difference in the water level. The credit for discovering the causes of the origin of these currents goes to Knudsen, an eminent Swedish oceanographer.
6. Precipitation and evaporation:
There is regional variation in the supply of fresh water in the form of precipitation. The reason is that certain areas receive a larger amount of precipitation than other areas.
Naturally, therefore, the water level in areas with greater amount of precipitation is higher than that in areas with lesser amount. This kind of difference in the water level creates a slope on the sea surface.
Thus, in order to eliminate the difference in water level ocean currents are produced. Besides the difference in the water level, the supply of fresh water results in the decrease of salinity, this in turn brings about a change in the density of sea water.
The precipitation, therefore, contributes a lot, directly or indirectly, in the initiation of ocean currents. The equatorial region for example, receives a larger amount of precipitation than the mid-latitude regions.
That is why ocean currents invariably flow from the equatorial regions towards the high latitudes, where the amount of precipitation is much less.
On the contrary, in the subtropical high pressure belt the climatic conditions permit a much greater rate of evaporation and the amount of precipitation is much less.
Since in this high-pressure region evaporation, exceeds precipitation, the salinity and density of the sea water are relatively higher. Therefore the surface currents start flowing from the areas with abundant precipitation to areas with very little precipitation.
To compensate the loss, the subsurface deeper currents flow in the opposite direction-from the subtropical high pressure region to the equatorial region.
7. Difference in temperature:
As we are aware, the amount of insolation is not the same everywhere on the sea surface. Thus, the variation in the amount of insolation is the main cause of non-uniformity in the rate of evaporation in different regions.
In the low latitude regions the amount of insolation received far exceeds that received in the high latitude regions. But due to the greater amount of cloudiness and precipitation, the rate of evaporation is much lower.
This causes both the salinity and density of sea water to be relatively lower. Thus, the light and warm water from the low latitude region is driven by the prevailing winds towards the high latitude regions.
8. Melt Water:
Melting of ice supplies fresh water to oceans so that their level is raised. Melt water reduces the salinity in ocean water. Thus, water movement from regions of high sea level to other regions of relatively lower sea level is quite natural.
The Eastern Greenland Current is initiated due to the supply of melt water. It may be stated that during winter a large amount of water is converted into ice. During summer, on the contrary, melting of ice takes place and, thus, a large amount of fresh water is added to the sea.
This not only raises the sea level, but also helps in decreasing the salinity. In a particular region, the melt water becomes the generating factor of ocean currents.