These oceanography notes for OPSC/UPSC (Part II) cover the movements of ocean water, namely waves, tsunami, tides and ocean currents, and then marine deposits, coral reefs and islands. Prelims asks classification and identification questions (warm or cold current, tide types, coral conditions), while mains uses the same topics for monsoon, cyclone, fisheries and Blue Economy answers. Relief, salinity, temperature and density are in Part I.
Oceanography Notes for OPSC/UPSC: Ocean Waves
Wave terms and how waves form
A wave is an oscillation of the water surface that carries energy forward while the water particles move in circles about a fixed position. The water itself does not travel forward.
| Term | Meaning |
|---|---|
| Crest and trough | The highest and lowest points of a wave |
| Wave height | Vertical distance from trough to crest |
| Amplitude | Half of the wave height |
| Wavelength | Horizontal distance between two adjacent crests or troughs |
| Wave period | Time taken by two successive crests (or troughs) to pass a fixed point |
| Fetch | Length of open water over which the wind blows |
- Generation: friction of wind on the water surface. Wave height depends on wind speed, duration of the wind in one direction and fetch.
- Speed: longer waves travel faster. Waves that move out of their area of origin are called swells.
- Wave refraction: as waves approach a shore the part in shallow water slows, so the crests bend to become parallel to the coast. Energy is concentrated on headlands and spread out in bays, which tends to smooth the coastal outline.
- Amplitude is half the wave height, not half the wavelength.
- Waves carry energy forward, not water.
Tsunami
A tsunami is a series of very long waves caused by a sudden displacement of water, most often by an undersea earthquake at a subduction zone, and also by undersea landslides and volcanic eruptions. Hurricanes and cyclones do not cause tsunamis; they cause storm surges. CDS 2010
- In the open ocean a tsunami has a very long wavelength and a height of only about a foot, and it travels at several hundred km per hour. It is hardly noticed there. CDS 2012
- In shallow water its speed falls, its wavelength shortens and its height rises sharply, sometimes to tens of metres. NDA 2013
- The sea often recedes before the first crest arrives, because the trough reaches the shore first. Tides play no role in generating a tsunami.
The earthquake off Sumatra on 26 December 2004 triggered the Indian Ocean tsunami that struck Indonesia, Sri Lanka, India and Thailand. After it India set up the Indian Tsunami Early Warning Centre at INCOIS, Hyderabad, which became operational in 2007.
IMAGE (Diagram): Tsunami generation at a subduction zone and wave growth near the shore
Labels: Subducting plate, Seabed uplift, Long wavelength in deep ocean, Slowing in shallow water, Increase in height, Run-up
Caption: A tsunami is low in the deep sea and grows as it reaches shallow water.
Find it: NOAA or INCOIS tsunami diagram; search tsunami generation subduction zone diagram
Tides Notes for OPSC/UPSC: Causes, Types and Theories
Why tides occur
Tides are the periodic rise and fall of sea level caused by the gravitational pull of the Moon and the Sun and the centrifugal force of the Earth-Moon system. UPSC 2015 The Moon dominates because it is much nearer, and the Sun modifies the effect. Most places have two high tides a day, about 12 hours 26 minutes apart, because the Moon rises about 52 minutes later each day. NDA 2010 Tidal range is the difference between high and low tide. UPSC 2009
Types of tides
| Type | When and what |
|---|---|
| Spring tide | At new moon and full moon, when Sun, Moon and Earth are in a line; about 20% higher than normal |
| Neap tide | At first and last quarter, when the Sun and Moon are at right angles (quadrature); about 20% lower than normal |
| Perigean tide | Moon nearest the Earth (perigee); higher than normal |
| Apogean tide | Moon farthest from the Earth (apogee); lower than normal |
| Equinoctial spring tide | Around the March and September equinoxes; recurs at an interval of six months |
| Tropical and equatorial tides | When the Moon is over the tropics, and when it is over the equator |
By frequency, tides are semi-diurnal (two highs and two lows a day), diurnal (one high and one low) and mixed.
- Spring tides have nothing to do with the season spring. They recur about every 14 days.
- Neap tides occur at the quarters, not at new moon.
- The direction of the Earth’s rotation is not the cause of tides.
Theories of tides
| Theory | Scientist |
|---|---|
| Equilibrium theory | Newton |
| Dynamic theory | Laplace |
| Canal theory | G. B. Airy |
| Stationary wave theory | R. A. Harris |
| Progressive wave theory | William Whewell |
A mismatch question swaps Harris and Whewell. Isostasy is not a theory of tides.
