Where the sea is salty: horizontal and vertical distribution of salinity

Detail

Basic terms

• Salinity is the amount of salt dissolved in sea water. It is written in parts per thousand (‰), which means grams of salt in 1 kilogram (1,000 g) of sea water. The average ocean is about 35‰ [2].
• Horizontal distribution of salinity means how salinity changes from place to place across the sea surface, by latitude and from ocean to ocean.
• Vertical distribution of salinity means how salinity changes as you go deeper, from the surface to the sea floor.
• Isohalines are lines on a map that join places with the same salinity. They are the main map tool for showing salinity (NCERT Class 11, Figure 12.5).

Horizontal distribution: the latitude pattern

• Highest in the subtropics (about 20°–30° N and S)
• Sinking air gives clear skies and dry trade winds.
• So evaporation is high and rain is low.

• Water leaves the sea, the salt stays behind, and salinity rises.

• Lower at the equator

• The equator is warm, but it rains almost every day.

• The rain dilutes the surface water, so salinity there is lower than in the subtropics.

• Lowest near the poles

• Evaporation is low because it is cold.

• Melting ice adds fresh water.

• Low at river mouths

• Large rivers pour in fresh water.

• NASA's Aquarius satellite (launched 10 June 2011; first global salinity map on 22 September 2011) confirmed this pattern from space. It showed [2]:

• higher salinity in the subtropics;
• higher average salinity in the Atlantic than in the Pacific and Indian Oceans;
• lower salinity in the rainy belts near the equator and in the northernmost Pacific;
• a sharp contrast between the dry Arabian Sea and the Bay of Bengal, which is freshened by the monsoon and the Ganga;
• a larger-than-expected spread of low-salinity water from the Amazon outflow.

Horizontal distribution: ocean by ocean

Ocean / area	Salinity (‰)	Key point
Atlantic	Average about 36, the saltiest ocean	Peak of about 37‰ between 20°N–30°N and 20°W–60°W (subtropical North Atlantic)
Indian Ocean	Average about 35	High in the Arabian Sea, low in the Bay of Bengal
Pacific	About 35 in general	Falls to 31‰ in the western North Pacific because of Arctic meltwater; about 33‰ beyond 15°–20°S
Red Sea	Up to 41	Among the saltiest open-sea water in the world
Estuaries and Arctic	Swing between 0 and 35	Changes with the seasons as ice forms and melts and as river flow rises and falls

• Why the Red Sea reaches 41‰
• It is almost landlocked. It opens to the ocean only through the narrow Bab-el-Mandeb strait.
• It lies in a hot desert belt, so evaporation is very high.

• Almost no rivers flow into it, and it gets very little rain.

• Why estuaries swing so widely

• An estuary is the wide mouth of a river where fresh river water meets salty sea water.
• In the flood or monsoon season, river water pushes salinity close to 0‰. In the dry season, sea water moves in and salinity rises towards 35‰.
• In the Arctic, ice formation in winter leaves salt in the water. Melting in summer adds fresh water.

Marginal seas: why nearby seas differ

A marginal sea is a partly enclosed sea at the edge of an ocean.

Sea	Salinity	Reason
North Sea	High for its latitude	The North Atlantic Drift, a warm current, brings in salty Atlantic water
Baltic Sea	Very low: under 10‰, and only 2–5‰ in the Gulf of Bothnia	Many rivers flow in, evaporation is low in the cool climate, and the only outlet is the narrow Danish straits
Mediterranean Sea	About 38–39‰	High evaporation in a hot, dry climate
Black Sea	Very low: about 17–18‰	Large rivers pour in: the Danube, Dnieper and Dniester, and the Don via the Sea of Azov

• Two-way flow at the Strait of Gibraltar (Mediterranean)
• Evaporation makes Mediterranean water salty and dense.
• This dense water sinks and flows out at depth through the Strait of Gibraltar into the Atlantic.

• Lighter Atlantic water flows in at the surface to replace it.

• Baltic vs Mediterranean is a common contrast: both are nearly enclosed, but climate decides the result. Cool and wet with many rivers makes the Baltic fresh. Hot and dry with high evaporation makes the Mediterranean salty.

India: Bay of Bengal vs Arabian Sea

Feature	Bay of Bengal	Arabian Sea
Surface salinity	Lower (fresher)	Higher (saltier)
River inflow	Very large: Ganga, Brahmaputra, Irrawaddy, Mahanadi, Godavari, Krishna	Small; few large rivers
Rainfall	Heavy monsoon rain	Less rain over the sea
Evaporation	Lower	High; dry winds blow from Arabia
Extra inputs	—	Salty water flows in from the Red Sea and the Persian Gulf

• How the fresh top layer helps cyclones form in the Bay of Bengal
• River water and rain form a light, fresh layer at the surface.
• This layer floats on denser, saltier water below and does not mix easily. This layering is called stratification.
• Cyclone winds normally stir up cool water from below and weaken the storm. Here, the stirring is blocked, so the SST (sea surface temperature) stays high.

