BharatNotes Why this chapter matters for UPSC: This is Chapter 13 in the current rationalised NCERT Fundamentals of Physical Geography (Reprint 2026-27); it was Chapter 14 in pre-2023 editions. It covers waves, tides and ocean currents. Mains GS1 has asked directly on the forces behind ocean currents and their effects twice (2015 and 2022), and Prelims has asked on the equatorial counter-current (2015) and on fishing grounds where currents meet (2013).
Contemporary hook: The IPCC Sixth Assessment Report (WGI, 2021) assessed that the Atlantic Meridional Overturning Circulation (AMOC), the Atlantic branch of the deep "conveyor" circulation described below, is very likely to weaken over the 21st century. It gave medium confidence that the AMOC will not collapse abruptly before 2100.
🧠 First Principles — Read This First
Ocean water moves in two ways: horizontally and vertically. Waves and currents are horizontal movements. Tides are a vertical one, a rise and fall of the sea surface. The upwelling of cold water from below and the sinking of surface water are also vertical movements.
A wave carries energy, not water. Watch a leaf on a pond when you throw in a stone: the ripples travel outward but the leaf only bobs up and down. In the sea, wind supplies the energy. Each water particle moves in a small circle as the wave passes, and the energy is released when the wave breaks on the shore.
A tide is the regular rise and fall of the sea level, once or twice a day. It is caused mainly by the Moon's gravitational pull, to a lesser extent the Sun's, together with the centrifugal force of the Earth–Moon system. Because the positions of the Earth, Moon and Sun are known exactly, tides can be predicted well in advance.
A current is like a river within the ocean: a large volume of water moving steadily along a definite path. Currents are set going by solar heating, wind, gravity and the Coriolis force. Differences in density (cold or salty water is heavier) drive the slow deep circulation.
Currents move heat. Warm currents carry tropical heat towards the poles and cold currents bring cooler water towards the tropics, which changes the climate of the coasts they pass. UPSC asks what drives currents and what they do to climate, fishing and navigation.
PART 1 — Quick Reference
Table 1: Characteristics of waves (NCERT)
| Term | NCERT definition |
|---|---|
| Crest and trough | Highest and lowest points of a wave |
| Wave height | Vertical distance from the bottom of a trough to the top of a crest |
| Wave amplitude | One-half of the wave height |
| Wave period | Time between two successive crests or troughs passing a fixed point |
| Wavelength | Horizontal distance between two successive crests |
| Wave speed | Rate at which the wave moves through water, measured in knots |
| Wave frequency | Number of waves passing a point in one second |
Source: NCERT, Fundamentals of Physical Geography, Class XI, Ch. 13, p. 109, Reprint 2026-27.
Table 2: Wave behaviour (NCERT)
| Point | NCERT statement |
|---|---|
| Source of energy | Wind; most waves are caused by wind driving against water |
| What moves | Energy moves forward; water particles travel in a small circle |
| Depth affected | Surface motion seldom affects the stagnant deep water |
| Formation | A breeze of 2 knots or less forms ripples on calm water; they grow until white caps appear |
| Maximum height | Set by wind strength, how long it blows, and the area over which it blows in one direction |
| Largest waves | In the open ocean |
| Steep waves | Young, probably from local wind |
| Slow, steady waves | From far away, possibly another hemisphere |
| Approach to shore | The wave slows because of friction with the sea floor, and breaks when water depth is less than half the wavelength |
| Mechanism | Wind pushes the water; gravity pulls crests down; falling water pushes former troughs up |
Table 3: Types of tides (NCERT)
| Basis | Type | NCERT description |
|---|---|---|
| Frequency | Semi-diurnal | Most common; two high and two low tides a day, of about equal height |
| Frequency | Diurnal | One high and one low tide a day, of about equal height |
| Frequency | Mixed | Tides varying in height; west coast of North America and many Pacific islands |
| Sun–Moon–Earth position | Spring tide | Sun, Moon and Earth in a straight line; higher tides; twice a month, at full moon and new moon |
| Sun–Moon–Earth position | Neap tide | Sun and Moon at right angles; their forces counteract; usually seven days after a spring tide |
| Moon's distance | Perigee (Moon closest, once a month) | Unusually high and low tides; greater tidal range |
| Moon's distance | Apogee (Moon farthest, two weeks later) | Tidal range less than average |
| Earth's distance from Sun | Perihelion (~3 January) | Much greater tidal range |
| Earth's distance from Sun | Aphelion (~4 July) | Much smaller tidal range |
| Term | NCERT definition |
|---|---|
| Ebb | Time between high tide and low tide, when water level is falling |
| Flow or flood | Time between low tide and high tide, when water is rising |
| Surge | Water movement caused by meteorological effects (winds, pressure changes); not regular like tides |
| Tidal current | Tide channelled between islands or into bays and estuaries |
| Highest tides | Bay of Fundy, Nova Scotia, Canada; tidal bulge 15–16 m |
Source: NCERT Ch. 13, pp. 109–110, Reprint 2026-27.
