Salinity of Ocean Water | UPSC

Salinity of Ocean Water

• Salinity refers to the total content of dissolved salts present in sea or ocean water.
• Salinity is calculated as the amount of salt dissolved in 1,000 grams (1 kg) of seawater.
• It is generally expressed as ‘parts per thousand’ (ppt), denoted by the symbol ‰.
• A salinity of 24.7 ‰ has been considered the upper limit to define ‘brackish water’ — water that is saltier than freshwater but less saline than seawater.
• Salinity is a significant factor in deciding several characteristics of the chemistry of natural waters and biological processes within the ocean.
• Isohalines are used on maps to show the salinity of different places. These are lines joining places having an equal degree of salinity. They help oceanographers visualize and compare salinity patterns across different ocean regions.
• The average salinity of the ocean is approximately 35.2 ‰ — meaning roughly 35 parts of salt are dissolved in every 1,000 parts of water.

The salinity of certain enclosed water bodies is far higher than the open ocean. For example, the salinity of the Great Salt Lake (Utah, USA) is 220 ‰, the Dead Sea is 240 ‰, and Lake Van in Turkey is 330 ‰. The oceans and salt lakes are becoming progressively saltier over time because rivers continuously dump more dissolved salts into them, while freshwater is lost through evaporation.

Role of Ocean Salinity

• Salinity determines several critical properties of ocean water including compressibility, thermal expansion, temperature, density, absorption of insolation, evaporation, and humidity.
• It also influences the composition and movement of sea water, the distribution of fish, and other marine resources. Certain marine species can survive only within specific salinity ranges, making salinity a key factor in marine biodiversity.
• Salinity directly affects the density of ocean water, which in turn drives the thermohaline circulation (the Global Ocean Conveyor Belt) — one of the most important circulatory systems on the planet.

The share of different salts dissolved in ocean water is as shown below —

• Sodium chloride — 77.7%
• Magnesium chloride — 10.9%
• Magnesium sulphate — 4.7%
• Calcium sulphate — 3.6%
• Potassium sulphate — 2.5%
• The remaining 0.6% includes trace amounts of other minerals like calcium carbonate, magnesium bromide, and other dissolved elements.

Factors Affecting Salinity of Ocean Water

There are parts of the ocean where hardly any rain falls, but warm dry winds cause large amounts of evaporation. This evaporation removes water — when water vapor rises into the atmosphere, it leaves the salt behind, so the salinity of the seawater increases. This causes the seawater to become denser.

The north and south Atlantic have high salinity — these are areas where there are strong winds and not much rain. The Mediterranean Sea in Europe has a very high salinity — 38 ppt or more. It is almost closed off from the main ocean, and there is more evaporation than there is rain or extra freshwater added from rivers. This makes it one of the most saline marginal seas in the world.

Rate of evaporation

• Compared to the temperate latitude ocean, the ocean between 20°N and 30°N latitudes has higher salinity because of a higher rate of evaporation (caused by high temperature and dry atmospheric conditions in the sub-tropical high-pressure belt).
• But this doesn’t mean that tropical oceans (near the equator) will have higher salinity. The reasons are explained in the next point — heavy rainfall near the equator offsets the evaporation effect.
• The relationship between evaporation and salinity is straightforward: higher evaporation = higher salinity, because water is removed but salt remains behind.

The amount of fresh water added by precipitation, streams and icebergs

• Places having high daily rainfall, high relative humidity, and a significant addition of freshwater from rivers have low salinity.
• E.g. Oceans into which huge rivers like the Amazon, Congo, Ganges, Irrawaddy, and Mekong drain have lower salinity near their mouths because of the enormous volume of freshwater discharged.
• The Baltic, Arctic, and Antarctic waters have a salinity of less than 32 ppt because much freshwater is added from the melting of icebergs, as well as by several large poleward-bound rivers.
• When sea ice forms, salt is excluded and left behind in the surrounding water, which increases the salinity of the remaining liquid water. When the same ice melts, it releases freshwater, which decreases salinity.

