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El Niño, the Indian Ocean Dipole, coral reefs and ocean resources
The normal Walker circulation and the Southern Oscillation; how El Niño and La Niña develop and affect rainfall worldwide and the Indian monsoon; the Indian Ocean Dipole and its interplay with El Niño; how coral polyps and their algae build reefs, the three types of reef, India's reef areas and coral bleaching; and the maritime zones of UNCLOS, Article 297, polymetallic nodules, the International Seabed Authority and India's Deep Ocean Mission.
Parts 1 and 2 described the ocean as it normally behaves. This last part looks at what happens when that normal pattern shifts, and at what the sea gives us. It begins with El Niño, La Niña and the Indian Ocean Dipole, the swings in ocean temperature that decide whether India's monsoon is generous or weak, and so touch farm output, food prices and water supply. It then turns to the coral reef, a living structure built by tiny animals in warm, clear seas, and to the threat of bleaching. It ends with the law of the sea: how far a country's control reaches into the ocean, and how the minerals of the deep seabed are shared.
To understand El Niño, start with a normal year in the tropical Pacific. The trade winds blow steadily from east to west, pushing warm surface water towards Indonesia and Australia, where it piles up into a deep pool of the warmest water on Earth. Off the coast of Peru, the water pushed away is replaced by upwelling cold water, which keeps the eastern Pacific cool. Over the warm west, air rises, forms clouds and brings heavy rain; over the cool east, air sinks and skies stay dry. High in the atmosphere the air returns eastward, completing a loop called the Walker circulation, after Gilbert Walker, a British scientist who headed weather observatories in India in the early twentieth century. He found a see-saw of air pressure between the eastern and western Pacific, known as the Southern Oscillation: when pressure rises at one end, it tends to fall at the other.
El Niño is what happens when this machine falters. Every few years the trade winds weaken, or even reverse in the western Pacific, and the pool of warm water spreads eastward across the central and eastern Pacific. Upwelling off Peru weakens, the coastal sea warms, and the fish that depend on cold, nutrient-rich water, such as anchovies, become scarce. The name, Spanish for 'the little boy' and a reference to the Christ child, comes from Peruvian fishermen who noticed warm water arriving around Christmas. Because the ocean change and the swing in air pressure are two sides of one event, scientists join them as ENSO, El Niño–Southern Oscillation. El Niño recurs irregularly, roughly every two to seven years, and an episode usually lasts nine to twelve months. NOAA declares El Niño conditions when the surface of the central equatorial Pacific stays at least 0.5°C warmer than its long-term average, along with matching changes in the atmosphere.
When the warm pool and its rain clouds shift east, rainfall patterns change across much of the tropics. Peru and Ecuador, normally dry, can receive heavy rain and floods, while Indonesia and eastern Australia often suffer drought and forest fires. For India the key effect is on the south-west monsoon. With rising air and cloud displaced towards the central Pacific, air tends to sink over the Indian region, so El Niño years are more likely to bring a weak monsoon and drought. The link is a tendency, not a rule: some El Niño years have still brought near-normal rain, while some weak monsoons came without El Niño, because other factors, including the Indian Ocean itself, also play a part. That is why the India Meteorological Department watches the Pacific closely before it issues its monsoon forecasts each year.
La Niña, 'the little girl', is the opposite phase. The trade winds blow harder than usual, more warm water is driven west, upwelling off Peru strengthens and the central and eastern Pacific turn cooler than normal. The rising branch of the Walker circulation over the western Pacific becomes stronger, and for India this usually means a normal or above-normal monsoon, though it can also bring floods. La Niña tends to bring wetter conditions to Indonesia and Australia and drier ones to parts of the Americas. Between the two phases lie neutral years, when neither pattern is established. A useful way to see the whole system is as a pendulum: El Niño and La Niña are its two extremes, and in many years the ocean sits near the middle.
India's own ocean has a see-saw too. The Indian Ocean Dipole (IOD) compares the sea-surface temperature of the western tropical Indian Ocean, near the Arabian Sea and East Africa, with that of the eastern part, near Sumatra and Java. In a positive phase the west is warmer than usual and the east cooler; rising air and rain shift towards the west, which tends to strengthen the Indian monsoon and bring heavy rain to East Africa, while Indonesia and Australia turn dry. A negative phase reverses this and tends to weaken the monsoon. The dipole was described by scientists in 1999 and usually develops around the middle of the year, peaking in September or October. Its importance for exams lies in its interplay with El Niño: in 1997 a strong positive IOD helped India escape the drought that a powerful El Niño might otherwise have brought.
