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Salinity


Q.Write short note on salinity patterns in Indian ocean.(12/11 )
Q.Give a reasoned account of the distribution of salinity in the oceans and partially enclosed seas.(60/93)
Salinity is defined as the total amount of solid material in grams contained in 1 gram of seawater.
FACTORS CONTROLLING SALINITY
  1. Influx of river water:Ex-Low salinity near mouth of  Ganga, Congo, Amazon etc
  2. Evaporation:Ex-Higher salinity in lower latitudes.
  3. Precipitation.Ex-Equatorial region has lowest salinity than the subtropical high pressure belt.
  4. Atmospheric pressure: High pressure results in low rainfall.Ex- subtropical high pressure belt.
  5. Prevailing Wind: Blowing wind facilitates process of evaporation.Ex- Southern hemisphere has low salinity than Northern hemisphere.
  6. Circulation of ocean water: Drives more saline water towards lower saline water.Ex-Equatorial warm currents drive salt from Western coastal areas towards eastern coastal areas.
HORIZONTAL DISTRIBUTION OF SALINITY
1.Hemisphere wise
  • Salinity of the northern hemisphere and Southern hemisphere are 34.5% and 35.5 % respectively.
  • Lesser salinity in Northern hemisphere is due to dilution effect of rivers and ice melts.
  • in Southern hemisphere, continents are nearly absent, so water addition is less. Also, unobstructed prevailing winds results in very high evaporation.
2.Latitudinal:.
  • On an average salinity decreases from equator towards the poles due to decrease in evaporation.
  • Near equator salinity is around average salinity  35% or slightly less. In spite of high temperature and evaporation, it witness conventional rainfall throughout the year.
  • In tropical and subtropical zone, salinity is higher than equator in both hemisphere  as anti cyclonic condition results in lower precipitation but high temperature leads to evaporation.
  • In middle latitudes, salinity further decreases because of lower temperature as well as mixing of warm and cold currents.
  • In higher latitudes in Northern hemisphere salinity is very low because of influx of ice melts ,cyclonic rainfall and very less evaporation.In Southern hemisphere salinity is lower because of very high speed prevailing winds and lower evaporation.
3.Regional
Jenkins has divided oceans on the basis of regional variations of salinity in three categories
  1. Seas having salinity above normal
  • Ex-Mediterranean Sea,Red Sea.etc.
  • Mediterranean sea and Red Sea have a very high salinity of 39% and 41% respectively because they are surrounded by dry landmass and there's hardly influx of river water.Also the rate of evaporation is very high
  1. Seas  having normal salinity.
  • Ex-Gulf of Mexico
  • Gulf of Mexico salinity is 35 -36% because the warm equatorial current increases the salinity.
  1. Seas  having salinity below normal .
  • Ex- Baltic Sea,Hudson bay,North sea.
  • Baltic Sea and Hudson Bay have very low salinity of 35 to 15% because of ice melts fresh water from the rivers and low evaporation.North Sea along the similar latitudes have greater salinity of 31-33% because of the salt brought by North Atlantic drift.
4.Indian Ocean
  • The spatial distribution of salinity in the Indian Ocean is more variable and complex than the Pacific and Atlantic Ocean
  • An average salinity of 35 % is found between 0°-10°N , but it gradually decreases northward in the Bay of Bengal because of influx of immense volume of fresh water brought by the Ganga river.
  • On the other hand, the Arabian Sea records higher salinity 36% than the Bay of Bengal because there is higher rate of evaporation due to relatively less humid condition and low influx of freshwater as compared to the Bay of Bengal.
  • The West Coast of Australia records highest salinity due to dry weather
  • The partially enclosed sea have higher salinity e.g it is 37% at the head and 40% in the interior of the Persian Gulf.Red Sea records highest salinity of 41% .



VERTICAL DISTRIBUTION
  • No definite trend of distribution of salinity with depth  can be spelt out because both the trends of increase and decrease of salinity with increasing depth have been observed.
  • Vertical distribution of salinity has following characteristics:
  1. salinity increases with increasing depth in high latitudes, where there is positive relationship between the amount of salinity and depth because of denser water below
  2. The trend of increase of salinity with increasing depth is confined to 200 fathoms from the surface in middle latitude, beyond which it decreases with increasing depth.
  3. Salinity is low at the surface at the equator due to high rainfall and transfer of water through equatorial current but highest salinity is noted below the water surface.It again become low at the bottom.
  4. Thermocline zone: Maximum salinity is found in the upper layer of the oceanic water and salinity decreases with increasing depth.Thus, the upper zone of maximum salinity and the lower zone of minimum salary is separated by a transition zone which is called thermocline zone on an average above which high salinity is found while low salinity is found below this zone.
salinityVertical.jpg
SIGNIFICANCE OF SALINITY
  • Freezing and boiling points are greatly affected and controlled by addition or subtraction of salt in seawater saline water freezes slowly in comparison to freshwater.
  • Salinity and density of seawater are positively correlated. This is why man is seldom drawn in the sea water with very high salinity.
  • Evaporation is controlled by salinity of the ocean.Salt in water lower the rate of evaporation in the ocean.
  • Spatial variation is sea water salinity is important factor in the origin of ocean currents .
  • Ocean salinity affects the Marine organisms and plant community

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