Salinity

Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Class 11, Ch 12 "Water (Oceans)"

Meaning

Salinity is the total amount of salt dissolved in seawater. It is measured as grams of salt in 1,000 g (1 kg) of seawater and written in parts per thousand (‰ or ppt). For example, 35‰ means 35 g of salt in 1 kg of seawater.

Salinity matters because it works together with temperature to decide the density of seawater (how heavy a given volume of it is). Dense water sinks, and this sinking helps drive deep ocean currents.

Explanation

How salinity is measured

• NCERT method: grams of salt in 1,000 g of seawater. This is written as ‰ and is also called "per mille".
• Practical Salinity Scale 1978 (PSS-78): the modern method.
• It works out salinity from the electrical conductivity of seawater, meaning how easily electricity passes through it [3].
• More salt lets more current pass [3].

• UNESCO accepted this scale and used it in its 1980 equation for seawater density, EOS-80 [3].

• TEOS-10 (Thermodynamic Equation of Seawater, 2010) added a new measure called Absolute Salinity. It still advises that data sent to national data centres be given as Practical Salinity on PSS-78 [3].

• Practical salinity has no unit. Some sources still write it as PSU (practical salinity unit). 1 PSU is about 1 g of salt in 1,000 g of water [2].
• Normal range: the open ocean is usually 33–37‰, with an average of about 35‰. NASA gives the range as 32–37 PSU [2].

Where the salt comes from and what it is made of

• Rock weathering: rain slowly breaks down rocks on land. Rivers carry the dissolved salts to the sea.
• Volcanic gases: volcanoes on land and on the sea floor add chlorine and other substances. Over geological time (millions of years), this adds salt to the ocean.
• Why the sea stays salty:
• When seawater evaporates, only the water leaves.
• The salt stays behind.

• So salt builds up in the ocean while the water keeps moving around the hydrological cycle.

• What the salt is made of:

Salt	Share
Sodium chloride (common salt)	about 78%, the largest share
Magnesium chloride	second
Sulphates (magnesium sulphate, calcium sulphate and others)	next

Factors that raise or lower salinity

The basic rule is simple. When fresh water is removed from the sea, salinity rises. When fresh water is added, salinity falls.

• Evaporation and precipitation: the main control on surface salinity
• Evaporation removes water and leaves the salt behind, so salinity rises.
• Rain and snow add fresh water, so salinity falls.
• So surface salinity follows evaporation minus precipitation (E – P). More evaporation gives saltier water, and more rain gives fresher water [2].

• About 86% of global evaporation and 78% of global precipitation happen over the oceans. This is why salinity works like a rain gauge for the ocean [2].

• Latitude pattern

• Equator: heavy rain, so salinity is only moderate.

• 20–30° N and S: the subtropical high-pressure belts (zones of dry, sinking air) have clear skies and strong evaporation. Open-ocean salinity is highest here.

• River inflow: big rivers pour fresh water into the sea, so salinity near the coast falls.

• Freezing and thawing in polar seas
• Sea ice is made mostly of fresh water. When seawater freezes, the salt is pushed out into the water below.
• That water becomes saltier and denser, so it sinks.

• When ice melts, it releases fresh water, so the surface becomes less salty.

• Winds

• Offshore winds (winds blowing from land towards the sea) push surface water away from the coast.

• Deeper water rises in its place. This is called upwelling, and it changes the local salinity.

• Ocean currents

• Warm currents from the subtropics, where evaporation is strong, bring saltier water.

• Cold currents from polar regions with melting ice bring fresher water.

• Link with temperature and density

• Salinity, temperature and density are linked, so a change in one changes the others.
• Warm, salty water can float on colder, fresher water if its density is lower.

Factor	Effect on salinity	Typical area
High evaporation	Raises	Subtropical oceans, Red Sea, Persian Gulf, Mediterranean
Heavy rain	Lowers	Equatorial belt, Bay of Bengal in the monsoon
River inflow	Lowers	Bay of Bengal, Baltic, near the Amazon mouth
Freezing	Raises	Arctic and Antarctic seas in winter
Thawing	Lowers	Polar seas in summer

Brackish water and the 24.7‰ limit

• Brackish water is saltier than fresh water but less salty than seawater.
• Upper limit (NCERT): 24.7‰. This number has a physical reason:
• At 24.7‰, the temperature at which water is densest is the same as its freezing point (about –1.3 °C).
• Below 24.7‰: water behaves like fresh water. It is densest above its freezing point, so cooled surface water sinks before it freezes.

• Above 24.7‰: water keeps getting denser until it freezes, which is how seawater behaves.

• Hypersaline water is saltier than normal seawater. In hot, dry regions, shallow lagoons with little inflow can reach about 70‰:

• Strong evaporation removes water.
• Little rain or river water comes in to replace it.
• So salt builds up.

Where it occurs

India

• Bay of Bengal: low salinity. It receives the Ganga–Brahmaputra and other large rivers, plus heavy monsoon rain.
• Arabian Sea: higher salinity. It gets less river water and has more evaporation.
• Brackish-water sites:
• Hooghly estuary, West Bengal. An estuary is a river mouth where fresh river water mixes with tidal seawater.
• Chilika Lake, Odisha, on the east coast. It is Asia's largest brackish-water lagoon. A lagoon is a shallow coastal water body cut off from the sea by a sandbar or spit.
• Pulicat Lake, on the Andhra Pradesh–Tamil Nadu border, on the east coast.

