Salinity
Published on 02 Feb 2025
Ocean water salinity refers to the concentration of dissolved salts in seawater. Vertical and horizontal distribution of salinity is influenced by the following factors,
Factors affecting salinity
Evaporation: Increases the salinity by the removal of water by vaporization.
Example: Tropical waters are more saline compared to the equator with high cloud cover and thus low evaporation.
Precipitation: Decreases salinity by the addition of water and dilutes salts.
Example: Equatorial waters with high precipitation are less saline compared to tropical waters.
Freshwater addition: Decreases salinity by diluting the water.
Example: The Black Sea is less saline compared to the Mediterranean Sea because of the discharge of the Don, Danube, and Dnieper rivers.
Ocean currents: Changes salinity by the mixing of waters.
Example: North Atlantic Drift increases the salinity along north-western Europe.
Ice: Freezing of water leads to an increase in salinity and thawing or melting of ice leads to a decrease in salinity.
Example: Polar region where ice melting occurs seasonally.
Prevailing winds: Strong onshore winds can cause upwelling and usually saltier water is forced to the surface. This can temporarily increase salinity levels in near-shore areas, particularly during seasonal or prolonged periods of strong winds.
Distribution of salinity
Surface distribution
Hemisphere: The Northern hemisphere is warmer than the Southern hemisphere and hence salinity is higher.
Latitude: Salinity decreases towards the poles from the equator. However, the tropics record high salinity compared to the equatorial region because of heavy rainfall, upwelling of water, and high cloud cover.
Modification: Semi-enclosed bodies of higher latitudes have lower salinity and vice versa.
Example: Baltic Sea at high latitudes have low salinity compared to open oceans; Red Sea and Persian Gulf at lower latitudes have high salinity compared to open oceans.
Inland lakes: Inland lakes of tropical regions have high salinity.
Example: Lake Van, Lake Urmia.
Vertical distribution: There is no definite trend of salinity but it depends upon the location of the ocean.
Lower latitudes: Salinity decreases with an increase in depth due to a high rate of evaporation at the surface.
Higher latitudes: Salinity increases with an increase in depth because of the loss of water to ice.
Density: Salinity generally increases with depth as lighter and less saline water floats above the denser and more saline water that sinks below.
Halocline: Zone of transition where there is a rapid vertical change in salinity over a short depth.
