Salinity: meaning, factors and the extremes

Detail

1. What salinity means

• Salinity is the total amount of salt dissolved in seawater.
• How it is measured (NCERT): grams of salt in 1,000 g (1 kg) of seawater.
• It is written as parts per thousand, shown as ‰ or ppt. This is also called "per mille".

• Example: 35‰ means 35 g of salt in 1 kg of seawater.

• Practical Salinity Scale 1978 (PSS-78): the modern way to measure salinity.

• It works out salinity from the electrical conductivity of seawater, meaning how easily electricity passes through it. 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. Even so, 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), where 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 a range of 32–37 PSU [2].

• Why salinity matters: salinity and temperature together decide the density of seawater (how heavy a given volume of it is).
• Saltier water and colder water are both denser.
• Dense water sinks, which helps drive deep ocean currents.

2. Where the salt comes from

• Rock weathering: rain slowly breaks down rocks on land (weathering). Rivers then 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.
• What the salt is made of:

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

• Why the sea stays salty: when seawater evaporates, only the water leaves. The salt stays behind. So over time salt builds up in the ocean, while the water keeps going round the hydrological cycle.

3. Brackish water

• Brackish water: water that is saltier than fresh water but less salty than seawater.
• Upper limit (NCERT): 24.7‰
• This is not a random number. 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. This is how seawater behaves.

• Where brackish water is found:

Setting	Example	Location
Estuary (a river mouth where fresh river water mixes with tidal seawater)	Hooghly estuary	West Bengal, India
Lagoon (a shallow coastal water body cut off from the sea by a sandbar or spit)	Chilika	Odisha, east coast; Asia's largest brackish-water lagoon
Lagoon	Pulicat	on the Andhra Pradesh–Tamil Nadu border, east coast
Mangrove creeks	Sundarbans creeks	Ganga–Brahmaputra delta, West Bengal
Semi-enclosed sea	Baltic Sea	northern Europe; large river inflow, low evaporation and a narrow link to the Atlantic

4. Factors affecting salinity

Salinity changes whenever fresh water is removed from the sea (salinity rises) or added to it (salinity falls).

(a) Evaporation and precipitation: the main control on surface salinity

• Evaporation removes water but leaves salt behind, so salinity rises.
• Precipitation (rain, snow) adds fresh water, so salinity falls.
• Result: surface salinity tracks evaporation minus precipitation (E – P).
• More evaporation than rain gives higher sea-surface salinity.
• More rain than evaporation gives lower sea-surface salinity [2].

• So salinity works like a rain gauge for the ocean. About 86% of global evaporation and 78% of global precipitation take place over the oceans [2].

• Pattern:

• Near the equator it rains heavily, so salinity is only moderate.

• Around 20–30° N and S lie the subtropical high-pressure belts (zones of dry, sinking air). Here there is clear sky and strong evaporation, so open-ocean salinity is highest.

• Satellite tracking: NASA's Aquarius mission was launched on 10 June 2011. It was the first mission built mainly to measure sea-surface salinity from space [2].

(b) River inflow

• Big rivers pour fresh water into the sea near the coast, so coastal salinity falls.
• India: the Ganga–Brahmaputra and other large rivers flow into the Bay of Bengal, which keeps it less salty. The Arabian Sea gets less river water and has more evaporation, so it is saltier.
• World: the Amazon, Congo and Ob–Yenisei rivers make the sea fresher near their mouths.

(c) Freezing and thawing in polar seas

• When sea water freezes:
• Sea ice is made mostly of fresh water.
• The salt is pushed out into the water below.

• So the water under the ice becomes saltier and denser, and it sinks.

• When ice thaws:

• Melting ice releases fresh water.

• So the surface water becomes less salty.

• Where: the Arctic Ocean and the Southern Ocean around Antarctica.

(d) Winds

• Winds push surface water from one area to another.
• Offshore winds blow surface water away from the coast. Deeper water rises in its place (upwelling) and changes the local salinity.
• Steady winds can pile up saltier water in one area and fresher water in another.

