In addition to the above major factors controlling the distribution of surface salinity, there are periodic or seasonal variations in it. In the North Atlantic the maximum salinity occurs in March and the minimum in November, when their values are 36.7 and 36.59% respectively. In fact, these variations are associated with the changes in evaporation and small changes of the sea- level.
According to Defant, if tidal effects are disregarded, the most obvious periodic changes in salinity to be taken into account are the diurnal and annual variations. However the limited space in the text book does not permit a detailed discussion of these variations in the surface water of the oceans.
Our knowledge of the annual salinity variation is scanty. Charts showing surface salinities for the Atlantic Ocean show that over the larger part of the open ocean surface away from the coastal areas the annual range in salinity in the temperate zone hardly exceeds 0.5%o, rather usually less than 0.25%o.
According to Bohnecke charts, a zone with more than 0.5%o and a core with more than 1% and occasionally over 1.5% extends right across the Atlantic from South America to Africa between 5° and 15°N and includes the area of the equatorial counter current. In the Gulf Stream region the annual variation in the surface salinity ranges from 0.5%o to about 1%0.
However, the maxima of annual variation are found near the mouths of large rivers in the coastal areas, where there is a large seasonal variation in the supply of fresh-water. Similarly, in the polar areas where there is a great seasonal variation in melt-water of the ice, annual variations in the surface salinity are large.
In the equatorial region, however, the maximum salinity is found in September and the minimum in May. This is because of the monsoon and the related changes in advection.
The annual variation in the surface salinity in the adjacent seas is controlled by the climatic conditions. If an adjacent sea has a humid climate with a large supply of fresh river-water or precipitation, the annual range is rather large.
On the other hand, if an adjacent sea is situated in an arid region with high evaporation rate and no supply of freshwater either from rivers or precipitation, the annual variation is small. However, sometimes heavy precipitation of long duration reduces the salinity of the surface water.
This reduction may reach the deeper layers by turbulent mixing. But after the precipitation stops, the salinity differences are removed by the same turbulent processes.