In humid regions precipitation exceeds evaporation. Thus, fresh water added to ocean water reduces salinity. In addition to this, the river discharge from the continents mixes with the saline ocean water and its salinity gets lowered.
On the other hand, in arid and semi-arid areas evaporation exceeds precipitation, so that fresh water evaporates leaving behind salt. The salinity in such areas is bound to be higher. That is why the distribution of salinity is found in latitudinal belts.
The belt of highest salinity is found between 20° and 25°N and S latitudes. These are the subtropical regions where precipitation is low, river discharge is negligible and the evaporation rate is high. From the subtropical belt of maximum salinity, it goes on decreasing towards the equator.
The salinity is relatively lower in the equatorial region despite the higher temperatures. This is due to the simple reason that the heavy equatorial precipitation dilutes the surface waters and evaporation is retarded due to cloudiness in the region. Besides, numerous large rivers pour out a large quantity of fresh water upon the surface of the oceans.
In the Polar Regions the temperatures are low throughout the year, so the rate of evaporation is also very low. In addition, at certain seasons fresh water is supplied by the melt water of snow and ice lying in thick layers on the ground. Naturally, therefore, the surface salinity of the sea water in this region is relatively lower.
In the middle latitude regions, the surface salinity of the oceans is relatively higher than in the Polar Regions. This is so because of two reasons. Firstly, the temperature of the ocean water in this region is relatively higher than that of the polar seas and secondly, the rate of evaporation is also higher.
In the Northern and Southern Hemispheres, the belts of maximum salinity are found between 20° and 25° N and S latitudes. In this subtropical belt of high pressure and descending air currents the amount of precipitation is negligible, and due to clear sky the rate of evaporation is high.
Besides, the inflow of fresh river water is very scant. As a matter of fact, the amount of salinity depends on the balance between influx of fresh water and evaporation. Since in the subtropical region evaporation exceeds precipitation, and input of fresh water is also very low, so the surface salinities are relatively higher.
In the Red Sea and the Mediterranean Sea the average surface salinities are 40-42%o and 38-39%o respectively. However, in these latitudes the surface salinity in the open ocean is about 36.5%.
The average salinity in the North and South Atlantic in the subtropical high pressure belt is 37.5%o and 37.0%o respectively. In the same belt the North and South Pacific exhibit an average salinity of 35.5%o and 36.5%o respectively. In the subtropical belt of the Indian Ocean the average salinity is close to 36.5%o.
From the above discussion of the surface salinity of the ocean water in different regions of the world, it is undoubtedly clear that between the two subtropical belts of maximum salinity, lays the equatorial belt of relatively lower salinity, where the average surface salinity varies from 34.0%o to 35.0%o In the polar region the average of the surface salinity generally falls below 34.0%o.
When the average salinities of the surface water of all the oceans of the Northern Hemisphere (34%o) are compared with those of the southern Hemisphere (35%o), there does not appear to be much difference.
However, due to the larger extent of the oceans in the Southern Hemisphere, its average salinity is slightly higher. In the open oceans of the world, generally the average salinity ranges from 33%o to 37%o. But in those regions where the large rivers discharge a large volume of fresh water, the average of the surface salinity falls below the average.