Ocean Salinity Part 5: The Indian Ocean and the Indian Context

The Indian Ocean: A Sea Like No Other

What Makes the Indian Ocean Unique

What is the significance of the Indian Ocean for the study of salinity: it is the ocean of India, the sea on which the monsoon, the cyclones and the coastal fisheries of the subcontinent all depend, and its salinity is shaped by a setting unlike that of any other ocean on Earth.

The Indian Ocean is, in a phrase, only half an ocean. Unlike the Atlantic and the Pacific, which run from pole to pole, the Indian Ocean is closed off to the north by the great landmass of Asia, so it has no cold northern end and no exchange with an Arctic sea. It is land-locked to the north, open only to the south.

It is also the warmest of the great oceans. Hemmed in by warm continents and capped in the north by the monsoon seas, the Indian Ocean holds the warmest surface water of any ocean, a vast store of heat that powers the monsoon and the cyclones. Its warmth and its salinity together make it the engine of the subcontinent's weather.

This shape gives the basin its special character. With no northern outlet, the seas of the north, the Arabian Sea and the Bay of Bengal, are nearly enclosed and feel the seasons of the land intensely, heating and cooling and drying and flooding far more sharply than an open ocean would. Salinity here is governed by monsoon rain and river flow more than by anything else. The map below sets out the salinity of these seas.

The most striking effect of the closed north is the reversal of the currents. Because the monsoon winds themselves reverse between summer and winter, the surface currents of the northern Indian Ocean reverse with them, a behaviour found in no other ocean and known as the monsoon drift. The next section follows this seasonal turning of the sea.

The Monsoon Drift: Currents That Reverse with the Season

What is the significance of the monsoon drift: it is the single feature that most sets the Indian Ocean apart, the reversal of its surface currents twice a year, and it was the subject of a UPSC question that turned on exactly this point.

In the other oceans the great surface currents hold a steady course. Driven by steady trade winds and the spin of the Earth, the gyres of the Atlantic and the Pacific circle in the same direction the year round. The northern Indian Ocean cannot, because the winds that drive it do not hold steady.

The monsoon winds reverse, and the currents reverse with them. In summer the south-west monsoon drives the surface water eastward in a broadly clockwise sweep; in winter the north-east monsoon turns it westward and anticlockwise. This twice-yearly turning is the monsoon drift, and it is why the Indian Ocean's circulation cannot be drawn as a single fixed pattern. The figure below shows the two seasons.

This is exactly what the 1997 examination tested. Asked why the regular direction of the ocean currents in the Indian Ocean changes, the answer is that the Indian Ocean has a monsoon drift, its currents bound to the reversing monsoon winds rather than running in a fixed gyre. A reader who has grasped the closed northern basin can see at once why this, and not the ocean's size or salinity, is the cause.

The reversal mixes and redistributes salinity. As the currents turn, they carry the fresh water of the Bay of Bengal and the salty water of the Arabian Sea along the coasts and around the tip of the peninsula, so the salinity map of the northern Indian Ocean itself shifts with the season, a restlessness peculiar to this ocean.

The strongest of these reversing flows is the Somali Current. Along the East African coast the summer monsoon drives a powerful northward current, among the swiftest in the world, which fades and reverses when the winter monsoon returns. With it comes a vigorous upwelling that cools the surface and feeds rich fisheries, a current that exists in this form nowhere outside the monsoon ocean.

India's Two Seas: A Study in Contrast

The Arabian Sea: Salty, Mixed and Evaporative

The Arabian Sea is the salty sea of the Indian coast. It lies under dry, descending air and strong north-westerly winds, it receives little fresh water from rivers, and it evaporates hard, so its surface is among the saltiest of the tropical oceans, around thirty-six to thirty-seven practical salinity units.

That saltiness drives the sea's behaviour. Salty surface water is dense, so it tends to sink and mix downward, forming the dense, well-mixed mass that oceanographers call Arabian Sea High Salinity Water. With deeper mixing, the Arabian Sea surface cools more readily and overturns more freely than the Bay of Bengal does.

The result is a sea of strong vertical exchange. The Arabian Sea mixes nutrients up from below, supports rich seasonal fisheries along the western coast, and, as it warms under climate change, has begun to raise more intense cyclones than it once did. Its salinity is the root of all these traits.

The Arabian Sea also holds one of the world's great oxygen-poor zones. Its high productivity and sluggish deep flow strip the mid-depth water of oxygen, creating an intense oxygen minimum zone where little can breathe and where nitrogen is lost from the sea. This zone, shaped in part by the sea's density structure, is among the most pronounced on Earth.

