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Quick recap: oceanography on one page
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.
Oceanography 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.
Part 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.
Part 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.
Part 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.
| 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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- 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