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Seawater Salinity and its effects on earth’s climate

Salinity is the saltiness or dissolved salt content of a body of water. The term salinity describes the level of different salts e.g. sodium chloride, magnesium and calcium sulphates etc dissolved in water.  Sea water has strong salinity due to its higher dissolved salts content. On an average, seawater has a salinity of about 3.5% (or 35 g/L). Sea water has almost all type of salts and other materials dissolved in it. This includes dissolved materials from Earth’s crust as well as materials released from biosphere (from living organisms). Sea Surface Salinity (SSS) can have dramatic effects on the water cycle and ocean circulation.

Salinity is expressed by the amount of salt found in 1,000 grams of water e.g. 1 gram of salt in 1,000 grams of water; the salinity is 1 part per thousand, or 1 ppt. The graph below shows the proportions of salts in sea water;

Chemical Ion Contributing to Seawater Salinity


Why does sea water have variation in  salinity?

Evaporation against precipitation: The relative amount of evaporation or precipitation in an area causes variations in ocean salinity. If there is more evaporation than precipitation, then the salinity increases; considering the fact that salts are not evaporated into the atmosphere along with water. If there is more precipitation than evaporation then the salinity decreases. As per the analysis of the GPCP version 2 data set (NASA Earth Science Applications Directorate), for the period 1979 to 1999, high precipitation was observed in the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), and the storm tracks in the North Pacific and Atlantic Oceans. Low precipitation was observed in the Polar Regions and in the subtropics of the East Pacific, East Atlantic, and the Southeast and Northwest Indian Ocean.Hence high precipitation regions like ITCZ, SPTZ and the storm tracks in the North Pacific and Atlantic Oceans have low salinity as compared to the regions where low precipitation occurs.

Large rivers meeting Oceans: When large rivers meet the oceans, they change the salinity of water.  Large rivers have more impacts on the sea water salinity as compared to the smaller rivers and streams. The runoff from the smaller streams and rivers is quickly mixed with ocean water by the currents and has little effect on salinity and in other hand, large rivers e.g. Nile, Amazon, Ganges, etc considerably affect the salinity due to its little or no salt content.

Thawing of large icebergs: Out of 3% freshwater available on earth, 67% is stored in glaciers, icebergs and ice caps. Large icebergs that are made up of frozen fresh water and without any salts can decrease the salinity of the ocean water upon melting in the ocean water.

The distribution of Salinity across the oceans: As per the information by NASA Science (EARTH), ‘highest concentrations (which is about 37 practical salinity units) of salt water are present the mid-Atlantic Ocean and lower-Atlantic off the coast of Brazil, the Mediterranean Sea and the Red Sea. Lower concentrations are found near the Arctic and Antarctic and the coastal regions of East Asia and western North America’.

Why is it important?

Salinity affects seawater density, which in turn governs ocean circulation and climate. We know that the wind drives upper ocean currents, however ocean current can also flow deep below the surface. These deep-ocean currents are driven by differences in the sea water density. As sea water density is controlled by temperature and salinity, these factors make the oceans very dynamic in nature. The term thermohaline circulation (THC) (thermo– temperature and haline – salt content) refers to a part of the large-scale ocean current circulation that is driven by global density gradients created by surface heat and salinity of the sea water. The thermohaline circulation is also known as the Ocean conveyor belt.

The Ocean Conveyor Belt is the principal mechanism by which the oceans store and transport heat. Since salinity is a key ingredient in the global thermohaline circulation, its measured value will help to discover how its variation induces change in global ocean circulation. Salinity is a major factor in ocean and atmospheric science because it affects the ocean currents that further control the climate of the planet.



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