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Quick recap: oceanography on one page

मराठीत स्पष्टीकरण

Oceanography: the ocean floor, ocean water and its movements · Quick recap · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

All three parts condensed for quick revision: the ocean floor, temperature and salinity; waves, tides and ocean currents; and El Niño, La Niña, the Indian Ocean Dipole, coral reefs and the maritime zones of UNCLOS.

Each idea is explained first in Marathi (मराठी), ending with the English and Hindi exam terms, followed by the same point in exam English. UPSC Prelims is in English or Hindi, so understand it in Marathi and revise it in English.

हे प्रकरण म्हणजे एकच गोष्ट: आधी भांडे (समुद्रतळ), मग त्यातले पाणी (तापमान, क्षारता), शेवटी त्या पाण्याची हालचाल. उष्णता आणि मीठ पाण्याचे वजन ठरवतात; वारा आणि वजनातला फरक पाणी हलवतात; त्या हालचालीतला बदल आपल्या मान्सूनपर्यंत पोहोचतो — चुलीची आच बदलली की आमटीची चव बदलते तसा. (→ English: salinity · हिन्दी: लवणता)

In exam EnglishOceanography moves from the container to the contents to the movement. Oceans cover about 71% of the Earth's surface and hold about 97% of its water. Part 1 covers the shape of the ocean floor and the temperature and salinity of sea water; Part 2 the movements of that water in waves, tides and currents; and Part 3 El Niño, the Indian Ocean Dipole, coral reefs and the law of the sea. Read the chapter as one story: heat and salt set the density of the water, winds and density set it moving, and changes in that movement reach India through its monsoon.

किनाऱ्यापासून समुद्रतळ पायऱ्यांसारखा उतरतो: उथळ खंडीय मंच (मासे, मुंबई हाय तेल), मग उतार, सपाट खोल मैदान. भूपट्ट खाली जातात तिथे गर्ता — पॅसिफिकमध्ये जास्त, मारियानात जवळजवळ 11 किमी खोल; दूर जातात तिथे रांगा. पाणी ध्रुवांकडे आणि खोल जावे तसे थंड. मीठ सरासरी हजारी 35; उपोष्ण कटिबंधात आणि तांबड्या समुद्रात जास्त, बाल्टिकमध्ये कमी; बंगालच्या उपसागरापेक्षा अरबी समुद्र जास्त खारा. (→ English: continental shelf · हिन्दी: महाद्वीपीय मग्नतट)

In exam EnglishPart 1: from the coast the floor descends as the continental shelf (shallow, rich in fish and in oil and gas such as Mumbai High), the continental slope, the rise and the abyssal plain. Ocean trenches mark where plates sink, mostly around the Pacific; the Challenger Deep in the Mariana Trench is nearly 11 km deep. Mid-ocean ridges mark where plates part. Sea temperature falls towards the poles and with depth, with a thermocline between warm surface water and cold deep water. Salinity averages about 35 parts per thousand; it is highest in the subtropics and in enclosed dry seas such as the Red Sea, low in the Baltic, and higher in the Arabian Sea than in the Bay of Bengal.

लाटेत ऊर्जा पुढे जाते, पाणी तिथेच गोल फिरते; त्सुनामी भूकंपामुळे, किनाऱ्याजवळच मोठी होते. भरती-ओहोटीचे मुख्य कारण चंद्र: दोन फुगवटे, दिवसातून दोन भरती, रोज सुमारे 50 मिनिटे उशिरा. अमावस्या-पौर्णिमेला उधाणाची भरती, अष्टमीला भांगाची भरती. प्रवाहांना वारा, कोरिओलिस बल आणि वजनातला फरक चालवतात; उत्तरेत घड्याळाच्या दिशेने, दक्षिणेत उलट चक्रे, उत्तर हिंदी महासागरात मान्सूनमुळे दिशाबदल. पूर्व किनाऱ्यावर उष्ण, पश्चिम किनाऱ्यावर थंड प्रवाह — थंड प्रवाहांजवळ अटाकामा, नामिब वाळवंटे. (→ English: tide · हिन्दी: ज्वार-भाटा)

In exam EnglishPart 2: in a wind wave the energy moves while the water mostly circles in place; a tsunami is a seismic wave that grows tall only near the coast. Each tide is caused mainly by the Moon; two bulges give most coasts two highs and two lows, about 50 minutes later each day. Spring tides come at new and full moon, neap tides at the quarters. Currents are driven by planetary winds, the Coriolis force and density differences; gyres turn clockwise in the north and anticlockwise in the south, and the north Indian Ocean reverses with the monsoon. Warm currents run along the eastern coasts of continents in low and middle latitudes and cold currents along the western coasts, where they help create deserts such as the Atacama and the Namib.