A coral reef is built by animals. Each coral polyp is a tiny creature, related to sea anemones and jellyfish, that draws calcium carbonate from sea water to build a hard limestone cup around itself; over thousands of years millions of these skeletons pile up into reefs. Inside the polyp's tissue live microscopic algae called zooxanthellae. The algae use sunlight to make food, share it with the coral and give it colour, while the coral gives them shelter. Because of this partnership, reef-building corals need warm, clear, shallow and salty water, mostly within the tropics, where sunlight reaches the algae. Muddy river mouths and very deep or cold water do not suit them.
Reefs come in three main forms. A fringing reef grows close to the shore with little or no water between it and the land. A barrier reef runs parallel to the coast but is separated from it by a wide, deep lagoon; the Great Barrier Reef off Queensland in Australia is the largest reef system in the world. An atoll is a ring or horseshoe of reef enclosing a lagoon, with no central land; it often forms around a volcanic island that has slowly sunk beneath the sea. India has reefs in four main areas: the Gulf of Mannar and the Gulf of Kachchh, which have fringing reefs, the Lakshadweep islands, which are atolls, and the Andaman and Nicobar Islands, which have extensive fringing reefs.
| Type | Form | Example |
|---|---|---|
| Fringing reef | Grows close to the shore, little or no lagoon | Gulf of Mannar, Gulf of Kachchh, Andaman and Nicobar |
| Barrier reef | Parallel to the coast, separated by a wide, deep lagoon | Great Barrier Reef, Australia |
| Atoll | Ring of reef around a lagoon, no central land | Lakshadweep |
coral bleaching happens when corals are stressed, above all by water that stays unusually warm for weeks. The stressed polyps expel their algae, and without them the white limestone skeleton shows through the clear tissue. A bleached coral is not yet dead: if temperatures fall soon, the algae can return and the coral recovers, but prolonged heat leaves it starving and it may die. Pollution, sediment from the land and very low tides can also cause bleaching. Warming seas have made mass bleaching more frequent, and El Niño years, which heat large areas of tropical ocean, have triggered several worldwide episodes. In April 2024 NOAA confirmed the fourth global bleaching event on record.
Who owns the sea? The United Nations Convention on the Law of the Sea (UNCLOS), adopted in 1982 and in force since 1994, divides it into zones measured from a baseline along the coast, in nautical miles (one nautical mile is about 1.85 km). The territorial waters extend up to 12 nautical miles; here the coastal state is sovereign, though foreign ships keep a right of innocent passage. The contiguous zone extends to 24 nautical miles, where the state may enforce its customs, tax, immigration and health laws. The exclusive economic zone reaches 200 nautical miles: the state has sovereign rights over fish, oil, gas and minerals, while other countries may still sail and fly through. In India, Article 297 of the Constitution vests all land, minerals and things of value in the territorial waters, the continental shelf and the exclusive economic zone in the Union.
| Zone | Extent | Rights of the coastal state |
|---|---|---|
| Territorial waters | Up to 12 nautical miles | Full sovereignty; innocent passage for foreign ships |
| Contiguous zone | Up to 24 nautical miles | Enforce customs, tax, immigration and health laws |
| Exclusive economic zone | Up to 200 nautical miles | Sovereign rights over living and mineral resources; others may navigate and overfly |
| High seas and the Area | Beyond national zones | Open to all; seabed minerals are common heritage, managed by the International Seabed Authority |
Beyond national zones lies the deep seabed, which UNCLOS calls the common heritage of mankind. Its minerals are managed by the International Seabed Authority, based in Kingston, Jamaica, which grants contracts for exploration. The best-known resource is polymetallic nodules: potato-sized lumps scattered over the abyssal plains and rich in manganese, nickel, cobalt and copper, metals needed for batteries and electronics. India was the first country to be registered as a pioneer investor for nodules, in 1987, and holds an exploration contract with the Authority in the Central Indian Ocean Basin. To go further, the Union Cabinet approved the Deep Ocean Mission of the Ministry of Earth Sciences in 2021. Its parts include deep-sea mining technology and Samudrayaan, a crewed submersible, MATSYA-6000, being built by the National Institute of Ocean Technology in Chennai to carry three people to a depth of 6,000 metres.
This part shows the ocean as a partner of the atmosphere and as a resource. El Niño and La Niña are two swings of one Pacific system, read through sea temperatures and the Southern Oscillation, and the Indian Ocean Dipole adds a second, closer influence that can soften or sharpen El Niño's effect on India's monsoon. Coral reefs depend on warm, clear water, and the same warming that drives strong El Niño years pushes them towards bleaching. The law of the sea then decides who may use what: sovereignty to 12 nautical miles, control of resources to 200, and a shared deep seabed managed for all. Together with Parts 1 and 2, this completes the picture of the oceans: their floor, their water, its movements and their place in India's weather and economy.
