All waters in nature, whether rainwater or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in seawater.

It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.

It is usually expressed as parts per thousand (o/oo) or ppt. Salinity is an important property of seawater.

The salinity of 24.7 o/oo has been considered as the upper limit to demarcate ‘brackish water’.


Ocean salinity is affected by the following factors :

  • The salinity of water in the surface layer of oceans depends mainly on evaporation and precipitation. When evaporation occurs in the ocean, it causes the salt to be left behind in the water, making the ocean water more salty. In contrast, when precipitation falls into the ocean, it can dilute the salt content in the water, making the ocean water less salty. The amount of evaporation and precipitation in a particular area can have a significant impact on the salinity of the ocean water in that area.
  • Surface salinity is greatly influenced in coastal regions by the freshwater flow from rivers, and in polar regions by the processes of freezing and thawing of ice.  

  • Wind also influences the salinity of an area by transferring water to other areas.  Wind can affect ocean salinity in a few ways. One way is through its effect on the surface of the ocean. When wind blows across the surface of the ocean, it can cause the surface water to become more mixed, which can lead to changes in salinity. This is because wind can cause the surface water to become more turbulent, which can lead to the mixing of water from different depths. Additionally, wind can also cause the surface water to become more stratified, which can lead to changes in salinity as well.
  • The ocean currents contribute to the salinity variations. ocean currents can also affect salinity by transporting water from one area to another. For example, the Gulf Stream carries warm water from the Gulf of Mexico to the North Atlantic, where it cools and becomes denser, causing it to sink and flow back towards the equator as deep water. This process, known as thermohaline circulation, can also affect the salinity of the ocean by mixing saltier water from the Mediterranean with the colder, fresher water of the North Atlantic.
  • Waves and tides: These can mix the water column, redistributing salt and fresh water.
  • Biological activity: The presence of certain types of marine organisms, such as phytoplankton, can affect salinity.
  • Upwelling and downwelling: Upwelling brings cold, deep, and nutrient-rich water to the surface, while downwelling pushes surface water to the ocean floor.

  • Human activities: Human activities such as damming rivers, diverting water, and desalination can also affect ocean salinity.
  • Climate change: Changes in temperature, precipitation, and evaporation due to climate change can affect ocean salinity.
  • Temperature: Warmer water can hold less dissolved salt than colder water, leading to variations in salinity.


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