Table 4: Forces behind ocean currents (NCERT)
| Force | Type | How it acts |
|---|---|---|
| Heating by solar energy | Primary | Water expands; near the equator sea level is about 8 cm higher than in middle latitudes, so water flows down this slight slope |
| Wind | Primary | Friction between wind and water surface pushes water along |
| Gravity | Primary | Pulls water down the "pile" and creates gradient variation |
| Coriolis force | Primary | Deflects water to the right in the NH and to the left in the SH; the resulting large circular flows are gyres |
| Density differences | Affects vertical movement | Salty water is denser than fresh, cold denser than warm; denser water sinks |
| Secondary forces | Influence the flow | NCERT does not list them in detail |
Source: NCERT Ch. 13, p. 111, Reprint 2026-27.
Table 5: Types of ocean currents (NCERT)
| Basis | Type | NCERT description |
|---|---|---|
| Depth | Surface currents | About 10% of ocean water; the upper 400 m |
| Depth | Deep water currents | The other 90%; moved by density and gravity; sink at high latitudes where cold raises density |
| Temperature | Cold currents | Bring cold water into warm areas. Found on west coasts of continents in low and middle latitudes (both hemispheres), and on east coasts in higher latitudes of the NH |
| Temperature | Warm currents | Bring warm water into cold areas. Found on east coasts of continents in low and middle latitudes (both hemispheres), and on west coasts in high latitudes of the NH |
| Speed ("drift") | — | Strongest at the surface, sometimes over 5 knots; at depth usually under 0.5 knots; most currents 5 knots or less |
Table 6: Major currents (NCERT Fig. 13.3)
| Current | Ocean | Warm or cold | Coast it affects |
|---|---|---|---|
| Gulf Stream and North Atlantic Drift | North Atlantic | Warm | East coast of North America; west coast of Europe (NCERT) |
| Labrador Current | North Atlantic | Cold | North-east coast of North America (NCERT) |
| Canary Current | North Atlantic | Cold | North-west Africa |
| Benguela Current | South Atlantic | Cold | South-west Africa |
| Brazil Current | South Atlantic | Warm | East coast of South America |
| Kuroshio Current | North Pacific | Warm | Japan |
| Oyashio Current | North Pacific | Cold | North-east Asia |
| California Current | North Pacific | Cold | West coast of North America |
| Peru (Humboldt) Current | South Pacific | Cold | West coast of South America |
| East Australian Current | South Pacific | Warm | East coast of Australia |
| Agulhas Current | Indian Ocean | Warm | South-east Africa |
| West Australian Current | Indian Ocean | Cold | West coast of Australia |
| North and South Equatorial Currents | All three oceans | Warm | Flow westward in the trade-wind belts |
| Equatorial Counter Current | Pacific, Atlantic, Indian | Warm | Flows eastward between the equatorial currents |
| West Wind Drift (Antarctic Circumpolar) | Southern Ocean | Cold | Flows eastward around Antarctica |
The Gulf Stream and Labrador Current examples come from NCERT's text (Ch. 12 p. 103). The other rows give only the names and warm or cold type of the major currents shown on standard current maps such as NCERT's Figure 13.3.
