Ocean Salinity Part 3: Climate System, Monsoons, ENSO, and Climate Change

Salinity and the Global Water Cycle

The Ocean as a Rain Gauge: Evaporation Minus Precipitation

What is the significance of salinity for the climate system: before it is anything else, the surface salinity of the ocean is a record of the water cycle, and that single fact makes it a climate variable rather than a mere chemical property.

Surface salinity records the balance of evaporation and rainfall. Where the atmosphere takes more water from the sea by evaporation than it returns as rain and river run-off, the salt left behind concentrates and the surface grows saltier. Where rain and rivers add more fresh water than evaporation removes, the surface is diluted and freshens.

Oceanographers write this balance as evaporation minus precipitation, or E minus P, and the surface salinity field is its slow, integrated fingerprint. Because the ocean covers seventy per cent of the planet and exchanges water ceaselessly with the air, the map of surface salinity is, in effect, a map of where the atmosphere rains and where it evaporates.

This idea explains the broad geography of surface salinity that Part 1 described. The subtropical belts near twenty-five to thirty degrees, where the great high-pressure cells bring clear skies and strong evaporation, hold the saltiest open-ocean surfaces, often above thirty-six practical salinity units. The figure below sets out the balance.

The equatorial belt, drenched by the rising air and heavy rain of the doldrums, and the high latitudes, cool and rainy with little evaporation, are markedly fresher. The Atlantic, ringed by arid subtropics and quietly losing water vapour westward across Panama to the Pacific, is the saltiest of the great oceans; the broad, rainy Pacific is the freshest.

The enclosed seas of the dry belts go furthest of all. The Red Sea and the Mediterranean, hemmed in beneath cloudless subtropical skies and fed by almost no rivers, evaporate so fiercely that their surface salinities climb above forty practical salinity units, the highest of the open marine world, a vivid case of evaporation running far ahead of rainfall.

Three features make salinity a uniquely useful climate record. First, it integrates the water cycle over time, smoothing the noise of single rain events into a steady signal. Second, it is conservative below the surface, so a salinity value carried into the deep ocean keeps a memory of the air-sea exchange that set it. Third, it is now measured from space by satellites such as NASA's Aquarius and SMAP.

The practical payoff is large. No network of rain gauges on land could ever sample the water cycle over the vast ocean, but surface salinity does it for free, and the Argo array of nearly four thousand profiling floats now reads the subsurface salinity of every basin. From these records the strength of the global water cycle, and its change under warming, can at last be measured directly.

Reading the Water Cycle from Space: Aquarius, SMAP and Argo

The water cycle is now read directly from the salt of the sea. For most of history the strength of the global water cycle could only be guessed at, because no instrument measured evaporation and rainfall over the open ocean, yet the surface salinity field records their balance, and reading the salt has become a way of reading the cycle itself.

Satellites measure the saltiness of the sea surface from orbit. NASA's Aquarius mission, flown from 2011 to 2015, and the later SMAP satellite and the European SMOS, sense the faint microwave glow of the surface, which dims as salt rises, and turn it into global maps of sea-surface salinity renewed every few days.

Below the surface the Argo float array keeps the record. Almost four thousand drifting floats sink to two thousand metres and rise again every ten days, radioing back the temperature and the salinity of the water column, so that for the first time the interior of every ocean basin is watched continuously rather than sampled by the occasional ship.

• Aquarius, SMAP and SMOS: satellites that map sea-surface salinity from microwave emission.
• Argo: nearly four thousand floats reading temperature and salinity to two thousand metres.
• RAMA and moored buoys: fixed stations watching the monsoon seas of the northern Indian Ocean.

This observing system has settled an old debate. The measurements show that the salty parts of the ocean have grown saltier and the fresh parts fresher over recent decades, which is the clearest direct evidence that the global water cycle is strengthening as the planet warms, a conclusion the section on climate change takes up below.