Exam traps
- During El Niño the central and eastern Pacific become warmer than normal and the trade winds weaken; during La Niña the trade winds strengthen and those waters become cooler.
- El Niño tends to weaken the Indian monsoon, but it does not always cause drought; La Niña usually favours a normal or above-normal monsoon.
- A positive Indian Ocean Dipole means a warmer western (Arabian Sea side) and cooler eastern Indian Ocean; it tends to help the Indian monsoon and can offset El Niño, as in 1997.
- The Southern Oscillation is a see-saw of air pressure across the Pacific, not an ocean current; together with El Niño it forms ENSO.
- Corals are animals; the colour comes from the algae (zooxanthellae) living in them. Bleaching means the algae are expelled, and a bleached coral can still recover.
- Lakshadweep consists of atolls; the Gulf of Mannar, the Gulf of Kachchh and the Andaman and Nicobar Islands have mainly fringing reefs. The Great Barrier Reef is a barrier reef off Australia.
- Territorial waters 12, contiguous zone 24, exclusive economic zone 200 nautical miles, all measured from the baseline. Foreign ships have innocent passage in territorial waters and freedom of navigation in the EEZ.
- Under Article 297 the resources of the territorial waters, continental shelf and EEZ vest in the Union, not in the coastal States.
Recap
- Normal year: trade winds pile warm water in the west Pacific, upwelling cools the east; the Walker circulation; Gilbert Walker and the Southern Oscillation.
- El Niño: trade winds weaken, warm water spreads east, upwelling off Peru fails; recurs every 2–7 years; El Niño plus Southern Oscillation = ENSO.
- El Niño effects: rain and floods in Peru, drought in Indonesia and Australia, a tendency towards a weak Indian monsoon.
- La Niña: stronger trade winds, cooler eastern Pacific, usually a good Indian monsoon.
- Indian Ocean Dipole: positive when the west is warmer; tends to help the monsoon and offset El Niño (1997); described in 1999.
- Coral polyps and zooxanthellae; need warm, clear, shallow, salty water; fringing reefs, barrier reefs and atolls.
- India's reefs: Gulf of Mannar, Gulf of Kachchh, Lakshadweep (atolls), Andaman and Nicobar; bleaching from heat stress, fourth global event confirmed in April 2024.
- UNCLOS 1982 (in force 1994): territorial waters 12, contiguous zone 24, EEZ 200 nautical miles; Article 297 vests these resources in the Union.
- Deep seabed is common heritage, run by the International Seabed Authority (Kingston); India first pioneer investor for nodules (1987); Deep Ocean Mission (2021), Samudrayaan, MATSYA-6000.
Practice questions
Q1. During an El Niño event, which one of the following usually happens?
- The trade winds over the tropical Pacific become stronger than normal
- Upwelling of cold water off the coast of Peru weakens
- The central and eastern Pacific become cooler than normal
- Indonesia receives unusually heavy rainfall
Show answer
Answer: B. Upwelling of cold water off the coast of Peru weakens
In El Niño the trade winds weaken, warm water spreads east and the upwelling of cold water off Peru weakens. Stronger trade winds and a cooler eastern Pacific describe La Niña, not El Niño. Indonesia usually turns dry during El Niño because the rain clouds shift east, so heavy rain there is the opposite of the typical effect.
Q2. Consider the following statements:
1. A positive Indian Ocean Dipole is marked by a warmer western and a cooler eastern tropical Indian Ocean.
2. A positive Indian Ocean Dipole tends to weaken the Indian south-west monsoon.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: A. 1 only
Statement 1 is correct: in the positive phase the western Indian Ocean, near the Arabian Sea and East Africa, is warmer than usual and the east, near Sumatra and Java, is cooler. Statement 2 is wrong: a positive dipole tends to strengthen the Indian monsoon, and it helped offset El Niño in 1997; it is the negative phase that tends to weaken it. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.
Q3. The term 'Southern Oscillation' refers to
- a see-saw of air pressure between the eastern and western tropical Pacific
- the seasonal reversal of currents in the north Indian Ocean
- the eastward flow of water around Antarctica
- the alternation of spring and neap tides
Show answer
Answer: A. a see-saw of air pressure between the eastern and western tropical Pacific
The Southern Oscillation, described by Gilbert Walker, is the see-saw of surface air pressure across the tropical Pacific; joined with El Niño it forms ENSO. The monsoon reversal of north Indian Ocean currents, the West Wind Drift around Antarctica and the cycle of spring and neap tides are different phenomena with no link to this term.