• Sundarbans mangrove creeks, in the Ganga–Brahmaputra delta, West Bengal.

• Hypersaline coast: the Rann of Kutch area in Gujarat has very salty coastal water and salt flats that are worked for salt.

World

• High salinity (strong evaporation): the subtropical oceans at 20–30° N and S, the Red Sea, the Persian Gulf and the Mediterranean.
• Low salinity:
• The Baltic Sea in northern Europe. It gets large river inflow, has low evaporation and has only a narrow link to the Atlantic.

• Areas near the mouths of the Amazon, Congo and Ob–Yenisei rivers.

• Polar seas: in the Arctic Ocean and the Southern Ocean around Antarctica, freezing raises salinity and thawing lowers it.

• Saltiest water bodies (NCERT order):

Rank	Water body	Location	Salinity (NCERT)
1	Lake Van	Turkey	330‰
2	Dead Sea	between Jordan and Israel (and the West Bank)	238‰
3	Great Salt Lake	Utah, USA	220‰

• Dead Sea:
• It is a landlocked salt lake (a lake with no outlet to the sea) in the Jordan Rift Valley.
• Its shore is more than 430 m below sea level, the lowest land on Earth.
• It is fed almost entirely by the Jordan River, which flows in from the north [4].
• Its water is so dense that the human body floats easily.

Don't confuse with

• Salinity vs density: salinity is only the amount of dissolved salt. Density depends on both salinity and temperature. Saltier water and colder water are both denser.
• Brackish vs hypersaline: brackish water is below seawater salinity, with an upper limit of 24.7‰. Hypersaline water is above normal seawater, for example lagoons at about 70‰.
• Salt lake vs soda lake:
• Lake Van is ranked first in NCERT's list, but it is actually the world's largest soda lake, rich in sodium carbonate rather than common salt [6].
• Its salinity is only about 23 g per litre, and its pH is about 10, which means the water is strongly alkaline [6].
• Its main salts are sodium carbonate and sodium sulphate [6].

• Modern sources usually give the Dead Sea as about 340‰. With that figure, the Dead Sea is the saltiest of the three.

• ‰ (NCERT) vs PSS-78 practical salinity: ‰ is a mass ratio of grams of salt per 1,000 g of seawater. PSS-78 is based on electrical conductivity and has no unit [3]. "PSU" is only an informal label [2].

Prelims Hooks

• The average ocean salinity is about 35‰, and the open-ocean range is 33–37‰. Sodium chloride makes up about 78% of sea salt, followed by magnesium chloride, then sulphates.
• The Practical Salinity Scale 1978 measures salinity from electrical conductivity and has no unit [3]. TEOS-10 (2010) added Absolute Salinity [3].
• NCERT gives the upper limit of brackish water as 24.7‰. At this salinity, the maximum-density temperature equals the freezing point.
• NCERT order of saltiest water bodies: Lake Van (330‰) > Dead Sea (238‰) > Great Salt Lake (220‰). Trap: Lake Van is really the largest soda lake, at about 23 g/L and pH about 10 [6].
• Freezing raises salinity; thawing lowers it. The Bay of Bengal is less salty than the Arabian Sea. Open-ocean salinity peaks at 20–30° N and S, not at the equator.
• NASA's Aquarius, launched on 10 June 2011, was the first satellite built mainly to measure sea-surface salinity from space [2].

Mains Points

• Salinity, density and climate (GS-I):
• Surface salinity follows evaporation minus precipitation, so it acts as a record of the water cycle [2]. Wet regions are becoming fresher and dry regions saltier, which helps track climate-driven changes in monsoon rain and drought.
• In polar seas, freezing makes water saltier and denser, so it sinks. This drives thermohaline circulation, the global system of deep currents driven by differences in temperature (thermo) and salt (haline).

• Faster melting of the Greenland and Antarctic ice adds fresh water, which can weaken this circulation.

• Water diversion and lake decline (GS-III, environment):

• Since the 1960s, Israel and Jordan have diverted much of the Jordan River's water for farms and towns [4].
• Lake water is also pumped into evaporation ponds to extract minerals [4][5].
• So the Dead Sea's level fell by about 0.7 m a year in the 1970s–1980s and about 1.2 m a year in recent decades [4].
• As the level falls, the lake grows saltier, and salt settles on the lake floor as a crust [4].

• India's brackish lagoons, such as Chilika and Pulicat, also depend on a balance between freshwater inflow and the sea mouth.

• Coastal economy and salinity ingress (GS-III):

• Brackish water supports estuarine fisheries, mangroves and shrimp aquaculture.
• Hypersaline coasts support salt production, as in Gujarat's Rann of Kutch.
• When river flow falls, seawater moves further inland. This salinity ingress harms farmland and drinking-water supplies.

Related concepts

• Brackish water
• Factors affecting salinity
• Dead Sea
• Lake Van

Read more

• Salinity: meaning, factors and the extremes