Salinity and the Monsoon

Two Seas, Two Regimes: the Bay of Bengal and the Arabian Sea

What is the significance of salinity for the monsoon: it helps fix how much heat the surface sea can store and hand to the atmosphere, and the two seas that flank the Indian peninsula behave very differently because their salinity regimes are opposite.

The Bay of Bengal is fresh and warm. It receives the enormous rainfall of the summer monsoon and the discharge of the Ganga, the Brahmaputra and the Irrawaddy, among the greatest river systems on Earth. This flood of fresh water spreads a thin, light, low-salinity layer over the surface, with values that fall to thirty-one or even twenty-eight practical salinity units near the river mouths.

Because fresh water is light, it floats on the saltier water beneath and resists mixing downward, so the Bay develops a thick barrier layer that seals the warm surface in place. The Bay therefore keeps a very warm skin through the monsoon season, and that warmth feeds the monsoon depressions and the tropical cyclones that form over it.

The Arabian Sea is salty and well mixed. It lies under dry north-westerly winds, evaporates strongly and receives little river water, so its surface is among the saltiest of the tropical oceans, around thirty-six to thirty-seven practical salinity units. The salty surface water is dense, sinks and mixes readily, forming the high-salinity mass that oceanographers call Arabian Sea High Salinity Water. The figure below sets the two regimes side by side.

With deeper mixing the Arabian Sea surface cools more easily, and its response to the monsoon winds differs from the Bay's. The contrast is a clean illustration of how salinity, by setting density, governs whether a sea traps heat at the surface or stirs it downward.

The outcomes are visible in the weather of the subcontinent. A large majority of the depressions and storms that bring monsoon rain to India form over the warm Bay of Bengal. The Arabian Sea, historically quieter, has shown a rising tendency to spawn intense cyclones as it warms, a change of real concern for the western coast.

Indian science now watches these seas closely. The Indian National Centre for Ocean Information Services monitors the salinity and the barrier layer of the northern Indian Ocean with Argo floats and the RAMA mooring array, precisely because the surface salinity of these seas is a control on the monsoon rainfall and on the cyclones that threaten the coast.

• The Bay of Bengal: fresh surface, thick barrier layer, warm skin, monsoon depressions and cyclones.
• The Arabian Sea: salty surface, deep mixing, Arabian Sea High Salinity Water, a cooler and more variable response.
• The common rule: salinity sets density, and density decides whether a sea traps heat or stirs it down.

The freshwater plume of the Bay can be followed from space. After the monsoon, the swollen rivers push a tongue of low-salinity water down the eastern coast and round the tip of the peninsula into the Arabian Sea, a movement that satellite salinity now traces season by season, tying the rainfall of the land to the chemistry of the sea.

The lesson is a general one. Wherever fresh water is added faster than it can be mixed away, as in the Bay, the sea develops a warm, stable cap; wherever evaporation dominates, as in the Arabian Sea, the surface turns dense and overturns. In this way salinity, through density, helps decide the heat and the weather of the seas around India.

The Barrier Layer, Sea-Surface Warmth and Cyclone Intensification

The barrier layer is the hidden hinge of the monsoon seas. Where a thin sheet of fresh water from rain and rivers lies over saltier water, it makes a stable lid that the wind cannot easily stir, and the band of water between the shallow fresh layer and the deeper temperature change is the barrier layer that seals warmth at the surface.

A sealed warm surface feeds storms. Because the barrier layer stops cool water from being mixed up from below, the skin of the Bay of Bengal stays warm even under a strong wind, and a warm sea surface is the fuel on which monsoon depressions and tropical cyclones grow.

The Arabian Sea is learning to make cyclones. Historically its saltier, well-mixed and cooler surface raised fewer severe storms than the Bay, but as the sea warms its surface now more often reaches the threshold for intense cyclones, and such storms have struck the western coast with growing force in recent years.

• Barrier layer: a fresh-capped, salinity-stratified layer that locks heat at the surface.
• Warm skin: the sealed surface fuels monsoon depressions and tropical cyclones.
• Warming Arabian Sea: a once-quiet basin now raising more intense storms.

