Salinity of Ocean Waters

All natural waters — rain water, river water, or ocean water — hold dissolved mineral salts in some quantity. Salinity is the term used to define the total content of these dissolved salts in sea water. It is calculated as the amount of salt, in grams, dissolved in 1,000 grams (1 kg) of sea water, and is usually expressed in parts per thousand (ppt). So a salinity of 35 ppt means 35 grams of salt in every kilogram of sea water.

Salinity is an important property of sea water. A salinity of 24.7 ppt has been taken as the upper limit to demarcate brackish water — water above this limit is considered truly saline, while anything below it is brackish.

Factors Affecting Ocean Salinity

Several factors influence the salinity of ocean waters:

• In the surface layer of the oceans, salinity depends mainly on evaporation and precipitation. High evaporation removes fresh water and leaves salt behind, raising salinity; heavy rainfall dilutes the surface water and lowers it.
• In coastal regions, surface salinity is greatly influenced by the fresh water flow from rivers, which lowers salinity near the coast.
• In polar regions, the processes of freezing and thawing of ice matter. When sea water freezes, salt is left out of the ice, making the remaining water more saline; when ice melts, it adds fresh water and lowers salinity.
• Wind influences salinity by transferring water from one area to another.
• Ocean currents contribute to salinity variations by moving water of different salinities across regions.

Salinity, temperature, and density of water are all interrelated. Any change in temperature or density affects the salinity of an area.

Horizontal Distribution of Salinity

For normal open ocean water, salinity ranges between 33 ppt and 37 ppt. But there are notable exceptions:

• In the land-locked Red Sea, salinity is as high as 41 ppt.
• In estuaries and the Arctic, salinity fluctuates seasonally from 0 to 35 ppt.
• In hot and dry regions where evaporation is high, salinity sometimes reaches as high as 70 ppt.

Among the world's highest salinity water bodies are Lake Van in Turkey (330 ppt), the Dead Sea (238 ppt), and the Great Salt Lake (220 ppt).

The Pacific Ocean shows salinity variations mainly due to its shape and larger areal extent. Salinity decreases from 35 ppt to 31 ppt on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region. Similarly, after 15°–20° south, it decreases to 33 ppt.

The Atlantic Ocean has an average salinity of around 36 ppt, with the highest salinity recorded between 15° and 20° latitudes. Maximum salinity of 37 ppt is observed between 20° N and 30° N and 20° W–60° W, and it gradually decreases towards the north.

Some regional seas show clear contrasts. The North Sea, despite its location in higher latitudes, records higher salinity because of the more saline water brought by the North Atlantic Drift. The Baltic Sea records low salinity due to the influx of river waters in large quantity. The Mediterranean Sea records higher salinity due to high evaporation. The Black Sea, on the other hand, has very low salinity because of the enormous fresh water influx by rivers.

The average salinity of the Indian Ocean is 35 ppt. A low salinity trend is observed in the Bay of Bengal due to influx of river water. On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

Vertical Distribution of Salinity …