As noted earlier, the average salinity of ocean water is approximately 35 %. However, numerous salinity variations are found in horizontal as well as vertical directions. These differences in salinity of the ocean water are certainly the results of different physical processes operating in the oceans.
When ocean water evaporates or freezes, most of the salt remains in the water. On the contrary, precipitation, influx of fresh water brought by the rivers from the continents, and melting water of snow and ice reduce the surface salinity.
However, oceans currents and other movements of sea water like tides and waves constitute the mixing processes. The mixing processes decrease the salinity differences both at the surface as well as through all the layers of ocean water.
Distribution of salinity on the map is shown by means of isohalines. Isohalines are lines drawn on the map joining places having the same amount of salinity. Remember that these lines exhibit only the surface salinity of the ocean water.
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Observational data confirm the fact that at a depth of 100 or 200 fathoms below the surface, the salinity distribution of surface salinity is, therefore, entirely different from the vertical distribution.
Variations in the distribution of salinity result from the interplay of mainly three factors supply of fresh water either through surface runoff, precipitation, influx of river water or melt water, the rate or rapidity of evaporation, and the mixing processes.