MOVEMENT OF THE OCEAN WATER

INTRODUCTION

The water of the ocean is never still; it is blown into waves by the winds; it rises and falls with the rides; and in many places there are definite currents either permanently in one direction, or changing with tide or with the season.
The water of the ocean is always being disturbed or being acted upon by certain factors, which give it movement. As a matter of fact, all these forces that come to play upon the water of the ocean, disturb its equilibrium and nature restores the equilibrium by giving movement to ocean waters.
The movement of ocean waters from one part to another part takes place due to the effects of the earth’s rotation, wind difference, salinity and density of sea waters.

MOVEMENTS

The water movement of the oceans is mainly horizontal, following the density stratification. It is only in high latitudes that there is fairly direct transference between surface and bottom. It is convenient to consider the circulation in two parts, viz. That of the upper layer and that of the lower layer.

1. Upper Layer

a. Winds
The movements of upper layers are mainly generated by the prevailing winds. The drag exerted by the winds blowing steadily across the ocean causes the surface layer of water to move. Thus, because winds are the primary driving force of surface layers, there is a relationship between the oceanic circulation and the general atmospheric circulation. When winds change direction, the surface layer movements also reverse direction.
The most regular winds at the surface of the earth are the south-east trade winds which towards Equator over the Atlantic Pacific and Indian Ocean and north-east trade winds which blow towards the Equator over the Atlantic and Pacific oceans. In the India Ocean, north of the equator, the north-east monsoon blows in winter and the south-east monsoon in summer. In the higher southern latitudes of all three oceans, the west winds blow continuously.
The trade winds generate the north and south equatorial currents, both flowing towards the west.

b. The Earth’s Rotation
Although winds are important in generating surface movements, other factors also influence the movement of ocean waters. The most significant of these is the earth’s rotation.
The earth rotates from west to the east at a tremendous speed. As a result of this, waters of the earth’s surface, particularly that of the middle part of the equatorial region, move from the east to the west.
In considering movements in the oceans, the effect of the rotation of the earth must be considered. This force, known as Coriolis Force, does not affect a particle at rest; but when the particle moves, its deflecting effect is proportional to the speed of the movement. Its value is zero at the equator, but increases towards the poles.
Due to the earth’s rotation, currents are deflected to the right of their path of motion in the Northern Hemisphere, and to the left in the Southern Hemisphere.
The Coriolis Effect is greater in high latitudes and diminishes toward the Equator. As a consequence, the direction of surface current does not coincide with the wind direction. In general the difference between wind direction and surface current direction varies from about 15o along shallow coastal areas to a maximum of 45o in the deep oceans.

2. Deep Water Movement

The circulation of deep water is maintained by changes of temperature and salinity taking place at the surface as well as by differences of wind stress.

a. Density
Unlike the wind-induced movements of surface and near-surface waters, deep ocean movement is governed by gravity and driven by density difference.
Two factors – temperature and salinity – are most significant in creating a dense mass of water. Sea water becomes denser with decreased temperature, increased salinity, or both. Consequently deep ocean circulation is called Thermohaline Circulation. Arctic and Antarctic represent the two major regions where dense water masses are created. Antarctic water is chilled in the winter. The temperatures here are low enough to form sea ice, and since salts are excluded from ice, the remaining water becomes saltier. The result is the densest water in all of the oceans.
The waters of the equatorial region are light, less dense because of high evaporation and high temperature, on the other hand, the density of sea water is quite high in the cold regions because of the low temperature and low evaporation. So waters of this region are heavy. In order to remove the density difference of these two regions, a circulation of water takes place.

b. Salinity
Salinity increases from one part of the ocean to another. It is quite possible that water with low salinity could be lighter than water with high salinity. This may result in a natural circulation of water.