Salinity is therefore a quiet partner in the weather of the coast. By deciding how deeply the wind can mix the sea, the freshness or saltiness of the surface helps set how warm it stays, and through that warmth it reaches into the rainfall and the storms that the monsoon brings to the land.

El Nino, La Nina and the Walker Circulation

The Coupled Swing of the Tropical Pacific

What is the significance of ENSO: the El Nino-Southern Oscillation is the single largest year-to-year swing in the climate system, a coupled dance of the tropical Pacific Ocean and the atmosphere above it that reorganises rainfall across the world and, for India, helps decide whether the monsoon is generous or fails.

In an ordinary year the trade winds drive a warm pool into the west. The easterly trade winds drag warm surface water across the tropical Pacific into a vast warm pool near Indonesia, while cold water wells up off South America in the east. Air rises over the warm western pool, flows east aloft, sinks over the cool east and returns west at the surface.

This great east-west overturning is the Walker circulation, first traced by Sir Gilbert Walker while he was studying the Indian monsoon, and it is the atmospheric partner of the ocean's warm pool. The same trade winds that pile up the warm water also pile up the fresh, rainy water of the low-salinity western Pacific fresh pool.

El Nino is the warm phase. Every few years the trade winds slacken, the warm pool and its heavy rain slide east into the central and eastern Pacific, and the cold upwelling off South America fails. The Walker loop weakens or reverses, rainfall shifts away from the western Pacific and the Indian Ocean, and the Indian summer monsoon has, in many such years, tended to be weak. The figure below sets out the two phases of the swing.

El Nino also leaves a salinity signature. The low-salinity fresh pool migrates eastward with the warm pool, a movement now visible from space in satellite salinity, so that the event can be read in the salt of the surface as well as in its temperature.

La Nina is the cool phase. In it the trade winds strengthen, the warm pool and its rain pile up further in the west, and cold upwelling intensifies in the east. The Walker loop strengthens, rainfall increases over Indonesia and often the Indian monsoon region, and a strong La Nina was suspected of the great Australian floods of 2010 to 2011.

A variant called El Nino Modoki complicates the picture. Whereas a canonical El Nino warms the eastern Pacific most, El Nino Modoki warms the central Pacific most, with cool flanks to east and west, and its effects on the monsoon and on world weather differ from the classic event. Each phase, classic or Modoki, carries its own salinity signature as the fresh pool shifts with the rain.

The consequences reach far beyond the Pacific. El Nino years carry a heightened risk of drought over India, Indonesia and Australia and of floods over Peru and the southern United States; La Nina years tilt the odds the other way. The phase of ENSO is, for this reason, among the most valuable single predictors used in the seasonal forecast of the Indian monsoon, which NOAA and the India Meteorological Department track without pause.

• El Nino (warm phase): trades weaken, warm pool shifts east, Walker loop weakens, monsoon tends to weaken.
• La Nina (cool phase): trades strengthen, warm pool in the west, Walker loop strengthens, monsoon tends to strengthen.
• El Nino Modoki: warming centred on the central Pacific, with effects distinct from the canonical event.

The atmospheric half of the swing has its own name. The see-saw of surface pressure between the eastern and western Pacific, high at one pole when low at the other, is the Southern Oscillation that Walker first measured; it is the air-pressure signature of the same coupled event, which is why the whole phenomenon is written as the El Nino-Southern Oscillation, ocean and atmosphere named together.

ENSO and the Indian Monsoon: A Weakening Teleconnection

The link between El Nino and the Indian monsoon is old and famous. Sir Gilbert Walker sought it a century ago, hoping to forecast the rains of India from the pressure pattern of the Pacific, and for most of the twentieth century El Nino years carried a clearly raised risk of a deficient monsoon.

Yet the connection is not a law. The strong El Nino of 1997 was followed by a near-normal monsoon, while some weaker events brought drought, so the Pacific alone has never been enough to forecast the Indian rains, and a single index can mislead.