सामान्य वर्षी पॅसिफिकमध्ये उबदार पाणी पश्चिमेला साचते, पूर्व थंड राहते. एल निनोमध्ये वारे कमकुवत होऊन उबदार पाणी पूर्वेकडे सरकते, आपला मान्सून कमकुवत होण्याची शक्यता; ला निनामध्ये उलटे. हिंदी महासागरात पश्चिम उबदार असलेला धन द्विध्रुव मान्सूनला मदत — 1997 मध्ये एल निनोचा तडाखा रोखला. प्रवाळ भिंती शेवाळासोबत राहणारे लहान जीव बांधतात; अति उष्णतेत शेवाळ जाऊन पांढऱ्या पडतात. सागरी कायदा: 12, 24, 200 सागरी मैल; अनुच्छेद 297 संपत्ती संघाची. (→ English: El Niño · हिन्दी: एल नीनो)

In exam EnglishPart 3: in a normal year the trade winds pile warm water in the western Pacific and upwelling keeps the east cool. In El Niño the trade winds weaken, warm water spreads east and the Indian monsoon tends to weaken; in La Niña the reverse happens and the monsoon is usually good. A positive Indian Ocean Dipole, with a warmer western Indian Ocean, tends to help the monsoon and can offset El Niño, as in 1997. Coral reefs are built by polyps living with algae in warm, clear, shallow water; they bleach when heat makes the polyps expel the algae. Under UNCLOS, territorial waters reach 12 nautical miles, the contiguous zone 24 and the exclusive economic zone 200. Separately, within India, Article 297 of the Constitution vests the resources of these waters and the continental shelf in the Union.

Oceanography at a glance
Topic	Must-know facts	Common trap
Ocean floor	Shelf, slope, rise, abyssal plain; Challenger Deep nearly 11 km	Mumbai High is on the shelf, not the abyssal plain
Salinity	Average about 35‰; highest in the subtropics	Not highest at the equator; Arabian Sea saltier than Bay of Bengal
Tides	Mainly the Moon; spring at new and full moon, neap at quarters	Spring tide has nothing to do with the season
Currents	Gyres clockwise in the north; warm on east coasts, cold on west coasts	North Indian Ocean currents reverse with the monsoon
ENSO	El Niño: weak trade winds, warm eastern Pacific; La Niña the opposite	El Niño tends to weaken the monsoon but does not always cause drought
Indian Ocean Dipole	Positive: warmer west; tends to help the monsoon	Can offset El Niño (1997)
Coral reefs	Polyps with zooxanthellae; fringing, barrier, atoll	Lakshadweep is atolls; bleached coral can recover
Maritime zones	12 / 24 / 200 nautical miles from the baseline	Free navigation for others in the EEZ

Exam traps

The Pacific is the largest and deepest ocean; most trenches ring the Pacific; the Mariana Trench is the deepest.
Surface salinity peaks in the subtropics, not at the equator; freezing raises salinity and melting lowers it.
The Moon, not the Sun, is the main cause of tides; spring tides come at new and full moon, twice a month.
In low and middle latitudes cold currents flow along western coasts (Peru, Benguela, Canary, California) and help form coastal deserts.
El Niño warms the central and eastern Pacific and tends to weaken the Indian monsoon; La Niña usually favours it.
A positive Indian Ocean Dipole (warmer west) tends to help the Indian monsoon.
Lakshadweep is made of atolls; the Great Barrier Reef is a barrier reef off Australia.
Territorial waters 12, contiguous zone 24, EEZ 200 nautical miles; Article 297 vests their resources in the Union.

Recap

Oceans: about 71% of the surface, about 97% of the Earth's water; Pacific largest, Arctic smallest.
Floor: shelf, slope, rise, abyssal plain; trenches where plates sink, ridges where they part.
Temperature falls poleward and with depth (thermocline); salinity about 35‰, highest in the subtropics.
Waves move energy, not water; tsunamis are seismic; tides mainly lunar, spring and neap.
Currents: winds, Coriolis force, density; thermohaline circulation; warm east coasts, cold west coasts.
El Niño weakens trade winds and tends to weaken the monsoon; La Niña the reverse; positive IOD helps.
Coral reefs: polyps plus algae; fringing, barrier, atoll; bleaching from heat stress.
UNCLOS zones 12 / 24 / 200 nautical miles; International Seabed Authority; India's Deep Ocean Mission.