Tidal bore, tidal ports and tidal energy
- Tidal bore: a wall of water that moves up a narrow, shallow estuary against the river flow, especially at spring tide. Examples are the Qiantang River in China and the Hooghly in India.
- Funnel-shaped gulfs with wide fronts and narrow rears amplify tides, as in the Bay of Fundy in Canada (about 15 m range). UPSC 2004
- Tidal ports: Kandla (Gujarat) and Diamond Harbour/Kolkata on the Hooghly depend on high tide for large ships, and tidal flow helps flush silt.
- Tidal energy: India’s potential, about 8,000 MW, is mainly in the Gulf of Khambhat and Gulf of Kutch, with a small share in the Sundarbans.
IMAGE (Diagram): Alignment of Sun, Moon and Earth for spring tide and neap tide
Labels: New moon, Full moon, First quarter, Last quarter, Spring tide, Neap tide, Tidal bulge
Caption: Spring tides occur when the Sun and Moon are in line, neap tides when they are at right angles.
Find it: NCERT Class 11 Fundamentals of Physical Geography, tides figure; search spring tide neap tide diagram
Ocean Currents Notes: Causes, Types and Distribution
Factors and patterns
A continuous, directed flow of ocean water along a definite path, at the surface or at depth.
- Planetary winds (trades, westerlies) are the dominant cause of surface currents. NDA 2012
- Temperature and salinity (density) differences drive deep thermohaline currents.
- Coriolis force deflects currents to the right in the Northern and left in the Southern Hemisphere, so gyres turn clockwise in the north and anticlockwise in the south. NDA 2011
- Coastline shape blocks and divides currents. Revolution of the Earth is not a cause. CDS 2013
The equatorial counter-current flows eastward because the North and South Equatorial Currents pile up water on the western side of the ocean, and the excess flows back east between them. UPSC 2015
Warm and cold currents by ocean
| Ocean | Warm currents | Cold currents |
|---|---|---|
| Atlantic | North and South Equatorial, Florida, Gulf Stream, North Atlantic Drift, Brazil | Canary, Benguela, Labrador, Falkland, South Atlantic Drift |
| Pacific | Kuroshio, North Pacific Drift, East Australian, North and South Equatorial | California, Peru (Humboldt), Oyashio, Kamchatka |
| Indian | Agulhas, Mozambique, Madagascar | West Australian |
- Agulhas is warm. Benguela, on the other side of Africa, is cold.
- Brazil is warm and Falkland is cold. Gulf Stream is a warm Atlantic current, not a Pacific one.
- The Gulf Stream meets the Labrador Current near Newfoundland, not Vancouver Island. The meeting gives plankton-rich fishing grounds (Grand Banks) and also dense fog.
- The Kuroshio is warm because it carries tropical water, not because of sea-floor volcanoes.
- The West Wind Drift is an ocean current, not a wind system.
IMAGE (Map): World map of major warm and cold ocean currents
Labels: Gulf Stream, North Atlantic Drift, Labrador, Canary, Benguela, Brazil, Falkland, Kuroshio, Oyashio, California, Peru, East Australian, Agulhas, West Australian, West Wind Drift, Equatorial Counter Current
Caption: Warm currents (red) and cold currents (blue) form gyres in each ocean basin.
Find it: NCERT Class 11 Fundamentals of Physical Geography, ocean currents map; search world ocean currents map warm cold
Indian Ocean: monsoon drift
The north Indian Ocean is unique because its currents reverse with the monsoon: they flow clockwise in summer under the south-west monsoon and anticlockwise in winter under the north-east monsoon. The Somali Current is a strong coastal current in summer with upwelling that makes the western Arabian Sea productive. The West Australian is the only major cold current. The Indian Ocean Dipole, the difference in sea surface temperature between its western and eastern parts, also affects monsoon rainfall.
Upwelling and El Nino
- Upwelling: cold, nutrient-rich water rises along the west coasts of continents (Peru, California, Canary, Benguela), making rich fisheries. The Oyashio is the usual odd one out in such lists.
- El Nino: abnormal warming of the central and eastern equatorial Pacific with weak trade winds. It suppresses the Peru upwelling, cuts the anchovy catch and is often linked with a weak Indian monsoon.
- La Nina: the opposite cool phase, often linked with a good monsoon.
Effects of currents
- Climate: warm currents bring rain and mild winters (the North Atlantic Drift keeps Norway’s coast ice free). Cold currents bring aridity and fog, forming the Atacama and Namib deserts.
- Fisheries: warm-cold mixing zones are the world’s best fishing grounds (Grand Banks, Newfoundland, and north-west Pacific).
- Navigation: currents save fuel but carry icebergs (Labrador) and fog.
