Ocean Salinity

Ocean Salinity

This article deals with ‘Oceanography: Ocean Salinity’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here


  • Salinity of Solution is defined as amount of salt in 1000 gram of water .
  • Salinity of ocean water is 35.5 ppt ( parts per thousand)  & maximum amount is of common salt.
Sodium chloride 78%
Magnesium Chloride 12%
Magnesium sulphate 3.5%
Calcium Sulphate 2.5%

Why Sodium Chloride (NaCl) is present in highest proportion ?

  • Every salt has cycle & they remain in sea water for specific time( called residual time) & then precipitated to bottom surface .
  • Sodium (Na) & Chlorine (Cl) has highest residual time in ocean water leading to very gradual removal => that is why they are present in highest proportion .

Salt Budget

  • Irrespective of absolute salinity,  proportion of above salts remain same in all parts of the world  .
  • Amount of addition or extraction of fresh water compared to salt content in ocean water decides absolute salinity of oceans .
  • Salt Budget  = Budget of addition of salt & removal of salt .

Sources of salts on ocean water.

  • Sediments carried by rivers (most important) .
  • Submarine volcanism at Mid Oceanic Ridge .
  • Chemical reaction between rocks of geothermal vent of volcano & cold water.
  • Erosion of oceanic rocks and wave erosion of coastal rocks  .

Removal of Salts in ocean water.

  • Physical Removal :  waves break at beaches ie salt spray .
  • Biological removal : marine life forms extract calcium from sea water for their bones & shells .

Factors effecting salinity

Evaporation Higher the rate of evaporation ,higher is salinity.
Temperature Warmer parts are more saline than frigid ones.
Precipitation Higher the precipitation, lower is the salinity.
Influx of Freshwater Influx of freshwater leads to lower salinity .
Atmospheric pressure  
Circulation of Ocean water Stagnant water has more salinity (Eg: Sargasso sea).
Windy situation Wind accelerate evaporation => windy situation = more saline

Salinity of Oceans

  • Standard  salinity of ocean water is 35.5 ppt ie salinity of Atlantic ocean .
Greater than 35.5 High saline.
Lower than 35.5 Less  saline.
  • Some highly saline lakes . Man  seldom drown in sea with high salinity because water is  highly dense .
Dead Sea 238 ppt West Asia
Lake Van 330 ppt Turkey
Great Salt Lake 220 ppt USA
Lake Urmia   Iran

Overall pattern of Salinity across world

Ocean Salinity

a. Latitudinal variation

  • Salinity is highest at tropics(not Equator)  & decreases on both sides.
  • This is due to Interplay of evaporation & precipitation & other complex interactions.

b. Hemispheric variation

  • Northern Hemisphere is warmer => high evaporation => more saline .
  • But southern Pacific => Roaring 40, furious 50 , Shreaking 60 , Screaming 70 => very fast winds . Hence in Pacific ocean , southern hemisphere has more salinity

c. Local Variation in Salinity

Warm ocean currents – Increases temperature of water => equivalent to High evaporation .
– Leads to more salinity.
Upwelling Cooler water from depth come to surface => low salinity.
Transport by currents The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift.

d. Enclosed Seas

Tropical region – Warmer than open sea .
– Will lead to high salinity.
– Eg Mediterranean Sea , Persian Gulf etc. 
High Latitude – Cooler than open sea .
– Lower salinity than open seas .
– Eg Baltic Sea, Gulf of Bothnia etc .

e. Inflow of large rivers

  • Ganga – Brahmaputra => flow into Bay of Bengal => large freshwater .
  • Bay of Bengal is less saline than Arabian Sea.

f. Glaciers

  • Those oceans /seas which receive greater glacier water are less saline
  • Baltic Sea is very less saline because of this reason .

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