Salinity of Ocean Water

• Salinity refers to the total amount of dissolved salts in the sea or ocean.
• It is measured as the amount of salt dissolved in 1,000 grams of seawater and is expressed as ‘parts per thousand’ (ppt).
• A salinity of 24.7 ppt is the upper limit for brackish water.
• Salinity affects the chemistry of natural waters and biological processes.
• Isohalines are lines on maps that connect areas with equal salinity.
• The average salinity of the ocean is 3.5% or 35 parts of salt per 1000 parts of water.

Role of Ocean Salinity

• Salinity is an important factor in determining several characteristics of the chemistry of natural waters and biological processes, including:
  • Compressibility
  • Thermal expansion
  • Temperature
  • Density
  • Absorption of insolation
  • Evaporation
  • Humidity
• It also influences the composition and movement of sea water and the distribution of fish and other marine resources. The share of different salts in seawater is as follows:
  • Sodium chloride (77.7%)
  • Magnesium chloride (10.9%)
  • Magnesium sulphate (4.7%)
  • Calcium sulphate (3.6%)
  • Potassium sulphate (2.5%)
• 

Factors Affecting Salinity of Ocean Water

• Some ocean sections have little rain but lots of evaporation due to warm, dry breezes.
• Evaporation leaves salt behind, making saltwater saltier.
• High salinity is found in the north and south Atlantic, which have strong winds and little rain.
• Rate of evaporation:
  • The ocean between 20°N and 30°N latitudes has more salinity than the temperate latitude ocean.
  • This is due to a higher rate of evaporation caused by high temperature.
  • However, tropical oceans may not necessarily have more salinity, as indicated in the next paragraph.
• Amount of Freshwater Added​:
  • Places with high daily rainfall, high relative humidity, and addition of freshwater have low salinity.
  • Oceans into which huge rivers like Amazon, Congo, Ganges, Irrawaddy, and Mekong drain have lower salinity.
  • Baltic, Arctic, and Antarctic waters have a salinity of <32 ppt due to freshwater added from the melting of icebergs and several large poleward bound rivers.
• The degree of water mixing by currents
  • Land-locked (enclosed by lands) areas have higher salinity due to no mixing of freshwater and continual evaporation
  • Examples of such areas include the Black Sea, the Caspian Sea, the Red Sea, and the Persian Gulf
  • Where there is free mixing of water by surface and subsurface currents, the salinity range is small

Horizontal distribution of salinity

• Normal open ocean: salinity ranges from 33 to 37 ppt.
• High salinity regions:
  • Landlocked Red Sea: salinity is as high as 41 ppt.
  • Hot and dry regions: salinity sometimes reaches 70 ppt due to high evaporation.
• Comparatively low salinity regions:
  • Estuaries and the Arctic: salinity fluctuates seasonally from 0 to 35 ppt due to freshwater influx from ice caps.
• Pacific:
  • Salinity variation is mainly due to its shape and larger area.
• Atlantic:
  • Average salinity is around 36-37 ppt.
  • Equatorial region: salinity is about 35 ppt due to heavy rainfall and freshwater from the melting of ice.
  • Polar areas: salinity is low, ranging between 20 and 32 ppt, due to little evaporation and large amounts of freshwater from melting ice.
  • Maximum salinity (37 ppt) is observed between 20° N and 30° N and 20° W – 60° W, gradually decreasing towards the north.
• Indian Ocean:
  • Average salinity is 35 ppt.
  • Bay of Bengal: salinity is lower due to freshwater influx from the river Ganga.
  • Arabian Sea: salinity is higher due to high evaporation and low influx of fresh water.
• Marginal seas:
  • North Sea: records higher salinity due to more saline water brought by the North Atlantic Drift.
  • Baltic Sea: records low salinity due to influx of river water in large quantities.
  • Mediterranean Sea: records higher salinity due to high evaporation.
  • Black Sea: salinity is very low due to enormous freshwater influx by rivers.
• Inland seas and lakes:
  • Salinity is very high due to regular supply of salt by rivers and evaporation.
  • Examples include Great Salt Lake (Utah, USA), Dead Sea, and Lake Van (Turkey).
• Cold and warm water mixing zones:
  • Salinity decreases from 35 to 31 ppt in western parts of the northern hemisphere due to influx of melted water from the Arctic region.

Vertical Distribution of Salinity

• Salinity changes with depth and depends on the location of the sea.
• Surface salinity can increase due to ice loss, evaporation, or decreased input of freshwater from rivers.
• Salinity at depth is fixed because water cannot be lost or salt added.
• Surface and deep zones of the ocean have marked differences in salinity.
• Lower salinity water is found above higher salinity, denser water.
• Salinity generally increases with depth, and there is a distinct zone called the halocline where salinity increases sharply.
• Increasing salinity causes seawater density to increase, leading to stratification by salinity.

Salt Budget

• The salt cycle involves the movement of salt from the ocean to the lithosphere, atmosphere, and back to the oceans.
• Water leaches minerals from rocks through surface erosion and adds them to rivers and streams which eventually reach the oceans.
• Some of the salts accumulate at the ocean bottom through sedimentation and turn into mineralized rocks that may be raised above the ocean surface by plate tectonics or volcanic activity, returning the salt to the lithosphere.
• Wind can spray salt from the ocean into the atmosphere, which returns to the lithosphere mixed with precipitation.
• The salt cycle operates over a long period of time.
• Around 3 billion tons of salt are added to the oceans from the land each year, with only a small fraction extracted by humans for consumption.