It is another factor which leads to diversity in the amount of salinity.
In equatorial regions, inspite of high temperature, the salinity is less because there are zones of heavy rainfall, which reduces the salinity.
North and south of equatorial regions, there is an Intra Tropical Convergence Zone of high temperature and zone of strong north-west and south-east Trade winds. Hence, there is an increase in evaporation resulting in an increase in salinity.
Again, in polar and sub-polar regions there is an excessive amount of precipitation in the form of snow. This accumulates on the land and solidifies in the form of ice.
When this ice passes into the sea as glaciers and icebergs and melts in the temperate latitudes, it adds fresh water to the sea and hence reduces salinity.
In the Atlantic and the Pacific Oceans, during northern summers the surface water is more salty in regions where evaporation is greatest (probably the sub-tropical belt) and less salty in regions where fresh water is added dilution by snowfall or rainfall (greatest in equatorial and westerly belts).
Around the poles there is a belt of low salinity, which is due to the melting of vast fields of ice. River Water
The amount of salinity in sea water also varies due to the influx of fresh water by river. Hence, near the mouths of rivers Amazon, Congo, Niger, Ganges and St. Lawrence, comparatively lower salinity is found.
This effect is also marked along the coasts in the closed seas. In the Baltic Sea, many rivers from the land pour down fresh water in the Gulf of Bothnia and reduce the salinity. Similarly, many big rivers, such as Danube, Dneister and Dneiper, fall into the Black Sea and the salinity is found to be 18%o only.
On the other hand, salinity is as high as 40%o in the Mediterranean where evaporation is more than the influx of fresh river water.
Further, in the enclosed dependent seas, during periods of maximum runoff from the land, salinity of surface waters will be less than usual, while during periods of minimum runoff it will be greater.