The teleconnection appears to have weakened in recent decades. From around the 1980s the once-tight inverse relationship between El Nino and a poor monsoon has loosened, and researchers tie the change partly to the rising influence of the Indian Ocean Dipole and to a warming background climate.

• Classical link: El Nino years tend to weaken, La Nina years to strengthen, the Indian monsoon.
• Not deterministic: 1997 saw a strong El Nino but a near-normal monsoon.
• Weakening since the 1980s: the dipole and a warming climate now share the controls.

The lesson for forecasting is to read more than one ocean. Because the Pacific signal can be blunted or reinforced by the Indian Ocean, the seasonal outlook for the monsoon now rests on watching ENSO and the dipole together, a theme the next section develops.

The Indian Ocean Dipole

The Dipole that Sways the Monsoon

What is the significance of the Indian Ocean Dipole: it is the Indian Ocean's own coupled mode, a see-saw of sea-surface temperature between the west and the east of the tropical basin that can strengthen or weaken the Indian monsoon, sometimes offsetting the influence of El Nino.

The dipole has a positive and a negative phase. In a positive Indian Ocean Dipole the western tropical Indian Ocean, towards the Arabian Sea and the African coast, becomes unusually warm, while the eastern tropical Indian Ocean, towards Sumatra, becomes unusually cool. In the negative phase the pattern reverses, with a warm east and a cool west.

The strength of the see-saw is measured by the temperature difference between the two poles, an index called the Dipole Mode Index. Because the warm and cool poles also change where the air rises and rains, the dipole leaves a contrast in evaporation, rainfall and therefore surface salinity between the two sides of the basin, just as ENSO does in the Pacific.

A positive dipole tends to help the monsoon. With its warm western pole close to the subcontinent, a positive Indian Ocean Dipole strengthens the Indian summer monsoon and can bring good rains even in an El Nino year, as it did in 1997. A negative dipole tends to weaken the monsoon, and the two phases can reinforce or oppose the effect of El Nino.

This is exactly why the monsoon cannot be forecast from the Pacific alone, and why the 2017 examination tested that the dipole is mentioned while forecasting the Indian monsoon. The strong positive dipole of 2019 contributed to a late but heavy Indian monsoon and, at the same time, to severe drought and bushfires in Australia, while positive-dipole years tend to bring wetter conditions to East Africa.

Seasonal forecasting now weighs the dipole alongside ENSO. Indian and international agencies issue dipole outlooks each year as part of the monsoon forecast, treating the Indian Ocean not as a passive recipient of Pacific signals but as an active partner with a climate mode of its own.

The dipole and ENSO can pull together or apart. A positive Indian Ocean Dipole arising alongside an El Nino can blunt its drying grip on the monsoon, as in 1997, while a negative dipole in a La Nina year can deepen a deficit. Because the two oceans can reinforce or oppose each other, the monsoon outlook rests on reading both basins at once.

The Coupled Feedback and the Discovery of the Dipole

The dipole sustains itself through a coupled feedback. When the eastern Indian Ocean cools a little, the equatorial winds shift, pushing warm water westward and lifting cool water in the east, which deepens the original difference, a self-reinforcing loop between ocean and wind known as the Bjerknes feedback.

The mode was named only recently. Although forecasters had long felt the Indian Ocean's moods, the dipole was identified as a distinct climate mode in 1999, when scientists showed that the basin has a coupled see-saw of its own, independent of the Pacific though able to interact with it.

The extreme event of 2019 showed its reach. A very strong positive dipole that year helped bring a late but heavy monsoon to India and, at the same moment, drove drought and bushfire across Australia and floods across East Africa, a vivid demonstration that one ocean mode can move the weather of three continents.

• Bjerknes feedback: a wind-and-ocean loop that strengthens the temperature difference.
• Identified in 1999 as a coupled Indian Ocean mode in its own right.
• The 2019 positive dipole: heavy Indian monsoon, Australian drought, East African floods.

The dipole has thus earned a permanent place in the monsoon forecast. It is no longer treated as background noise but as a leading actor, watched each season beside El Nino, because its phase can decide whether a Pacific warming is felt in India as drought or as rain.