The Bay of Bengal: Fresh, Stratified and River-Fed

The Bay of Bengal is the fresh sea of the Indian coast. It catches the full force of the summer monsoon rain and the discharge of some of the greatest rivers on Earth, the Ganga, the Brahmaputra, the Mahanadi, the Godavari and the Krishna, so its surface salinity falls to around twenty-eight to thirty-three units, far below the Arabian Sea.

The fresh surface makes the Bay strongly stratified. Because the light, fresh layer floats on the saltier water beneath and resists mixing, the Bay develops a thick barrier layer that seals warmth at the surface, keeping a very warm skin through the monsoon season. This warmth is the fuel of the monsoon depressions and tropical cyclones that form over it.

The Bay is therefore a sea of weather. Its warm, stratified surface breeds the bulk of the storms that bring the monsoon rains and the cyclones to the eastern coast, and its fresh plume can be followed by satellite as it spreads down the coast and round the peninsula. Its low salinity is the source of this restless, storm-making character.

The Bay has written some of the darkest pages of cyclone history. Its warm, stratified waters and the low, crowded coasts around its head have made the Bay of Bengal the deadliest cyclone basin on Earth, where past storms have taken hundreds of thousands of lives. The same barrier-layer warmth that feeds the monsoon also feeds these disasters, which modern warning systems now work to blunt.

Why the Two-Sea Contrast Matters for India

What is the significance of the contrast between the two seas: it means that the same monsoon breaks over two very different oceans, and the difference in their salinity, by setting how each sea stores and gives up heat, helps decide the rains, the storms and the fisheries of the two coasts.

The two seas store and release heat differently. The fresh, barrier-layered Bay traps warmth and feeds storms, while the salty, well-mixed Arabian Sea exchanges heat through a deeper column. The figure below sets the two regimes against each other. This difference in heat storage is written, at root, in salt.

For India the consequences are immediate. The Bay of Bengal raises most of the country's monsoon depressions and its deadliest cyclones; the Arabian Sea, long quieter, is now stirring; and the seasonal upwelling of the western coast, governed by the mixing that salinity allows, underpins major fisheries. To read the two seas is to read much of India's coastal life.

• The Arabian Sea: salty (36 to 37 PSU), well mixed, evaporative, Arabian Sea High Salinity Water, rising cyclones.
• The Bay of Bengal: fresh (28 to 33 PSU), stratified, river-fed, barrier-layered, the chief storm sea of India.
• The common rule: salinity sets density, and density decides whether a sea traps heat or stirs it down.

The two seas even trade their stormy season. The Bay of Bengal is most dangerous before and after the peak monsoon, while the warming Arabian Sea has begun to spawn its fiercest cyclones in the same shoulder months, so the calendar of risk along India's coasts is set by the changing warmth and salinity of the two seas as much as by the monsoon itself.

The Water Masses of the Indian Ocean

High-Salinity Tongues: Red Sea and Persian Gulf Water

What is the significance of the Indian Ocean's water masses: the basin is fed by two of the saltiest enclosed seas on Earth, the Red Sea and the Persian Gulf, whose intensely evaporated water spills out and spreads through the Indian Ocean as distinct tongues of high salinity at depth.

The Persian Gulf and the Red Sea are evaporating basins. Hemmed in beneath cloudless desert skies and fed by almost no rivers, both evaporate so fiercely that their water grows extremely salty and dense, far saltier than the open sea. This heavy water sinks and pours out over the sills at their mouths into the Arabian Sea.

The outflows spread as salinity maxima at depth. Persian Gulf Water settles at a few hundred metres and Red Sea Water deeper still, each forming a tongue of unusually salty water that can be traced across the northern Indian Ocean. The figure below shows these high-salinity layers beneath the fresher surface. They are the fingerprints of the enclosed seas to the north-west.

These salty tongues shape the sea above them. By laying dense, salty water at mid-depth, the Persian Gulf and Red Sea outflows help set the stratification of the northern Indian Ocean, influencing how the surface mixes and how the oxygen-poor zone of the Arabian Sea is maintained. A signal born of evaporation in a distant gulf is felt in the open sea.

Low-Salinity Water and the Indonesian Throughflow

Against these salty tongues, the surface of the basin is freshened from several sides. The monsoon rain and the great rivers freshen the Bay of Bengal, while from the east a current of relatively fresh water enters the Indian Ocean from the Pacific, threading between the islands of Indonesia.

This eastern inflow is the Indonesian Throughflow. It is the only place where warm, fresher water passes from one great ocean to another through the tropics, carrying Pacific water into the Indian Ocean and helping to keep the surface of the eastern basin less salty and warm. It is a key piece of the global circulation.