Practice questions

Q1. Where is the surface salinity of the open ocean generally highest?

At the equator
In the subtropics
Near the poles
Off the mouths of large rivers

Show answer

Answer: B. In the subtropics

Surface salinity peaks in the subtropics, where strong evaporation and little rain concentrate salt. At the equator heavy rain dilutes the water, near the poles melting ice and low evaporation keep salinity lower, and river mouths add fresh water, so all three are wrong.

Q2. Consider the following statements:
1. Most of the world's ocean trenches lie in the Pacific Ocean.
2. Offshore oil fields such as Mumbai High lie on the abyssal plain.
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: trenches form at subduction zones, which ring the Pacific. Statement 2 is wrong: Mumbai High lies on the shallow continental shelf off western India, not on the deep abyssal plain. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q3. Neap tides occur when

the Sun and the Moon pull at right angles to each other
the Sun, the Moon and the Earth are in a straight line
the Moon is at perigee
there is a full moon

Show answer

Answer: A. the Sun and the Moon pull at right angles to each other

Neap tides come at the first and last quarter, when the pulls of the Sun and Moon act at right angles and partly cancel, giving the smallest range. A straight-line alignment and the full moon produce spring tides, and perigee makes tides larger, not smaller.

Q4. Which one of the following is a cold ocean current?

Kuroshio
Gulf Stream
Benguela
Agulhas

Show answer

Answer: C. Benguela

The Benguela is a cold current flowing north along south-west Africa, beside the Namib Desert. The Kuroshio off Japan, the Gulf Stream off the eastern United States and the Agulhas off south-east Africa are all warm currents.

Q5. Consider the following statements:
1. During La Niña the trade winds over the tropical Pacific are stronger than usual.
2. La Niña years usually bring a weak Indian 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 La Niña the trade winds strengthen and the eastern Pacific turns cooler than normal. Statement 2 is wrong: La Niña usually favours a normal or above-normal Indian monsoon; it is El Niño that tends to weaken it. So '2 only' and 'both' are wrong, and 'neither' rejects a true statement.

Q6. Which of the following Indian reef areas consists of atolls?

Gulf of Kachchh
Gulf of Mannar
Lakshadweep
Sundarbans

Show answer

Answer: C. Lakshadweep

The Lakshadweep islands are atolls, rings of reef around lagoons. The Gulfs of Kachchh and Mannar have fringing reefs, and the Sundarbans is a mangrove delta, not a coral reef area.

Q7. Consider the following statements:
1. A positive Indian Ocean Dipole tends to support the Indian monsoon.
2. Coral bleaching happens when corals expel the algae living in their tissue.
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. A positive dipole, with a warmer western Indian Ocean, tends to strengthen the monsoon and can offset El Niño; bleaching is the loss of the symbiotic algae under stress, mainly unusually warm water. So '1 only', '2 only' and 'neither' each leave out a true statement.

Q8. Under UNCLOS, the exclusive economic zone of a coastal state extends up to

12 nautical miles from the baseline
24 nautical miles from the baseline
200 nautical miles from the baseline
the edge of the abyssal plain

Show answer

Answer: C. 200 nautical miles from the baseline

The exclusive economic zone extends up to 200 nautical miles from the baseline. Twelve nautical miles is the limit of the territorial waters and 24 that of the contiguous zone, while the abyssal plain is a feature of the deep-sea floor, not a legal boundary.

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Sources

NOAA National Ocean Service, Ocean Exploration and Climate.gov: the ocean floor, tides, currents and ENSO (US government) — https://oceanservice.noaa.gov
NASA ocean-salinity material (Aquarius and SMAP missions): salinity and its distribution (US government) — https://salinity.oceansciences.org
NASA Earth Observatory: ocean circulation and coastal climate (US government) — https://earthobservatory.nasa.gov
India Meteorological Department: ENSO, the Indian Ocean Dipole and the monsoon — https://mausam.imd.gov.in
NOAA Coral Reef Conservation Program and Coral Reef Watch: corals, reef types and bleaching (US government) — https://coralreefwatch.noaa.gov
United Nations Convention on the Law of the Sea, 1982, and Constitution of India, Article 297 — https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf