Salinity and Temperature of Ocean Water | Geography

After reading this article you will learn about the salinity and temperature of ocean water.

Salinity of Ocean Water:

We know that sea water is salty. It is seen from the chemical analysis that the sea water on the average contains 3½ per cent of salt, it means every 1,000 grams of sea water contains nearly 35 grams of salt. Of the total salt content in sea water, about 77 per cent is sodium chloride or common salt.

Scientists believe that much of the salt was dissolved in the water when the seas were initially formed during the early part of the earth’s history. Besides, the rivers bring down some quantities of salt and that is why the sea gets slightly saltier every year. This state of saltiness of sea water is called salinity.

Distribution of Salinity:

The salinity of all seas is not equal. It varies from one part of the ocean to the other.

There are two main factors for this variation:

(i) Amount of evaporation of sea water, and

(ii) Supply of fresh water by rivers, rainfall, melting ice, etc.

We know that near the tropics of cancer and capricorn temperature is very high. Here the sky is usually clear and the oceans are exposed to the onshore trade winds. As a result of these factors, evaporation is very high near the tropics. This causes concentration of salts.

For this reason, areas of the highest salinity are seen near the tropics (about 38 gm per kg). But the salinity near the equatorial region is relatively low because of higher rainfall. Apart from higher rainfall in this belt, many great rivers carrying fresh water enter the ocean. So, higher rainfall, higher relative humidity, and supply of fresh water do not allow active evaporation. As such, salinity is relatively low here.

Average salinity of this region is about 35 per cent. Away from the tropics, the salinity decreases poleward because of low evaporation and low temperature. In addition to this, the melting of ice yielding fresh water leads to decrease in salinity in the polar region. Average salinity near the poles is 34 per cent.

From the above discussion, we have an idea about the salinity of sea water. Besides these, there are many other reasons affecting the distribution of salinity. The seas which get fresh supply of water from many large rivers like the Amazon, Mississippi, Congo, Ganges, Brahmaputra, Yangtze Kiang, Hwang Ho, etc., are less salty. Some of the enclosed or partly enclosed seas are very salty while others have relatively more fresh water.

For example, the Red Sea has a salinity of more than 30 per cent because there are no big rivers entering into it. Moreover, evaporation is also high. The Baltic and Black Sea receive many large rivers and the evaporation is also small.

So, salinity of the Baltic Sea near Swedish coast is only 11 per cent and near the Gulf of Bothnia is only 2 per cent. The salinity of the black sea is 17 per cent. The inland lakes like the Dead Sea, the Caspian Sea and the Great Salt Lake of USA are more salty.

Temperature of Ocean Water:

The surface temperature of the ocean water mainly depends on the latitude and season. It is to be noted that water is heated and cooled rather slowly. Moreover, the water of the ocean is constantly moving and mixing by the action of waves and currents. This results in little variation of temperature of the surface water in different period.

Usually, the temperature of ocean water decreases from the equator to the polar regions. Thus, the average surface temperature of ocean water in the equatorial region is 27°-28°C; at latitude 44°-55°, the temperature is 10°-13°C; at latitude 60°, it is 0°C; and it is below the freezing point in the polar areas. In fact, the surface water 60° latitude to the polar seas remains permanently frozen.

The temperature of the sea water decreases with depth. At little depth, however, the temperature is more or less uniform. The rate of fall of temperature is well marked to a certain depth. The average temperature of sea water at depth of 200 metres is 12°C and at 1000 metres is 3.5°C.

But, below this the rate of decrease becomes very slight and gradual. The average temperature of sea water at the depth of 5000 metres is only 1°C and below this, there is very little changes of temperatures.

Temperature of the water of the polar seas increases with the increase in depth. The surface water of this region forms extensive ice-fields. Below the ice there is a layer of warm water. Unusually, the ice remains floating on this layer water.