(e) Ocean currents

• Currents carry water of a different salinity into an area.
• Warm currents coming from the evaporation-heavy subtropics bring saltier water.
• Cold currents coming from polar regions with melting ice bring fresher water.

(f) Temperature and density link

• Salinity, temperature and density are interrelated.
• Warm, salty water can float on top of colder, fresher water if its density is lower.
• Any change in one of the three changes the others.
• Together they decide whether water sinks or stays at the surface.

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
Winds and currents	Move salty or fresh water around	Everywhere

5. The extremes

Saltiest water bodies (NCERT box), highest to lowest:

Rank	Water body	Country	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

• What it is: a landlocked salt lake (a lake with no outlet to the sea) in the Jordan Rift Valley, between Jordan to the east and Israel and the West Bank to the west.
• Lowest land on Earth: its shore is more than 430 m below sea level.
• Inflow: almost all its water comes from the Jordan River, which flows in from the north [4].
• Why its level is falling:
• 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 level fell by about 0.7 m a year in the 1970s–1980s and about 1.2 m a year in recent decades [4].

• Effects of the falling level:

• The lake is becoming saltier, especially near the surface.

• Salt is coming out of the water and settling on the lake floor as a crust [4].

• People float easily: very salty water is very dense, so the human body floats on it.

Caution: the figures disagree

• Lake Van, in eastern Turkey, is actually the world's largest soda lake (a lake rich in sodium carbonate rather than common salt) [6].
• Its salinity is only about 23 g per litre, far below 330‰ [6].
• Its pH is about 10, which means the water is strongly alkaline [6].

• Its main salts are sodium carbonate and sodium sulphate [6].

• The Dead Sea is usually given as about 340‰ in modern sources. If that figure is used, the Dead Sea, not Lake Van, is the saltiest of the three.

• Exam rule: in NCERT-based questions, use NCERT's order (Van > Dead Sea > Great Salt Lake). In open or analytical answers, mention the disagreement.

Hypersaline lagoons

• Hypersaline means 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.

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

Prelims Hooks

• Salinity is measured in parts per thousand (‰): grams of salt in 1,000 g of seawater. The Practical Salinity Scale 1978 is based on electrical conductivity and has no unit [3].
• The average ocean salinity is about 35‰, and the open-ocean range is 33–37‰.
• Sodium chloride makes up about 78% of sea salt. It is followed by magnesium chloride, then sulphates.
• The upper limit of brackish water (NCERT) is 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 the largest soda lake, at only about 23 g/L and pH about 10 [6].
• Dead Sea: the lowest land point on Earth, more than 430 m below sea level. It is fed almost entirely by the Jordan River [4].
• Freezing raises salinity; thawing lowers it. Sea ice is mostly fresh water.
• The Bay of Bengal is less salty than the Arabian Sea because of the Ganga–Brahmaputra inflow and monsoon rain.
• Aquarius (launched 10 June 2011) was the first satellite built mainly to measure sea-surface salinity [2].

Mains Points

• Salinity as an indicator of the water cycle (GS-I): surface salinity follows evaporation minus precipitation [2].
• Wet regions are becoming fresher and dry regions saltier.

• This is a way to track climate-driven changes in monsoon rain and drought.

• Salinity → density → ocean circulation (GS-I):

• Freezing in polar seas makes water saltier, so it sinks.
• This sinking drives thermohaline circulation, the global pattern of deep currents driven by differences in temperature (thermo) and salt (haline).

• Faster melting of Greenland and Antarctic ice adds fresh water. This can weaken that circulation.

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

• The Dead Sea falls about 1.2 m a year after Jordan River water was diverted from the 1960s onwards [4]. Lake water is also pumped into evaporation ponds for minerals [5].

• Similar threats exist for India's brackish-water lagoons, where freshwater inflow and the sea mouth must stay in balance. Chilika and Pulicat are examples.

• Coastal and economic links (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 farms and drinking water.