Topic Ocean and Seas,,, Movement of ocean water

Movement of ocean water

        Ocean water never remains stagnant. It tends to move. Main causes of this movement are waves, currents and tides. Let us have a look on nature and causes of these movements.

1.     Waves

        The movement of surface water is called wave. Waves in an ocean are generated due to different natural factors. Actually it is the movement of energy which is transferred in water particles from one to another.

Nature

        The top of the wave is called crest and the bottom is called Trough. The vertical distance between crest and trough is called wave height. While the horizontal distance between two crests or troughs is called wave length. As the wave travels towards the shallow waters of the coast, its height increases and length decreases. The crest gets narrower and sharp. This called Breaker. The sharp pointed crest travels ahead of the bottom part and breaks to make foam. This is called surf.

Causes

        Winds are the main cause of generating waves. Cyclones and tornadoes which originate and travel across the ocean also create waves. An earthquake near coastal areas or beneath the ocean floor can also generate huge waves.

2.     Currents

        Rivers flow on land. Similarly when the ocean water moves permanently in a specific direction, it is called current.

Nature

        The currents which flow from equator to polar areas are called Warm currents. They increase the temperatures of coastal areas e.g. Gulf Stream along the eastern coast of North America in the Atlantic Ocean is a warm current. The currents which flow form polar areas to equator are called cold currents e.g. Labrador Current in Atlantic and Kamchatka current in the Pacific Ocean are cold currents. Currents move clockwise in the northern hemisphere and anti-clock wise in the southern. A larger current forming by the joining of two currents is called Drift. Where warm currents meet cold currents, fog is produced which is harmful for the sailing ships.

Causes

        Following are the major causes of current circulation.

i)      Permanent winds

        The most important cause of current circulation is winds. Winds force the ocean water to circulate in their general direction e.g. trade winds blow from east to west, while western winds blow from west to east. So the currents move eastwards under trade winds and vice versa under western wind.

ii)     Temperature of the ocean water

        Temperature difference id another cause of current circulation. Warmer water of equatorial regions moves upwards, while colder water of Polar Regions sinks downwards due to greater density.

iii)    Salinity of ocean water

        Salinity variation also causes the ocean water to circulate. Water of inland seas is more saline than the water of open seas and oceans. So more saline water due to its greater density sinks do downwards while less saline water moves upward.

3.     Tides

        There is a continuous rise and fall in sea level. Twice a day, the level rises and twice a day the level falls as well. This is called tides.

Causes

        The basic cause of tides is the gravitation of moon. This reality was presented by Newton in his “Theory of Gravitation” in the seventeenth century. According to this theory every two celestial bodies attract each other. So the moon, which is nearest to the earth, creates tides on earth inspire of its lesser mass as compared to the sun. Sun, as compared to moon, is 390 times more distant from the earth. So the effect of sun’s gravitation is 46% of the moon’s gravity on the earth.

Nature

        According to nature, there are two types of tides.

i)      Daily tides

        Daily tides occur twice with the interval of 12 hours 25 minutes. The effect of gravitation is maximum on the other side. But the centrifugal forces of the earth maintain balance of tides on the other side too. So the effect of gravitation is equal on both sides of the earth.

ii)     Monthly Tides

        These are of two types.

        Spring tides

        The earth revolves around the sun and the moon revolves around the earth. During this revolution it happens twice on the 1^st and 14^th date in a lunar month that the moon, earth and sun are in the same plane. The combined gravitation of sun and moon create very high tides. These are called spring tides.

        Neap tides

        Twice in the lunar month on the 7^th and 21^st date, it happens that the moon and sun are perpendicular to each other with reference to the earth. Gravitation of both bodies counteracts each other resulting in the creation of low tides. These are called Neap tides.