Marine Deposits: Terrigenous and Pelagic
| Class | Source and location | Examples |
|---|---|---|
| Terrigenous (littoral and shallow water) | Land-derived, on the shelf and slope | Gravel, sand, silt, clay; blue mud, green mud (glauconite), red mud, volcanic mud, coral mud |
| Pelagic (deep water) | Remains of marine organisms and fine clay, deep basins | Calcareous oozes (globigerina, pteropod), siliceous oozes (diatom, radiolarian), red clay |
- Blue mud is the most widespread terrigenous deposit and red clay the most widespread pelagic deposit.
- Calcareous oozes dissolve below a certain depth (the carbonate compensation depth, about 4,000-5,000 m), so the deepest floors have red clay or siliceous ooze.
- Seabed resources: polymetallic nodules (manganese, nickel, cobalt, copper) lie on the abyssal plains. India holds a 75,000 sq km site in the Central Indian Ocean Basin, allotted by the International Seabed Authority.
Coral Reefs Notes: Conditions, Types and Theories
Conditions for growth
Reef-building corals are small animals (polyps) that live in symbiosis with algae called zooxanthellae. They need:
- Warm water: tropical seas, with the textbook lower limit about 20°C and best growth around 23-29°C.
- Shallow, sunlit water: usually less than about 50-70 m, because the algae need light.
- Clear, sediment-free water: silt clogs the polyps.
- Normal salinity: about 27-40‰. Fresh water and low salinity hinder growth, and a salinity of 6% is wrong. CAPF 2017
- A firm platform to attach to, and more growth on the seaward side with food-rich clear water.
Reefs occur between about 30°N and 30°S, mostly on the eastern margins of continents where warm currents flow, while cold currents off the western margins discourage them.
Types of reefs and theories
| Type | Description | Example |
|---|---|---|
| Fringing reef | Attached to the coast or island shore | Reefs of the Andaman islands |
| Barrier reef | Separated from land by a lagoon | Great Barrier Reef, Queensland, over 2,000 km long |
| Atoll | Ring-shaped reef around a lagoon | Lakshadweep islands |
- Darwin’s subsidence theory (1842): a fringing reef becomes a barrier reef and then an atoll as a volcanic island slowly sinks and the coral grows upward.
- Daly’s glacial control theory: sea-level changes during the ice age shaped reefs.
IMAGE (Diagram): Darwin’s subsidence theory showing fringing reef, barrier reef and atoll stages
Labels: Volcanic island, Fringing reef, Lagoon, Barrier reef, Atoll, Sea level, Subsidence
Caption: A subsiding island turns a fringing reef into a barrier reef and then an atoll.
Find it: search Darwin subsidence theory atoll formation diagram; Wikimedia Commons
Coral reefs in India and threats
- India has four main reef regions: Gulf of Mannar, Gulf of Kutch, Lakshadweep, and the Andaman and Nicobar Islands.
- The muddy Gulf of Khambhat has no reefs because of turbid, silt-laden water. CAPF 2018 CDS 2019
- Coral bleaching occurs when stressed corals expel their zooxanthellae. Marine heat waves and ocean acidification are major threats, and mild bleaching can reverse if the stress passes.
Islands: Continental, Oceanic and Indian Islands
| Type | Description | Examples |
|---|---|---|
| Continental islands | On the continental shelf, geologically part of the continent | Great Britain, Sri Lanka, Newfoundland |
| Volcanic oceanic islands | Rise from the deep sea floor | Hawaii, Iceland |
| Coral islands | Built by coral on a platform | Lakshadweep, Maldives |
- Greenland is the largest island in the world. Cyprus lies in the Mediterranean, Madagascar in the Indian Ocean, the Falklands in the South Atlantic and the Chagos group in the central Indian Ocean. CAPF 2017
- From west to east the Indonesian islands run Sumatra, Java, Bali, Lombok.
- The Andaman and Nicobar Islands in the Bay of Bengal are peaks of a submerged mountain chain, and Lakshadweep in the Arabian Sea is a group of coral atolls.
Mains Analysis: Oceanography Concepts for GS Answers
- Ocean currents and climate: explain the role of warm and cold currents in rainfall, deserts, fog and fisheries, with Atacama, Namib and Grand Banks as examples.
- Indian monsoon: link El Nino, La Nina, the Indian Ocean Dipole and sea surface temperature to monsoon rainfall and to cyclone intensity in the Bay of Bengal.
- Tides and ports: why tidal range matters for ports, dredging, tidal energy and storm surge risk.
- Tsunami preparedness: early warning (INCOIS), community readiness, coastal zone regulation and bio-shields such as mangroves.
