NCERT Notes on Salinity of Ocean Water is an integral topic of Geography subject for the UPSC IAS exam. The salinity of ocean water or oceanic salinity is mainly defined as the concentration of various salts in the water. All water present on the Earth surface whether they are in the form of rain water, ocean water all contains dissolved mineral salts.

• The oceanic salts can be calculated as the amount of salt dissolved in 1,000 gm of seawater.
• Salinity is generally expressed as ppt i.e. “ Part Per Thousand.” If the Salinity level is 24.7%, that water is considered as the upper limit to fix ‘brackish water’.
• The average oceanic salinity is about 35 parts per thousand.

Salinity is also known to be a significant factor for deciding several characteristics of the chemistry of natural waters and biological processes. Along with that it also has its influence on the composition and movement of the sea: water and the distribution of fish and other marine resources.

Along with UPSC exams, these NCERT notes are equally important for other competitive exams like PO, SSC, state civil services exams and so on. In this article we will be covering all aspects of salinity of ocean water.

Salinity of Ocean Water (UPSC Geography) NCERT Notes: Download PDF Here!

Composition of Different Salt in Ocean Water

sodium chloride — 77.7%

magnesium chloride—10.9%

magnesium sulphate —.4.7%

calcium sulphate — 3.6%

potassium sulphate — 2.5%

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Sources of Salinity

• The salt quantity which is present in ocean waters have their origin on the continental landmasses. They come into oceans from different sources like rain, rivers, groundwater table, sea-waves, winds, and glaciers.
• Along these sources, some salt also comes from the ocean bottom. As the layer is present beneath the Earth crust contains minerals in a molten state which can reach the crust either due to volcanic activity or due to their outgassing from the fissures present at the bottom of the ocean.
• Some of salinity is also contributed from the dead and decomposing organic matter also adds to the salinity of the oceans.

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Factors Affecting Salinity of Ocean Water

Salinity of ocean water is dependent on several factors and because of these factors they keep varying with the place and time of measurement. Some of the major factor on which salinity of ocean is dependent are listed below:

• Evaporation: In terms of evaporation it is known that salinity is higher at places with high rates of evaporation. For example: the Red Sea, Persian Gulf etc., have high salinity as they experience the highest rates of evaporation.
• Temperature: There is a direct relationship present between temperature and salinity. As regions with high temperature are also the regions with high salinity. Like: the Torrid Zone has a higher salinity level than the Frigid zone.

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• Precipitation: There is an inverse relationship between precipitation and salinity. A region with high precipitation has a low salinity level. Because this region is the only equatorial region that is as hot as the sub-tropics, it records lower salinity than the sub-tropics since the former receives heavy precipitation in a day.
• Ocean current: Ocean current also plays a crucial role in determining the distribution of salt in the ocean water. As warm currents present  near the equatorial region force away the salts from the eastern margins of the oceans and accumulate them near the western margins. In a similar way, ocean currents present in the temperate regions increase the salinity of ocean waters near the eastern margins. For example: Gulf Stream in the North Atlantic Ocean increases the salinity of ocean waters along the western margins of the Atlantic Ocean whereas the North Atlantic Drift increases the salinity of waters in the North Sea.
• The influx of fresh water: Area where all major rivers meet, their salinity level is low. For example: at the mouths of rivers such as the Amazon, Congo, Ganga etc. Along with that the ocean surface salinity is found to be lower than the average surface salinity. Same happens in polar regions, when melting of glaciers occurs during summer at that time there is an influx of fresh water into the surrounding ocean which reduces the surface salinity.

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Horizontal And Vertical Distribution Of Salinity

Horizontal Distribution of Salinity:

The surface salinity of oceans decreases on either side of the tropics. For example: the surface salinity along the Tropic of Cancer is measured to be 36 parts per thousand (ppt) whereas surface salinity at the equator it’s around 35 parts per thousand. Further on the basis of their salinity level, seas across the world can be categorized as follows:

• Seas with salinity levels below the normal: Sea which have low salinity level because of the influx of freshwater. Such salinity level oceans are: the Arctic Ocean, Southern Ocean, Bering Sea, Sea of Japan, Baltic Sea etc. The salinity level of these oceans are below 21 ppt. 
• Seas with normal salinity levels: If the salinity level is between 35 ppt to 36 ppt, then it is considered to be normal salinity level. For example: Caribbean sea, Gulf of Mexico, Gulf of California, yellow sea, etc have normal salinity level. 
•  Seas with salinity levels above the normal: When salinity level is in between 37 ppt- 42 ppt then it is considered as higher salinity level. Such high salinity is because of their location in regions with higher temperatures leading to greater evaporation. For example: Red Sea, Persian Gulf, Mediterranean Sea, etc have high salinity levels.

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Vertical Distribution of Salinity:

• There is change in salinity with depth but the way of changing relies on the position of the sea. Whereas salinity at the surface of the sea is decreased by the input of fresh waters or increased by the loss of water to ice or evaporation.
• Salinity at the depth is constant as there is no addition of salt to it. 
• There is a difference in the salinity level between the surface zones and the deep zones of the oceans.
• Higher saline dense water is present beneath the lower saline water. 
• There is decrease in salinity with depth but there is a distinct zone called the halocline, where salinity increases abruptly.
• With increase in salinity of seawater there is an increase of density, which causes stratification by salinity.

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