Maarg Manthan · Topic 6.5

Salinity of Ocean Water: Factors Affecting Salinity, Vertical Distribution

Geography › Oceanography · Topic 6.5

Salinity is the amount of dissolved salts in sea water, expressed as grams of salt per kilogram of water, or parts per thousand. The average salinity of the oceans is about 35 parts per thousand, ranging from 33 to 37 in the open sea. It is highest in the subtropics where evaporation exceeds rainfall, lowest near the equator, the poles and river mouths, and it controls the density of water.

6 min read · UPSC Prelims · Geography · Oceanography

At a Glance

Average About 35 parts per thousand
Open ocean range 33 to 37 parts per thousand
Major salt Sodium chloride, about 78 per cent
Maximum zone Subtropics, around 20 to 30 degrees
Extreme examples Dead Sea very high; Baltic Sea low
Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This topic follows ocean temperature and describes the dissolved salts of the sea. Salinity, with temperature, decides the density of sea water, which is the subject of the next topic. Previous topic: Temperature of Ocean Water: Vertical and Spatial Variations, the Thermocline. Next topic: Density of Ocean Waters.

What Is Salinity and What Are Sea Salts Made Of?

In short: salinity is how salty the sea is, and the average is 35 grams of salt in every kilogram of sea water.

Salinity is the total quantity of dissolved salts in a given mass of water. It is expressed in grams per kilogram, or as parts per thousand, written as per mille. The average salinity of ocean water is about 35 parts per thousand, which means that 1 kg of sea water contains about 35 g of salt. In the open ocean it ranges from about 33 to 37. The salts come from the weathering of rocks and the gases and minerals from volcanoes and vents on the sea floor, and they have built up over millions of years. Sodium chloride, ordinary table salt, makes up about 78 per cent of the total.

Salt or ion	Approximate share of the dissolved salts
Chloride	55 per cent
Sodium	30.6 per cent
Sulphate	7.7 per cent
Magnesium	3.7 per cent
Calcium	1.2 per cent
Potassium	1.1 per cent

The proportions of the main salts are almost the same everywhere, which is why the salinity can be calculated from the measurement of chlorinity or electrical conductivity. The salinity affects the properties of the water: the more salt, the higher the density, and the lower the freezing point, which is about minus 1.9 degrees Celsius for water of 35 parts per thousand. Both the density and the temperature decide whether the water floats or sinks, as in Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents.

What Factors Control Salinity?

Evaporation: raises salinity, because water vapour leaves and the salt stays; it is highest in the hot, dry subtropical regions under the high pressure belts, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.
Precipitation: lowers salinity by adding fresh water, as near the equator, where rain is heavy.
River water: dilutes the salinity near river mouths, as in the Bay of Bengal, where the Ganga-Brahmaputra and the Irrawaddy reduce it to as low as 30 or less, and in the Baltic Sea.
Melting and freezing of ice: melting ice reduces the salinity in the polar regions in summer, and freezing, which excludes salt, raises the salinity of the water below and makes cold dense brine that sinks, driving the deep currents.
Winds and currents: winds blow the saline surface water away, and currents carry water of different salinity, mixing and spreading it.
Enclosed basins and exchange: a sea with a narrow link to the ocean, such as the Mediterranean, keeps its salty water, while the Baltic is too fresh. Waves and tides mix the water.

How Is Salinity Distributed?

Pattern	Description
Latitudinal	Lowest at the equator, about 34 to 35, because of heavy rain and cloud; rises to a maximum of 36 to 37 at around 20 to 30 degrees in both hemispheres, where evaporation is high; falls again to 33 or less toward the poles
By ocean	The Atlantic is the saltiest of the major oceans; the Arctic is the least saline because of low evaporation, melting ice and rivers
Seas with high salinity	Red Sea and Persian Gulf, around 40 or more; Mediterranean, about 38; Dead Sea, about 340, and Lake Van in Turkey, about 330, in which bathers float
Seas with low salinity	Baltic Sea, about 7; Black Sea, about 18; Arctic coastal seas, below 30
Vertical	Salinity changes with depth, and a layer where it changes rapidly is called the halocline; in the tropics the surface salinity is high, and it decreases to a minimum at around 1,000 m; in the high latitudes it increases with depth