Salinity and Climate Change

The Amplifying Water Cycle and the Salinity Fingerprint

What is the significance of climate change for salinity: a warming world is speeding up the global water cycle, and because salinity is the ocean's record of that cycle, the salinity pattern is sharpening in a way that has become one of the clearest fingerprints of the changing climate.

A warmer atmosphere holds more water. By the Clausius-Clapeyron relation, the air can carry about seven per cent more water vapour for each degree Celsius of warming. A moister atmosphere drives a stronger water cycle, with more evaporation where it is already dry and more rainfall where it is already wet.

The consequence for the ocean is caught in a memorable phrase, fresh gets fresher and salty gets saltier: the salty subtropical surfaces grow saltier still, while the rainy tropics and the cool high latitudes grow fresher. The salinity contrast between the wet and the dry regions of the ocean intensifies as the cycle speeds up.

This sharpening has actually been observed. Comparisons of salinity measurements since the 1950s show that the salty regions of the surface ocean have become measurably saltier and the fresh regions fresher, exactly as a strengthening water cycle predicts. The chain from warming to a sharper salinity pattern is set out in the figure below, and the comparison table of the chief climate drivers makes it plain.

The Intergovernmental Panel on Climate Change reports this salinity-contrast change with high confidence as evidence that the global water cycle has intensified, a finding all the more striking because it comes from the ocean, far from the cities and industries that drive the warming.

There is a further and graver linkage. The sinking of cold, salty water in the North Atlantic drives the deep limb of the global conveyor that Part 2 described. As the high latitudes freshen, from extra rainfall and from the melting of Greenland's ice, the surface water grows less dense and harder to sink.

The Atlantic overturning is therefore projected to weaken through this century, so that a salinity change at the surface can reach down and slow the great circulation of the deep. What changes at the sea surface in the rain and the run-off does not stay at the surface; it is carried into the engine room of the climate.

The threads of this part converge here. Salinity records the water cycle, couples to the monsoon, swings with ENSO and the dipole, and now sharpens under warming, with consequences that run from the deep overturning to the marine ecosystems and the human economies that the later parts of this series take up.

• Warming: a warmer atmosphere holds about 7 per cent more water vapour per degree.
• Cycle: more evaporation in dry zones, more rainfall in wet zones; salty saltier and fresh fresher.
• Fingerprint: the observed salinity contrast, and the projected weakening of the Atlantic overturning.

Sea level carries a salinity signature too. As the ocean warms and freshens in places, its density falls and the water expands, so that part of the rise in sea level is a steric swelling rather than added mass alone, another way in which a change written in salt is felt at the coast.

Polar Freshening, Ice Melt and the Overturning Risk

The poles are freshening fastest of all. The high latitudes are warming more quickly than the tropics, the extra rain and the melting of land and sea ice are pouring fresh water into the polar seas, and the surface there is growing markedly less salty, the other end of the sharpening pattern from the saltier subtropics.

Fresher polar water is harder to sink. The deep conveyor of the ocean is driven by cold, salty water becoming dense enough to plunge in the North Atlantic and around Antarctica, so a freshening of these surfaces makes the water lighter and slows the sinking that powers the global circulation.

The melt of Greenland adds to the danger. As the Greenland ice sheet loses mass, its meltwater spreads a fresh cap over the northern Atlantic, and this is among the reasons the assessment reports project the Atlantic overturning to weaken through the present century, with consequences for the climate of Europe and the tropics alike.

• Polar amplification: the high latitudes warm and freshen faster than the tropics.
• Ice melt: Greenland and sea-ice meltwater cap the surface with fresh water.
• Overturning risk: a lighter, fresher surface slows the deep Atlantic circulation.

A change written in salt thus reaches the engine room of the climate. What begins as extra rain and meltwater at the surface can slow the deep overturning circulation, so that the freshening of the poles is not a local curiosity but a lever on the heat budget of the whole planet.