The Indian Ocean is thus layered in salt. A fresher surface, fed by monsoon rain, by the rivers and by the Indonesian Throughflow, lies over saltier water at depth fed by the Red Sea and the Persian Gulf, so the basin holds its salinity in distinct layers, each with its own origin and its own path.

The Bay's fresh water itself travels far. After the monsoon, the low-salinity plume of the Bay of Bengal is carried round the tip of the peninsula and up into the south-eastern Arabian Sea, freshening it for a season and changing how that sea mixes. Even the fresh and the salty seas of India are joined by the currents that the monsoon drives.

Observing and Using the Indian Ocean

Watching the Sea: INCOIS, Argo and RAMA

What is the significance of observing the Indian Ocean: because the monsoon on which the country depends is set in part by the temperature and salinity of these seas, India watches them closely, and that watching has become a pillar of the nation's ocean science.

India reads its seas through a network of instruments. The Indian National Centre for Ocean Information Services, INCOIS, gathers data from drifting Argo floats that profile temperature and salinity to two thousand metres, from the moored buoys of the RAMA array across the tropical Indian Ocean, and from satellites. Together they map the salinity of the basin in near real time.

This observation feeds directly into forecasts. The salinity and the barrier layer of the Bay of Bengal, the warming of the Arabian Sea and the state of the Indian Ocean Dipole all enter the seasonal forecast of the monsoon and the warnings for cyclones, so the science of these seas is woven into the safety and the harvest of the coast.

The same science guards the coast against the sea's violence. After the Indian Ocean tsunami of 2004, India built a tsunami warning system run by INCOIS that watches the basin for earthquakes and waves, and the centre also issues ocean-state and high-wave forecasts for fishers. Reading the ocean, in temperature, in salinity and in motion, has become a matter of saving lives.

Salinity, Currents and the Fisheries of the Coast

What is the significance of salinity and currents for fisheries: the same forces that move the sea and set its salinity also decide where the fish gather, so the currents of the Indian Ocean are, for the coastal economy, the difference between a rich sea and an empty one.

Ocean currents drive the fisheries of the world. Where currents and winds force deep, cold, nutrient-rich water up to the sunlit surface, the process of upwelling, they fertilise a bloom of plankton that feeds the whole food chain, so the great fishing grounds of the world lie over upwelling zones. The forces behind the currents are therefore the forces behind the fisheries.

The forces that move the currents are several. The winds drag the surface, the spin of the Earth deflects the flow through the Coriolis effect, differences in temperature and salinity set the water's density and drive the deep circulation, and the shape of the coasts steers the result. Along the Indian coast the monsoon winds drive a seasonal upwelling that underpins the western fisheries.

• Winds: the prevailing and monsoon winds drag the surface water along.
• Coriolis effect: the Earth’s rotation deflects the moving water and shapes the gyres.
• Density: differences in temperature and salinity drive the deep, thermohaline circulation.
• Coasts and upwelling: the shape of the coast steers currents and lifts nutrient-rich water to feed fisheries.

For India this is the heart of the blue economy. The seasonal upwelling of the Arabian Sea and the productivity of the Bay support a marine fishery that feeds and employs millions, and managing it well demands an understanding of the currents and the salinity that govern where the fish are found.

India's two coasts fish two different seas. The western coast, off Kerala, Karnataka and Gujarat, draws on the monsoon upwelling and the productivity of the Arabian Sea for its great pelagic catches, while the eastern coast works the calmer, fresher Bay. The currents and the upwelling set by wind and density decide, season by season, where the shoals will run.

Salinity, Navigation and the Strategic Sea

Salinity has a quieter bearing on navigation and strategy. The saltier and denser the water, the higher a ship floats and the more it can carry, so mariners have always reckoned with the changing salinity of the seas, and the density structure of the ocean shapes how sound and submarines move through it.

The Indian Ocean is among the most strategic seas on Earth. Across it run the sea lanes that carry much of the world's oil and trade through the chokepoints of Hormuz, Bab-el-Mandeb and Malacca, so the knowledge of its currents, its water masses and its acoustics is of real consequence to commerce and to security alike. The salinity that this series has traced is, in the end, woven into the strategic life of the ocean.

The monsoon drift once carried the trade of half the world. For centuries the sailors of the Arabian Sea timed their voyages to the reversing monsoon winds and currents, riding the summer drift to India and the winter drift back, so that the monsoon shaped the commerce of the Indian Ocean long before it was understood. The sea that this part describes is also a sea of history.