• Warm water gives the storm more heat and moisture, so cyclones form and grow stronger.

• Example: Cyclone Yaas (May 2021) grew stronger over Bay of Bengal water with SST of 31.5–32 °C. Large fresh-water inflow, strong stratification and high heat stored in the upper ocean helped it grow [4].

• This is one reason the Bay of Bengal gets more and stronger cyclones than the Arabian Sea.

Vertical distribution of salinity

• At the surface, salinity changes a lot
• It goes up when: evaporation removes water, or sea ice forms. Ice is mostly fresh, so it leaves its salt in the water.

• It goes down when: fresh water comes in from rain, rivers or melting ice.

• At depth, salinity hardly changes

• Deep water does not evaporate and gets no rain or river water.

• So no water is lost and no salt is added.

• Salinity generally increases with depth. Lighter, fresher water rests on top of denser, saltier water.

Halocline

• Halocline is a layer in the water column where salinity changes sharply with depth. It lies below the well-mixed surface layer [3].

• The word comes from Greek: hals means "salt" and klinein means "to slope" [3].

• How it forms

• Fresh water from rivers or melting ice spreads over salty sea water.
• The fresh water is lighter, so it stays on top.

• A thin zone of fast salinity change separates the two layers.

• Result: the water column forms layers (stratification), and mixing between the top and bottom is limited.

• Where it is seen: haloclines are most common where fresh water from rivers or melting ice meets sea water, for example in estuaries, the Arctic, the Baltic and the Bay of Bengal [3].
• Watch out: in the Atlantic, salinity can decrease by several ‰ from the base of the surface layer down to about 1 km depth [3]. This happens because the subtropical surface water is made very salty by evaporation. So "salinity increases with depth" is the general rule, but it does not hold everywhere.
• Link to ocean currents: differences in salt and heat change the density of water. Cold, salty, dense water sinks and drives the thermohaline circulation (deep ocean currents driven by differences in temperature and salinity).

Prelims Hooks

• Surface salinity is highest in the subtropics, not at the equator. The equator is fresher because of daily heavy rain.
• Atlantic is the saltiest ocean (average about 36‰). Its peak is about 37‰ between 20°–30°N and 20°–60°W.
• Red Sea, up to 41‰, is saltier than the Mediterranean (about 38–39‰). Both are much saltier than the Black Sea (about 17–18‰) and the Baltic (under 10‰).
• Saltiest to freshest: Red Sea > Mediterranean > Atlantic average > Black Sea > Baltic (Gulf of Bothnia, 2–5‰).
• North Sea is salty for its latitude because of the North Atlantic Drift.
• Strait of Gibraltar: salty Mediterranean water flows out at depth, and Atlantic water flows in at the surface.
• Black Sea rivers: Danube, Dnieper, Dniester, and the Don via the Sea of Azov.
• Bay of Bengal is fresher than the Arabian Sea. The Arabian Sea gets salty inflow from the Red Sea and Persian Gulf.
• Isohalines join places of equal salinity. Do not confuse them with isotherms (equal temperature) or isobaths (equal depth).
• NASA Aquarius (launched 2011) is a satellite mission that mapped sea surface salinity [2]. Its partner salinity satellite is ESA's SMOS.

Mains Points

• Controls on salinity distribution: evaporation versus rainfall (subtropics compared with the equator), river inflow (Bay of Bengal, Baltic, Black Sea), ice formation and melting (Arctic), ocean currents (North Atlantic Drift) and how enclosed a sea is (Red Sea, Mediterranean). Together these explain the zonal pattern shown by isohalines and the differences between neighbouring seas.
• Bay of Bengal versus Arabian Sea: a fresh, stratified surface layer keeps SST high and helps cyclones form and grow, as seen in Cyclone Yaas in 2021 [4]. This matters for disaster planning in Odisha, West Bengal and Andhra Pradesh, and for cyclone forecasting by IMD and INCOIS (GS-I and GS-III, disasters).
• Vertical layering: surface salinity changes with the climate, while deep water stays nearly constant. The halocline limits mixing, which affects how nutrients move up and how productive fisheries are. Salt-driven density differences drive the thermohaline circulation, which moves heat around the globe and can change as more ice melts with warming.
• Satellite monitoring: missions such as Aquarius and SMOS track salinity as a signal of the water cycle. Saltier dry zones and fresher wet zones show how rainfall and evaporation are changing [2].