The degree of water mixing by currents

• Regions that are land-locked (enclosed by land on most or all sides) have higher salinity because of no mixing of freshwater from the open ocean + continuous evaporation. E.g. — Black Sea, Caspian Sea, Red Sea, Persian Gulf.
• The range of salinity is negligible where there is free mixing of water by surface and subsurface currents. Open ocean areas with active current circulation tend to have more uniform salinity levels.
• Ocean currents also redistribute salinity — warm currents from low latitudes carry more saline water poleward, while cold currents bring less saline water equatorward.

The salinity of some important water bodies is given below for comparison:

Horizontal distribution of salinity

The salinity for normal open ocean ranges between 33 and 37 ppt.

High salinity regions

• In the landlocked Red Sea (not to be confused with the Dead Sea, which has much greater salinity), salinity is as high as 41 ppt. The Red Sea’s high salinity is a result of very high evaporation rates, minimal rainfall, and limited freshwater inflow from rivers.
• In hot and dry regions, where evaporation is extremely high, the salinity sometimes reaches up to 70 ppt.

Comparatively Low salinity regions

• In the estuaries (enclosed mouths of rivers where fresh and saline water get mixed) and the Arctic, the salinity fluctuates from 0 – 35 ppt seasonally, depending on the amount of freshwater coming from ice caps and seasonal river discharge.

Pacific

• The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent. Being the largest ocean, the Pacific exhibits wide variations in salinity from one region to another depending on local conditions of evaporation, precipitation, and freshwater input.

Atlantic

• The average salinity of the Atlantic Ocean is around 36-37 ppt — the highest of all the major oceans.
• The equatorial region of the Atlantic Ocean has a salinity of about 35 ppt.
• Near the equator, there is heavy rainfall, high relative humidity, cloudiness, and the calm air of the doldrums (the Inter-Tropical Convergence Zone). These factors reduce the net evaporation and keep salinity relatively lower.
• The polar areas experience very little evaporation and receive large amounts of freshwater from the melting of ice. This leads to low levels of salinity, ranging between 20 and 32 ppt.
• Maximum salinity (37 ppt) is observed between 20°N and 30°N and 20°W – 60°W. This high-salinity belt corresponds to the sub-tropical high-pressure zone where evaporation is maximum and rainfall is minimal. Salinity gradually decreases towards the north from this belt.

Indian Ocean

• The average salinity of the Indian Ocean is 35 ppt.
• A low salinity trend is observed in the Bay of Bengal due to the influx of river water by the Ganga, Brahmaputra, Irrawaddy, and other major river systems that discharge enormous quantities of freshwater.
• On the contrary, the Arabian Sea shows higher salinity due to high evaporation rates and a low influx of freshwater from rivers. The Arabian Sea receives far less river discharge compared to the Bay of Bengal.
• This contrast between the Bay of Bengal and the Arabian Sea is a frequently tested topic in UPSC examinations.

Marginal seas

• The North Sea, in spite of its location in higher latitudes, records higher salinity due to the more saline water brought by the North Atlantic Drift (a warm ocean current).
• The Baltic Sea records low salinity (as low as 7 ppt) due to the influx of river waters in large quantities from numerous rivers draining into it from the surrounding Scandinavian and Eastern European countries.
• The Mediterranean Sea records higher salinity due to high evaporation and its semi-enclosed nature with limited exchange of water with the Atlantic Ocean through the narrow Strait of Gibraltar.
• Salinity is, however, very low in the Black Sea due to the enormous freshwater influx by rivers, especially the Danube, Dnieper, and Don rivers.

Inland seas and lakes

• The salinity of inland seas and lakes is very high because of the regular supply of salt by rivers falling into them. Since these water bodies have no outlet to the ocean, the salt accumulates continuously.
• Their water becomes progressively more saline due to evaporation — water evaporates but the dissolved salts remain behind, concentrating over time.
• For instance, the salinity of the Great Salt Lake (Utah, USA), the Dead Sea, and Lake Van in Turkey are 220, 240, and 330 ppt respectively.
• The oceans and salt lakes are becoming saltier as time goes on because the rivers continuously dump more salt into them, while freshwater is lost due to evaporation.

Cold and warm water mixing zones

• Salinity decreases from 35 to 31 ppt in the western parts of the Northern Hemisphere because of the influx of melted water from the Arctic region.
• Where warm and cold currents meet, the mixing of waters of different salinities creates transition zones. These zones are important for marine biodiversity, as the mixing of nutrient-rich cold water with warm water creates ideal conditions for plankton growth.