Salinity and India's Coastal Future

Climate Change in the Indian Ocean

What is the significance of climate change for the Indian Ocean: the basin is warming faster than the global average, and as it warms its salinity pattern is shifting in ways that bear directly on the monsoon, the cyclones and the coast of India.

The warming Indian Ocean is changing the monsoon seas. The Bay of Bengal grows warmer and, with heavier rain and run-off, fresher at the surface, deepening its barrier layer; the Arabian Sea warms enough to raise more frequent and more intense cyclones than the western coast has known. The salinity contrast between the two seas is sharpening with the wider water cycle.

These changes reach the people of the coast. A stormier Arabian Sea, a more variable monsoon and a rising sea together threaten the dense populations of the Indian littoral, so the salinity and the warming of these seas are not an abstraction but a matter of lives and harvests, taken up in the final stress below.

The Indian Ocean is warming faster than any other. Observations show its surface heating more quickly than the global ocean average, lengthening and intensifying the marine heatwaves that bleach coral and disturb fisheries, and adding energy to the cyclones of both seas. A warming basin magnifies every salinity-linked stress this part has traced.

Sea-Level Rise and Salt Intrusion on India's Coast

The last and nearest stress is the creep of salt onto the land. As the sea rises and the fresh outflow of the rivers weakens, salt presses inland into the estuaries, the soils and the coastal groundwater of India's deltas, a slow advance that the earlier part on salinity stress described and that the Indian coast feels acutely.

The great deltas of the Bay of Bengal are most exposed. In the Sundarbans, in the deltas of the Krishna, the Godavari and the Mahanadi, reduced upstream flow and a rising sea drive salt into farmland and wells, forcing changes in crops and livelihoods, so that the salinity this series has studied becomes, on the Indian coast, a question of food and water security.

The remedies are as much about rivers as about the sea. Holding back the salt means keeping enough fresh water flowing to the deltas, maintaining the embankments and mangroves that buffer the coast, and managing the over-pumping of coastal aquifers, so that the defence of India's coast against the advancing salt is a task of water policy as well as of climate adaptation.

The series closes its Indian chapter here. From the salt of the open ocean to the salt creeping up an Indian delta, salinity has run as a single thread through the climate, the ecosystems and now the human geography of the sea, and the parts that follow draw these threads into the processes and the comparisons that the examination rewards.

UPSC Relevance and Exam Focus

Where the Indian Ocean Fits in the UPSC-CSE Syllabus

This topic sits at the meeting of physical geography and Indian geography in General Studies Paper I, and it is heavily examined because the Indian Ocean, the monsoon and the seas around India recur across the Prelims and the Mains as questions of both oceanography and the geography of India.

The questions most often test the uniqueness of the Indian Ocean, the reversal of its currents with the monsoon, the contrast between the Arabian Sea and the Bay of Bengal, and the forces that drive ocean currents and the fisheries they support.

Several linked points recur and are worth holding in working memory:

• Half an ocean: the Indian Ocean is closed to the north, so its northern currents reverse with the monsoon (the monsoon drift).
• Arabian Sea: salty (36 to 37 PSU), well mixed, evaporative, Arabian Sea High Salinity Water.
• Bay of Bengal: fresh (28 to 33 PSU), stratified, river-fed, barrier-layered, the chief storm sea.
• Water masses: high-salinity Red Sea Water and Persian Gulf Water at depth; the Indonesian Throughflow freshens the surface.
• Forces on currents: winds, the Coriolis effect, density (temperature and salinity), and the coasts; upwelling drives fisheries.
• Observation: INCOIS uses Argo floats and the RAMA array to watch the basin.

A 1997 Prelims question asked why the regular direction of the ocean currents in the Indian Ocean changes, the answer being that the Indian Ocean has a monsoon drift; a reader who has fixed that the closed northern basin binds the currents to the reversing monsoon winds can answer it at once.

A 2022 Mains question asked for the forces that influence ocean currents and their role in the fishing industry; this part supplies both halves, the winds, the Coriolis effect, density from temperature and salinity, and the coasts as the forces, and the upwelling of nutrient-rich water as the link from currents to the great fisheries of the world.

Sources and Further Reading

• INCOIS: Indian Ocean observation (Argo, RAMA)
• NOAA: Ocean currents tutorial
• India Meteorological Department: Monsoon and seasonal forecasts
• NASA SMAP: Sea surface salinity
• Wikipedia: Indian Ocean
• Wikipedia: Arabian Sea
• Wikipedia: Bay of Bengal
• NCERT Class 11, Fundamentals of Physical Geography (Movements of Ocean Water)