Q4. Consider the following statements about corals:
1. Corals are plants that make their own food by photosynthesis.
2. A bleached coral is not necessarily dead and can recover if the stress ends soon.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: B. 2 only
Statement 1 is wrong: corals are animals related to jellyfish and sea anemones; it is the algae (zooxanthellae) living in their tissue that photosynthesise. Statement 2 is correct: bleaching means the algae have been expelled, and if temperatures fall soon the algae can return and the coral recovers. So '1 only' and 'both' include a false statement, and 'neither' rejects a true one.
Q5. Match the following:
A. Atoll — 1. Great Barrier Reef
B. Barrier reef — 2. Lakshadweep
C. Fringing reef — 3. Gulf of Kachchh
Select the correct answer using the code given below:
- A-2, B-1, C-3
- A-1, B-2, C-3
- A-3, B-1, C-2
- A-2, B-3, C-1
Show answer
Answer: A. A-2, B-1, C-3
Lakshadweep is made of atolls, the Great Barrier Reef off Australia is a barrier reef, and the Gulf of Kachchh has fringing reefs. The second code wrongly calls the Great Barrier Reef an atoll and Lakshadweep a barrier reef; the third makes the Gulf of Kachchh an atoll; the fourth swaps the barrier and fringing examples.
Q6. Under the United Nations Convention on the Law of the Sea, a coastal state may enforce its customs and immigration laws in the contiguous zone, which extends up to how many nautical miles from the baseline?
- 12
- 24
- 200
- 350
Show answer
Answer: B. 24
The contiguous zone extends up to 24 nautical miles. Twelve nautical miles is the limit of the territorial waters, 200 is the limit of the exclusive economic zone, and 350 is the outer cap that can apply to a continental shelf extending beyond 200 miles, not to the contiguous zone.
Q7. Consider the following statements:
1. Under Article 297 of the Constitution of India, things of value in the exclusive economic zone vest in the Union.
2. In its exclusive economic zone a coastal state may bar foreign ships from navigating.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: A. 1 only
Statement 1 is correct: Article 297 vests all lands, minerals and other things of value in the territorial waters, the continental shelf and the exclusive economic zone in the Union. Statement 2 is wrong: in the EEZ the coastal state has sovereign rights over resources, but other countries keep freedom of navigation and overflight. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.
Q8. With reference to India's activities in the deep ocean, consider the following statements:
1. India holds an exploration contract with the International Seabed Authority for polymetallic nodules in the Central Indian Ocean Basin.
2. MATSYA-6000 is a crewed submersible being developed under the Deep Ocean Mission.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: C. Both 1 and 2
Both statements are correct. India, the first registered pioneer investor for nodules (1987), holds an exploration contract with the International Seabed Authority in the Central Indian Ocean Basin, and MATSYA-6000, the vehicle of Samudrayaan under the Deep Ocean Mission, is designed to carry three people to 6,000 metres. So '1 only', '2 only' and 'neither' each leave out a true statement.
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- NOAA Climate Prediction Center and NOAA Climate.gov: ENSO, El Niño and La Niña, the Walker circulation, the Southern Oscillation and the 0.5°C threshold (US government) — https://www.climate.gov/enso
- India Meteorological Department, Ministry of Earth Sciences: monsoon forecasts and the influence of ENSO and the Indian Ocean Dipole on Indian rainfall — https://mausam.imd.gov.in
- NASA Earth Observatory: El Niño, sea-surface temperature and global rainfall patterns (US government) — https://earthobservatory.nasa.gov
- Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Sciences: the Indian Ocean Dipole and sea-surface temperature of the Indian Ocean — https://incois.gov.in
- NOAA Coral Reef Conservation Program and Coral Reef Watch: coral polyps and zooxanthellae, reef types, coral bleaching and the fourth global bleaching event (April 2024) (US government) — https://coralreefwatch.noaa.gov
- Ministry of Environment, Forest and Climate Change, Government of India: coral reefs of India (Gulf of Mannar, Gulf of Kachchh, Lakshadweep, Andaman and Nicobar Islands) — https://moef.gov.in
- United Nations Convention on the Law of the Sea, 1982: Parts II, V, VI and XI (territorial sea, contiguous zone, exclusive economic zone, continental shelf, the Area and the International Seabed Authority) — https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf
- Constitution of India, Article 297 — https://legislative.gov.in/constitution-of-india/
- Ministry of Earth Sciences and Press Information Bureau: the Deep Ocean Mission (approved 2021), Samudrayaan and MATSYA-6000, and India's polymetallic-nodule programme in the Central Indian Ocean Basin — https://moes.gov.in