Question.
What are the controlling factors of salinity? Explain its horizontal and vertical distribution in the ocean. ( 63rd BPSC, 2019)
Answer.
Salt is one of the main minerals dissolved in ocean water. Although the average salinity of ocean water is 35 parts per thousand, salinity is not equally distributed everywhere. Salinity in the ocean water varies both horizontally and vertically in the ocean water.
The following are controlling factors of salinity:
Evaporation and precipitations:
Evaporation and precipitation have a great influence on the Salinity of surface water.
For example,
- The temperate ocean has lower salinity due to low evaporation.
- Tropical ocean between 20 to 30 degrees latitude has high salinity due to high evaporation and low precipitation.
- The equatorial ocean has relatively low salinity than the tropical ocean( 20 to 30 degrees latitudes) due to heavy precipitation.
Amount of Freshwater addition:
- Ganga, Mekong, Congo, and Amazon rivers add hue fresh water in the ocean resulting in the lower salinity in the ocean.
Ocean current:
- Enclosed seas have relatively higher salinity due to the absence of a mixing agent that is the ocean current. For example, the salinity of the Caspian sea, black sea, etc are higher.
Freezing and thawing of the ice:
- As we know that ice do not contain salt. In the polar region, salinity is greatly influenced by the freezing and thawing of the ice.
Horizontal distribution of Salinity:
- Normal ocean salinity is generally between 33 to 37.
- In landlock such as the Red sea, dead sea, and black sea, it is higher than 40 due to higher evaporation and no mixing.
- Estuaries and Arctic sea, it fluctuates between 0 to 35 depending on freshwater addition and ice.
- In the hot and dry regions where the evaporation of water is higher, it may reach 70 ppm.
Vertical Distribution of Salinity:
- Salinity in the depth is generally fixed as salinity factors only worked on surface water.
- Salinity generally increases with an increase in depth.
- There is a distinct zone called Halocline, in this zone, there is a sharp increase of salinity.
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