In the North Atlantic Ocean the surface water between 20° and 30°N latitude is more saline than that in any other ocean of the world. The salinity in this latitudinal belt touches a high-value of 37%.
In the Atlantic Ocean the distribution of salinity is largely controlled by the ocean currents, but the influence of evaporation and precipitation cannot be ignored. The average salinity of the surface water in the North Atlantic is 35.5%o, whereas in the South Atlantic it is only 34.5%o.
The major factor responsible for such a difference in the average salinity of the North and South Atlantic is the rapidity of evaporation in the Mediterranean Sea.
In the preceding discussion we have learnt that the highly saline water passing through the Strait of Gibralter mixes with the surface water of the open ocean, so that the degree of salinity is always higher than that in any other open ocean.
Generally the average salinity in a 5° broad latitudinal belt in this ocean is directly proportional to the difference in the amount of precipitation and evaporation. In the equatorial zone of this ocean supply of fresh water by heavy precipitation more than compensates the loss of water through evaporation.
That is why the average surface salinity is 35%o. On the contrary, at about 20°-25° N and 20°S latitude the evaporation exceeds precipitation so that in a wider area the average salinity is relatively higher (37%o).
From the subtropical region pole-ward the precipitation once again is in excess of evaporation. This results in gradual decrease in the salinity of the surface water. In higher latitudes the average salinity in extensive areas is 35%o.
Let us remember that the maritime influence is superimposed on the above mentioned characteristics of the distribution of salinity in this ocean, more so in the North Atlantic. The maritime influence produces certain anomalies in the general distribution of salinity.
At this point let us examine the role of the Gulf Stream, a warm ocean current, in controlling the salinity distribution in the North Atlantic. This warm ocean current drives warm and saline water (35%o) to Spitzbergen located at 78°N latitude.
Under the influence of this warm current along with its other branches salinity in the North Sea and the coastal waters of Norway is relatively higher. On the other hand, cold currents flowing from the polar area towards the equator bring with them Arctic water with 34%o salinity in the vicinity of 45°N latitude.
It is because of the influence of these currents that pole-ward from 40°N latitude the isohalines show the north-south alignment, while the same lines to the south of 45° S latitude show the east west trend.
In the polar areas, low temperature, retarded evaporation rate and supply of melt – water are some of the factors that lower the salinity of the surface water. The average salinity ranges from 30%o to 33%o.
Coastal lagoons and shallow seas are characterised by low salinity, which is less than 34%o. For example, in the Atlantic Ocean around Newfoundland, the surface salinity is as low as 34%o. Here, fresh water is poured in by the St. Lawrence and Labrador rivers.
Another factor that helps in lowering the salinity is the melt – water of numerous ice-bergs that are brought down by the Labrador Cold Current. On the contrary, in the central part of this ocean at about 25°N latitude the Sargasso Sea has a higher salinity of about 57%0 during summer.
Gulf of Guinea, even though situated in the subtropical region on the west coast of Africa-has a low surface salinity because of the upwelling of cold water mass there. In the South Atlantic, the surface salinity in the western part is relatively higher than in the eastern part.
It is particularly so between 10°S, and 30°S latitude, because it is in these latitudes that there is upwelling of cold and less saline water near the eastern coast of the ocean.
Besides, in the Atlantic Ocean, low salinities are found near the mouths of great rivers like the Amazon, Cong Niger and St. Lawrence etc. Salinity, however, is subjected to seasonal variations. Near the mouths of Amazon, Congo and Niger the amount of salinity is 15%o, 34%0 and 20%0 respectively.