Ocean Salinity upsc

Salinity means amount of salt content dissolve per unit mass of water. It is expressed as number of gram of dissolve salt in 1000gm of seawater. The average salinity of seawater is 35 part per thousand (symbolically 35‰). It means that in 1000gm of seawater 35gm of salt are present.

Fresh Waters Salinity : Its salinity can range from 0 to 5 ppt.

Brackish Water Salinity : Brackish waters salinity can range from 0.5 to 35 ppt. It is mixture of seawater and fresh water.

Sea Water Salinity : Sea waters salinity is more than 35 ppt.

Role of Ocean Salinity :

* Salinity affects  thermal expansion, density, evaporation, humidity, temperature and compressibility.

* It influences the composition and movement of the seawater and determine the distribution of fish and other organism.

Highest salinity in the world :

Lake Van in Turkey - 330‰

Dead Sea of Palestine - 238‰

Great Salt Lake of USA - 220‰

Dissolve Salts in Sea Water (gm of Salt per kg of water)

Chlorine Sodium Sulphate Magnesium Calcium Potassium Bicarbonate Bromine Borate Strontium

18.97 10.47 2.65 1.28 0.41 0.38 0.14 0.06 0.02 0.01

Composition of Salt :

In one cubic kilometre of seawater 41 million tonnes salt are present. The share of different salts in the ocean -

* Sodium Chloride (NaCl) : 77.7%

* Magnesium Chloride (MgCl2) : 10.9%

* Magnesium Sulfate (MgSO4) : 4.7%

* Calcium Sulphate (CaSO4) : 3.6% 

* Potassium Sulphate (K2SO4) : 2.5%

Factors Affecting Ocean Salinity :

* The salinity of surface layer of ocean depend mainly on evaporation and precipitation.

* Surface salinity of specific costal regions diluting by fresh water flow from rivers, melting of ice.

* The ocean currents affects the ocean salinity.

* High wind velocity, high temperature and greater is the rate of evaporation lead high salinity.

* Salinity, temperature and density of water are interrelated. Hence, any variation in the temperature or density influences the salinity of an area.

Distribution of Salinity :

Horizontal Distribution of Salinity :

The distribution along latitudinal line known as Horizontal distribution. The amount of salinity vary from one part of the ocean to other. This variation in the amount of salinity is due to -

* Differences between evaporation and precipitation.

* Streams/river fall in sea (mixing of fresh water).

* Freezing and thawing of glacier.

* Wind velocity.

* Ocean currents.

Vertical Distribution of Salinity :

* In higher latitude the salinity increases with depth.

* In mid-latitude - salinity increases upto 35 metre and after decrease.

* Equatorial area the sub-surface salinity is low.

* Salinity, generally, increases with depth and there is a different zone  called the halocline  (compare this with thermocline), where salinity increases sharply.

High Salinity Zone :

* In the land locked sea e.g. Caspian sea - 180 part per thousand(enclosed sea).

* Maximum salinity in between 10 degree North to 40 degree North latitude and 10 degree South to 40 degree South latitude.

Note : Salinity decreases from Tropical zone to both area - Equator and Poles.

At Equator : 

Lower salinity due to -

* High temperature and high rainfall.

At Tropics : 

Highest salinity due to -

* Active role of evaporation.

* High temperature.

* Low rainfall.

* Steady trade wind.

At Polar Area :

Lower salinity due to -

* Rate of evaporation low.

* Melting of glacier taken place.

* High albedo.

Pacific Ocean Salinity :

The salinity of the surface waters of the Pacific influenced majorly by wind and by precipitation and evaporation patterns.

* At the equatorial area salinity is low 34 due to high rainfall, little evaporation and low wind speed.

* Southeastern area of Pacific ocean is  highest surface salinity 37

* Pacific waters near Antarctica have salinity of less than 34

* In the northern most area have salinity of less than 32

Atlantic Ocean Salinity :

* The average salinity of Atlantic Ocean in between 33 to 37‰.

* Near the equator, the Atlantic Ocean has a salinity of about 35‰.

* In the Polar areas, salinity ranging about 20 and 32‰.

* Maximum salinity at 20 degree North & 30 degree North of 20 degree West to 60 degree West. It decreases towards north.

Indian Ocean Salinity :

* Average salinity of the Indian Ocean is 35‰.

* Low salinity zone in the Bay of Bengal due to influx of number of big rivers.

* Arabian Sea has higher salinity due to high high evaporation and low influx of fresh water.

The salinity is less pronounced in the open sea than in the partly or completely enclosed sea. The salinity in Baltic lake is only 11 part per thousand while in the Black Sea the salinity is 18 part per thousand in Black Sea many rivers fall. The average salinity of Red Sea 40‰ due to high rate of evaporation and no rivers falling into it.