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Why Salinity of Ocean water varies from place to place

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  • Salinity is the saltiness of sea water. Sea water is salty because it contains a number of dissolved salts. Most of them get there from rivers carrying different salts dissolved out of rock and soil.
  • The salinity is calculated as the number of grams of dissolved salts in 1,000 grams of sea water. It is usually expressed as parts per thousand or ppt.
  • The average salinity of sea water is about 35 per thousand. It means that in one kilogram of sea water there are 35 grams of dissolved salts.

Distribution of Salinity

Salinity of Open Sea

  • Salinity near Tropic of Cancer and Tropic of Capricorn:
    1. Near Tropic of cancer and Tropic of Capricorn highest salinity is found. This due to
      1. High evaporation due to intense Sun’s heat because of clear skies
      2. High temperature
      3. Steady trade winds
      4. Small number of rivers which brings fresh water to Ocean.
    2. Salinity is about 37 per thousand in these areas. From the Tropics, salinity decreases both towards the equator and the poles.
  • Salinity at Equator:
    1. At equator salinity is low as compared to tropics. This is due to:
      1. Heavy rainfall
      2. Influx of fresh water to the ocean by big rivers like Amazon and Zaire
      3. Due to high humidity evaporation of ocean water is quite slow
    2. Here salinity is 35 per thousand.
  • Salinity at poles:
    1. Salinity is very low as compared to tropics or equator at poles. This is due to:
      1. Very little evaporation because of very low temperature.
      2. Influx of fresh water to ocean by melting of ice.
    2. Here salinity ranges between 20 to 32 per thousand.

Salinity of Partially Closed Seas

  • Salinity of partially closed sea also varies due to different factors. Some of them are:
    1. Due to absence of any large rivers influx of fresh water are limited in some of the partially closed sea like Mediterranean Sea, Red Sea and Persian Gulf. Evaporation rates are also high due to high temperature.
    2. In Black sea salinity of water is low because of low evaporation due to low temperature and influx of fresh water by rivers like Danube, Dnepr and Don.
    3. In Baltic Sea salinity is quite low, which is around 11 per thousand. Reason for such a low salinity is low evaporation due to low temperature and influx of fresh water by melting of ice from Scandinavian mountain.

Salinity of Enclosed Seas and Lakes

  • Enclosed seas have high salinity because of very less or almost no source of influx of fresh water. Continuous evaporation makes water further salty.
  • Caspean sea which is a closed sea, however is having a low salinity because of influx of sufficient fresh water from Volga and Ural rivers. So the salinity here is only 14 to 17 per thousand.

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