Sub-Surface Salinity

• With depth, the salinity also varies, but this variation is again subject to latitudinal differences. The pattern of decrease or increase in salinity with depth is also influenced by cold and warm currents operating in the region.
• In high latitudes, salinity generally increases with depth. This is because the surface water at high latitudes is diluted by freshwater from melting ice and heavy precipitation, keeping surface salinity low. The deeper water, however, retains a higher salt content.
• In the middle latitudes, salinity increases up to about 35 metres depth and then it decreases. The surface water in these regions tends to have moderate salinity, with a slight increase just below the surface due to subsurface mixing.
• At the equator, surface salinity is lower compared to the sub-tropical regions. This is because of heavy rainfall and high freshwater input at the surface in the equatorial belt. Below the surface, salinity tends to increase as the effect of freshwater diminishes.

Vertical Distribution of Salinity

• Salinity changes with depth, but the way it changes depends upon the location of the sea and the local oceanic conditions.
• Salinity at the surface increases by the loss of water to ice or evaporation, or decreases by the input of freshwater, such as from rivers, rainfall, or melting ice.
• Salinity at depth is very much fixed and stable because there is no direct mechanism for water to be ‘lost’ or for salt to be ‘added’ at great depths. There is a marked difference in the salinity between the surface zones and the deep zones of the oceans.
• The lower salinity water rests above the higher salinity dense water. This is because denser (saltier) water naturally sinks below lighter (less saline) water.
• Salinity generally increases with depth, and there is a distinct zone called the halocline, where salinity increases sharply over a relatively short vertical distance. (Compare the halocline with the thermocline — where temperature changes steeply — and the pycnocline — where density changes sharply.)
• Other factors being constant, increasing the salinity of seawater causes its density to increase. High salinity seawater generally sinks below the lower salinity water.
• This leads to stratification by salinity — the layering of ocean water into distinct zones based on their salinity and density characteristics.
• The halocline acts as a barrier to vertical mixing. In regions with a strong halocline, surface and deep waters remain relatively separated, which affects the distribution of nutrients, dissolved oxygen, and marine life.
• The depth and intensity of the halocline varies by latitude and regional conditions. In polar regions, the halocline is well-developed due to the significant difference between low-salinity surface water and higher-salinity deep water. In tropical and sub-tropical regions, the halocline may be less pronounced.

Salt Budget

The salt budget is also known as the salt cycle. It involves all the processes through which salt moves from the ocean into the lithosphere, to a certain extent into the atmosphere, and back into the oceans. Understanding the salt cycle helps explain why the salinity of the ocean has remained relatively stable over geological time scales, even though rivers continuously add salt to the oceans.

• Moving water, including groundwater, leaches minerals from the rocks through the process of surface erosion and chemical weathering. The mineral-laced water joins the rivers and streams which eventually reach the oceans. These minerals add to the salinity levels of the ocean waters.
• Some of the salts in the ocean waters accumulate at the ocean bottom through the process of sedimentation, turning into mineralized rocks over time. Over a period of millions of years, some of these rocks get raised above the ocean surface due to plate tectonics or volcanic activity. This process brings the salt back to the lithosphere in the form of mineral deposits and rocks.
• Salt from the oceans also gets sprayed into the atmosphere due to the action of wind — especially in areas of strong wave activity and ocean spray. This atmospheric salt returns to the lithosphere mixed with precipitation (rain). However, this constitutes only a tiny fraction of the total salt moving between the land and the sea.
• The salt cycle operates over a very long period of time — spanning millions of years. It maintains a rough equilibrium where the amount of salt being added to the ocean from rivers is approximately balanced by the amount being removed through sedimentation and tectonic uplift.
• Submarine volcanic activity and hydrothermal vents on the ocean floor also contribute salts and minerals directly into the ocean water, adding another source of dissolved salts.

Every year, around 3 billion tons of salt gets added to the oceans from the land through rivers, groundwater, and surface runoff. A tiny fraction of this salt is extracted by humans for daily consumption, industrial use, and chemical manufacturing.