Oceanography: Ocean Salinity

Basics

• Salinity means the total content of dissolved salts in Sea or Ocean.
• Salinity is calculated as the amount of salt dissolved in 1,000 gm of seawater.
• It is generally expressed as ‘parts per thousand’ (ppt).
• A salinity of 24.7 % has been regarded as the upper limit to fix ‘brackish water’.
• The average salinity of the ocean is 35 parts of salt in 1000 parts of water.

Water Bodies with the Highest Salinity

Share of Salt Dissolved in Ocean Water

Role of Ocean Salinity

• Salinity determines compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation, and humidity.
• It also influences the composition and movement of the sea water and the distribution of fish and other marine resources.

Factors Determining Ocean Salinity

1. Evaporation
   • The salinity of water in the surface layer of oceans depends mainly on evaporation. Where the evaporation is greater, the salinity is higher.
   • For example –  Mediterranean sea because the sea is surrounded by landmass.
2. Freshwater Inflow
   • Where the freshwater flow into the oceans is greater, the salinity is lower.
   • Major source of freshwater inflow is huge rivers and thawing of ice.
   • For example – The Bay of Bengal is less saline than the Arabian Sea because Bay of Bengal receives huge inflow of freshwater from Ganga river system and east flowing peninsular rivers.
3. Temperature and Density
   • Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of an area.
   • The regions with high temperatures are also regions with high salinity.
4. Ocean Currents
   • The warm currents near the equatorial region push away the salts from the eastern margins of the oceans and accumulate them near the western margins.
   • Ocean currents 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.
5. Atmospheric Pressure and Wind Direction
   • Anti-cyclonic conditions with stable air and high temperature increase salinity of the surface water of oceans

Horizontal Distribution of Salinity

• The salinity for normal open ocean ranges between 33 and 37.
• Landlocked water bodies like the Red Sea have a salinity of 41.
• In the estuaries and the Arctic, the salinity fluctuates from 0 – 35, seasonally.

Vertical Distribution of Salinity

• Salinity changes with depth, but the way it changes depends upon the location of the sea.
• Salinity at the surface increases by the loss of water to ice or evaporation or decreases by the input of freshwaters, such as from the rivers.
• Salinity at depth is very much fixed because there is no way that water is ‘lost’, or the salt is ‘added.’ There is a marked difference in the salinity between the surface zones and the deep zones of the oceans.
• The lower salinity water rests above the higher salinity dense water.
• Salinity, generally, increases with depth and there is a distinct zone called the halocline (compare this with thermocline), where salinity increases sharply.
• Other factors being constant, increasing the salinity of seawater causes its density to increase. Higher salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.