Human Influence on the Indian Monsoon: Warming Seas and a Changing Land Surface

The monsoon is changing under human pressure from two directions. One is the ocean that this part has traced, the warming of the Bay of Bengal and the Arabian Sea and the intensifying water cycle; the other is the humanised land surface over which the monsoon breaks, reshaped by farming, by cities, by forests felled and by skies thick with haze.

Aerosols dim the sunlight and shift the rain. The haze of soot, dust and sulphate over South Asia, the so-called atmospheric brown cloud, reflects and absorbs sunlight, cools the surface relative to the air aloft and weakens the land-sea heat contrast that draws the monsoon inland, tending to reduce and to redistribute the seasonal rainfall.

Clearing forests and spreading cities leave their own mark. Deforestation reduces the moisture that the land recycles back to the air and weakens the orographic lifting that wrings rain from the hills, while urban heat islands and vast irrigated tracts alter local heating, humidity and convection, nudging where and when the rain falls.

• Warming seas: a hotter Bay of Bengal and Arabian Sea, an intensifying water cycle and a freshening Bay.
• Aerosols: the South Asian haze dims sunlight, cools the surface and weakens the monsoon’s land-sea contrast.
• Land-surface change: deforestation, urban heat islands and irrigation reshape local heating and convection.

The balance of the argument is one of degree. The human imprint on the monsoon is real and growing, working through warmer seas, a stronger water cycle and a transformed land surface, yet the year-to-year swing of the rains is still set chiefly by the natural ocean drivers, ENSO and the Indian Ocean Dipole, so the honest verdict is one of partial agreement.

UPSC Relevance and Exam Focus

Where Salinity and Climate Fit in the UPSC-CSE Syllabus

This topic sits at the meeting point of oceanography and climatology in General Studies Paper I, and it is among the most heavily examined themes of physical geography, in both the Prelims and the Mains, because it links the ocean to the monsoon on which the country depends.

The questions most often test the ocean-atmosphere interaction, the difference between El Nino and La Nina, the Indian Ocean Dipole and its monsoon link, and the role of the seas in cyclone formation and in the changing monsoon.

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

• E minus P: surface salinity is the ocean’s record of evaporation minus precipitation, the water cycle.
• The Bay of Bengal: fresh and barrier-layered, it stays warm and feeds the monsoon and cyclones; the Arabian Sea is salty and well mixed.
• El Nino: warm phase, weak Walker loop, tends to weaken the Indian monsoon; La Nina is the cool, strengthening phase.
• El Nino Modoki: central-Pacific warming, distinct from the canonical eastern-Pacific El Nino.
• The Indian Ocean Dipole: a positive dipole (warm west, cool east) strengthens the monsoon and is used in monsoon forecasting.
• Climate change: fresh gets fresher and salty gets saltier; the salinity contrast is a fingerprint of the strengthening water cycle.

A 2017 Prelims question on the Indian Ocean Dipole turned on the difference between the two tropical Indian Ocean poles and on the dipole's power to modulate the effect of El Nino on the monsoon; a reader who has fixed that the dipole is an Indian Ocean see-saw, not an Indian-versus-Pacific contrast, can sort the statements correctly.

A 2015 Mains question asked how far the behaviour of the Indian monsoon has been changing under a humanised landscape; the salinity-and-climate material of this part supplies a key strand of the answer, the warming of the seas, the freshening of the Bay of Bengal and the intensifying water cycle that are reshaping the rains, set beside the land-use and aerosol changes the question points to.

Sources and Further Reading

• IPCC AR6 Working Group I, Chapter 9: Ocean, Cryosphere and Sea Level Change
• NOAA Climate.gov: ENSO (El Nino / La Nina)
• India Meteorological Department: Monsoon and seasonal forecasts
• INCOIS: Indian Ocean observation (Argo, RAMA)
• NASA SMAP / Aquarius: Sea surface salinity
• Wikipedia: Indian Ocean Dipole
• Wikipedia: El Nino-Southern Oscillation
• NCERT Class 11, Fundamentals of Physical Geography (Movements of Ocean Water)