Salinity is measured by Argo floats, ships and satellites, such as the SMOS and SMAP missions. It matters in several ways. The contrast between the salty subtropics and the fresh tropics and poles is increasing with the strengthening of the water cycle, which scientists use as a sign of climate change. The density of water, which depends on temperature and salinity together, drives the thermohaline circulation, as in Major Ocean Currents of the World and Their Effects. Salt levels also limit marine life and corals, as in Coastal Plains, Coral Reefs and Islands of India (Andaman and Nicobar Islands, Lakshadweep), and salinity ingress into rivers and groundwater harms coastal agriculture, as in the delta of the Ganga-Brahmaputra. Desalination, the removal of salt from sea water, provides drinking water to coastal cities such as Chennai, but it is energy-intensive and releases brine that harms marine life. As an analytical point, salinity is the quiet variable of the ocean: it is hardly visible in daily life, but together with temperature, it moves the deep ocean and records changes in the global water cycle.

Why Is This Topic Important?

Concepts Average, factors, distribution
Link Density and currents
Application Desalination and coasts

What Came Next?

The next topic explains the density of ocean waters and how it layers and moves the sea. Next topic: Density of Ocean Waters.

Key People and Terms

Salinity Dissolved salt content of water
Halocline Layer of rapid change in salinity
Brine Very salty water
Per mille Parts per thousand
Chlorinity Chloride content used to estimate salinity
Desalination Removal of salt from water

Exam Corner

Points to Remember

Average salinity is 35 parts per thousand.
Salinity is highest at 20 to 30 degrees latitude.
The Atlantic is the saltiest ocean.
The Dead Sea is far saltier than the oceans.
The Baltic Sea is brackish.

Do Not Confuse With

Halocline and thermocline Salinity versus temperature change
Salinity and density Salt content versus mass per volume
Dead Sea and ocean About 340 versus 35 parts per thousand

Memory Hook

Evaporate and it salts, rain and rivers and ice dilute; subtropics are the saltiest belt.

Mains Angle

Questions ask for factors and distribution. These points add depth.

Balance Evaporation minus rainfall
Distribution Latitude and enclosed seas
Use Density, currents, desalination

A Question You May Face

An original practice question, not a past paper question.

“Explain the factors that affect the salinity of ocean water and describe its latitudinal distribution.”

How to Answer

Definition Average and composition.
Factors Evaporation and inflow.
Pattern Latitude and seas.
Importance Density and coasts.

GS Relevance

Prelims: average salinity, factors, saltiest and least salty seas, composition of salts. GS Paper 1: oceanography.

Frequently Asked Questions

What is the average salinity of the oceans?

The average salinity of the oceans is about 35 parts per thousand, meaning that each kilogram of sea water contains about 35 grams of dissolved salts. In the open ocean it ranges from about 33 to 37 parts per thousand, with sodium chloride forming most of the salts.

What factors affect ocean salinity?

Ocean salinity is affected by evaporation, which increases it, and by rainfall, river inflow and melting ice, which decrease it. Winds, ocean currents, the freezing of sea ice, and the degree of enclosure of a sea also influence salinity by mixing, removing or adding water.

Why is the salinity highest in the subtropics?

The subtropics, around 20 to 30 degrees latitude, lie beneath the high pressure belts with clear skies, strong sunshine and little rainfall. Evaporation exceeds precipitation there, and as water vapour leaves, the salts stay behind, so the surface salinity is the highest in the open ocean.

Which sea is the saltiest?

The Dead Sea is the saltiest, with a salinity of about 340 parts per thousand, nearly ten times that of the ocean, and people can float easily on it. Among the open seas, the Red Sea and Persian Gulf are very salty, at 40 parts per thousand or more.

Why is the Baltic Sea less salty than other seas?

The Baltic Sea receives a large amount of fresh water from rivers and rain, but has little evaporation in the cool climate. Its connection to the North Sea is narrow and shallow, so less salt water enters, and its salinity is only about 7 parts per thousand.

PYQ Practice — Statement Analysis

1 The average salinity of ocean water is about 35 parts per thousand.

True

That is 35 grams per kilogram.

2 Salinity is highest at the equator.

False

It is highest in the subtropics.

3 Sodium chloride is the main salt in sea water.