- Coral reefs: importance for biodiversity, fisheries, tourism and shoreline protection, and threats from warming, acidification, pollution and sedimentation.
- Blue Economy: India’s EEZ of about 2 million sq km, the Deep Ocean Mission (approved in 2021 with an outlay of about ₹4,077 crore), seabed nodules, fisheries and offshore energy.
Warm-cold current questions are the commonest in prelims. Learn the table above by ocean, and remember the exceptions: Agulhas is warm and South Atlantic Drift is cold.
Oceanography in Odisha: Tides, Currents and Coast
- East India Coastal Current: it flows along the Odisha coast, northward for roughly February to September and reversing southward in the north-east monsoon, so the currents off Odisha follow the monsoon cycle.
- Cyclones and surge: the 1999 Super Cyclone crossed the coast near Paradip on 29 October 1999. A surge arriving at spring high tide gives the worst flooding, so tide tables matter in cyclone warnings.
- Tides and mangroves: tidal creeks of the Mahanadi, Brahmani and Baitarani delta feed the mangroves of Bhitarkanika, which also protect the coast.
- Chilika: exchange of tidal water through the lagoon mouth sets the salinity gradient of Asia’s largest brackish water lagoon. The inlet was widened in 2000 to restore tidal flow.
- Tsunami readiness: in 2020 two Odisha villages, Venkatraipur in Ganjam and Noliasahi in Jagatsinghpur, were recognised under the UNESCO-IOC Tsunami Ready programme.
- Reefs and islands: the turbid, river-fed coastal water of Odisha is unfavourable for coral reefs, which is why India’s reef regions lie elsewhere. Dr. A. P. J. Abdul Kalam Island (formerly Wheeler Island) off Bhadrak and Kalijai Island in Chilika are the best-known Odisha islands.
Oceanography Practice Questions and PYQs
What explains the eastward flow of the equatorial counter-current?
- The Earth’s rotation on its axis
- Convergence of the two equatorial currents
- Difference in salinity of water
- Occurrence of the belt of calm near the equator
Show answer
(b) - the two equatorial currents pile water in the west, and the excess flows back east. UPSC 2015
Tides occur in the oceans and seas due to which among the following? 1. Gravitational force of the Sun 2. Gravitational force of the Moon 3. Centrifugal force of the Earth
- 1 only
- 1 and 2 only
- 2 and 3 only
- 1, 2 and 3
Show answer
(d) - the gravity of both bodies and the centrifugal effect of the Earth-Moon system act together. UPSC 2015
Which one of the following statements is not correct?
- Gulf with narrow fronts and wider rears experience high tides.
- Tidal currents take place when a Gulf is connected with the open sea by a narrow channel.
- Tidal bore occurs when a tide enters the narrow and shallow estuary of a river.
- The tidal nature of the mouth of the river Hooghly is of crucial importance to Kolkata as port.
Show answer
(a) - high tides build up in gulfs with wide fronts and narrow rears. UPSC 2004
Which one among the following is not correct regarding Tsunami?
- Tsunamis have a very long wavelength.
- Tsunamis have a slight swell of about twelve inches above the normal sea surface.
- Tsunamis grow in height when they reach shallower water.
- Tides also play an important role in the generation of Tsunamis.
Show answer
(d) - tsunamis are generated by seismic or volcanic events and landslides, not by tides. CDS 2012
Which of the following are warm ocean currents?
- Kuroshio and California Current
- North Atlantic Drift and Brazil
- Canaries and Benguela Current
- West Wind Drift and Falkland Current
Show answer
(b) - both North Atlantic Drift and Brazil are warm, while California, Canaries, Benguela, West Wind Drift and Falkland are cold. CDS 2020
Which of the following theories of the origin of tides is not correctly matched?
- Equilibrium Theory - Newton
- Canal Theory - Airy
- Stationary Wave Theory - William Whewell
- Dynamic Theory - Laplace
Show answer
(c) - the stationary wave theory is associated with R. A. Harris, and Whewell with the progressive wave theory.
Which one of the following sites does not have coral reef formation?
- Gulf of Cambay/Khambhat
- Gulf of Mannar
- Lakshadweep
- Andaman and Nicobar Islands
Show answer
(a) - turbid, silt-laden water prevents coral growth in the Gulf of Khambhat. CAPF 2018
Which one of the following pairs of island and sea/ocean is not correctly matched?
- Cyprus - Mediterranean Sea
- Falkland - Atlantic Ocean
- Chagos - North Pacific Ocean
- Islas Cocos - Indian Ocean
Show answer
(c) - the Chagos archipelago lies in the central Indian Ocean. CAPF 2017