Properties of Ocean Waters, Movements of Ocean Waters part – 2

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Properties of Ocean Waters

The temperature and salinity are two important aspects of the ocean waters which affect their movements.

Temperature of Ocean Waters

The temperature of the surface water of the oceans varies in the same way as that of the land surface. This is because the insolation is responsible for the varying quantities of heat which are received at different latitudes and in different seasons. Usually, the temperature is higher near the equator and gradually decreases towards the poles. The mean annual temperatures of about 27°C or higher are common in the tropical seas.

There is a general decrease towards the poles where the mean temperature of around 1.8°C is found. The decrease of temperature of surface water towards the poles or increase towards the equator is not uniform because drifting warm water from the tropical seas may move into the higher latitudes or vice versa and gives a local increase or decrease in temperature. The upwellings of deep, cold water also reduce locally the surface temperature of tropical and subtropical sea waters.

There is also variation in the vertical distribution of temperature. Temperature decreases with increase in depth. As the rays penetrate the water, their intensity is reduced by scattering, reflection, and diffusion. The rate of decrease in the temperature is not equal at all depth. Up to a depth of about 100 metres, the temperature of water is same as that of the surface, while it falls from 15°C to about 2°C between the surface and a depth of 1,800 metres. The decrease between 1,800 and 4,000 metres is from 2°C to about 1.6°C.

The main process of heating the ocean waters are:

  • By absorption of heat from the sun

  • By convection of heat through the ocean bottom from the interior of the earth

The cooling processes are:

  • By loss of heat to the atmosphere

  • By evaporation

Salinity of the Ocean Waters

One of the most striking characteristics of the ocean water is its salinity or saltiness. The dominant salts are sodium chloride and magnesium chloride with 77.7% and 10.9% respectively. Due to the free movement of ocean water, the proportion of different salts remains remarkably constant in all oceans and even to great depth. But the degree of concentration of the salt solution in oceans does vary appreciably in different seas.

Salinity is defined as the weight in grams of solid material left after the evaporation of 1000 grams of sea water. If the weight of solid material is 35 grams, the salinity would be shown 350/00 or 35 per thousands.

In the Baltic Sea, fresh water enters it from the surrounding land and reduces the salinity to 70/00. But great evaporation combined with very dry climate in the Red Sea region gives the water of this sea a high salinity of 410/00 - 420/00. In enclosed seas, which are areas of inland drainage such as the Caspian Sea, the salinity is very high. It is 180/00 in the Dead Sea of Jordan. The variation of salinity in different seas and oceans is affected by:

  • The rate of evaporation

  • The amount of fresh water added by streams and icebergs

  • Mixing of the ocean waters

Movements of Ocean Waters

The oceans exhibit three major types of movements:

Waves

Waves are oscillatory movements that result in the rise and fall of water surface. The movement of each water particle in a wave is circular. A wave has two major parts. The raised part is called the crest. Between the two crests are low areas called troughs. The vertical distance between trough and crest is called the wave height. The horizontal distance between two crests or two troughs is called the wave length. The time it takes for two crests to pass a given point is called the wave period. Fast moving waves have short period while slow moving waves have long period.

Wave Anatomy

Wave Anatomy

Wave Anatomy

The size and force of a sea wave depends on three factors:

  • Velocity of the wind

  • The length of time the wind blows

  • Distance that the wind has travelled across the open sea termed a fetch.

Waves are an important agent of erosion. When waves are associated with storms or volcanic eruption, they are very violent and cause damage on coastal areas. They are also a source of energy and efforts are being made to harness their energy.

Tides

Along the coasts, we observe the sea water moving both upwards and downwards at rates varying from place to place. At the time of a rising sea level, the incoming tide towards the land is known as a flow tide or a flood tide. At the time of a falling sea level after a few hours, the tide water going out or withdrawn, is an ebb tide or low tide. The flood tide is a high tide and the ebb tide is a low tide. Twice a day regularly at constant intervals, a tide flows in and twice a day it ebbs away. Twice a month, flow tides are higher and the ebb tides are lower than the average. Also, twice a month flow tides are lower and the ebb tides are higher than the average.

The regular interval between two high tides or between two low tides is 12 hours and 25 minutes. Each day or in 24 hours the high tide arrives about 51 minutes later than on the previous day. It is so because each day the rising and setting of the moon also falls behind by 51 minutes. It takes 24 hours and 50 minutes for the rotating earth to bring the same meridian vertically below the moon every day.

The factors responsible for bringing about such a variation in the regulation and the size of tides are:

  • The location of the sun, the moon and the earth in relation to each other which is rarely in a straight line;

  • The distances of the sun and the moon from the earth are not constant;

  • Our globe is not entirely covered with water;

  • The outline or shape of the coast may help or hinder the tides.

The moon and the sun both exert their gravitational force on the earth. The sun which is bigger in mass than the moon is also at a greater distance from the earth than the moon. Therefore, the gravitational attraction of the moon is more effective on the earth than the gravitational attraction of the sun. Since the water is liquid and mobile, its bulging in the direction facing the moon is easily noticed, yet a lower tidal bulge also develops on the other side of the earth farther from the moon because of moon’s least attraction.