PART 2 — Concepts & Narrative
Waves
Waves are energy moving through water (Tables 1 and 2). Wind supplies the energy. Gravity then pulls each crest back down, the falling water lifts the trough ahead, and so the wave form advances while the water itself goes round in a circle. A floating object is carried up and forward as the wave arrives, then down and back as it passes.
As a wave reaches shallow water, friction with the sea floor slows it. NCERT says it breaks when the depth becomes less than half the wavelength. In physics terms, that depth is where a wave first begins to "feel" the bottom and steepen, and breaking follows as it moves into still shallower water. For the exam, use NCERT's wording.
Wave height vs amplitude. Wave height is the full vertical distance from trough bottom to crest top. Amplitude is half the wave height. A wave 2 m high has an amplitude of 1 m.
Tsunami (beyond NCERT). A tsunami is not a wind wave. Most are set off by vertical movement of the sea floor in a large submarine earthquake. NOAA notes that tsunamis move through the entire depth of the ocean, unlike wind waves, which affect only the surface. In deep water they can travel faster than 800 km/h with wavelengths of hundreds of kilometres, while rarely rising more than about a metre. Near land they slow to about 30–50 km/h, shorten and grow in height (NOAA JetStream, "Tsunami Propagation"). The 26 December 2004 tsunami followed a magnitude 9.1 earthquake on the megathrust where the India Plate descends beneath the Burma micro-plate at the Sunda Trench, with local run-ups of nearly 32 m (USGS). In India, the Home Minister told Parliament in 2005 that 10,273 people had died and 5,823 were missing and feared dead, most of the missing from the Nicobar Islands (PIB, MHA statement, 2005). That statement used the magnitude estimate then current, 8.6. Mains GS3 2017 (gs3-pyq-2017-38) asked about this event.
Tides
The Moon's gravity is the main cause of tides and the Sun's is secondary. NCERT says the Moon's attraction is "more than twice as strong as the sun's". The other force is the centrifugal force of the Earth–Moon system.
Why there are two tidal bulges
The tide-generating force is the difference between the Moon's gravitational attraction and the centrifugal force.
- On the side facing the Moon, the Moon's pull is greater than the centrifugal force, so the net force makes a bulge towards the Moon.
- On the opposite side, the Moon is farther away and its pull is weaker, so the centrifugal force dominates and the net force makes a second bulge away from the Moon.
As the Earth rotates, a coastal point passes through both bulges, giving most places two high tides and two low tides a day. NCERT adds that the horizontal tide-generating forces matter more than the vertical ones in building the bulges.
Local modifications (NCERT). Tidal bulges are higher on wide continental shelves and lower at mid-ocean islands. Funnel-shaped bays and estuaries greatly increase tidal range. This is why the Bay of Fundy has the world's highest tides, with a bulge of 15–16 m. NCERT works through the arithmetic: with two high and two low tides a day, the tide comes in within about six hours, rising roughly 240 cm an hour.
How big a tide is. This depends on the relative positions and distances of the Sun, Moon and Earth (Table 3). Spring tides come at full moon and new moon, when the three bodies are in line. Neap tides come at the quarter moons, when the Sun and Moon are at right angles. Tides are also larger when the Moon is at perigee and the Earth at perihelion (around 3 January), and smaller at apogee and aphelion (around 4 July).
Perigee/apogee vs perihelion/aphelion. Perigee and apogee describe the Moon's distance from the Earth (once a month each). Perihelion and aphelion describe the Earth's distance from the Sun (once a year each). NCERT's exercise asks both: the Moon is closest at perigee, and the Earth reaches perihelion in January.
Importance of tides (NCERT).
- Navigation: tides are predictable, so sailors and fishermen can plan around them. Tidal height decides when ships can cross the shallow "bars" at harbours near rivers and in estuaries.
- Estuaries: tides help remove silt and flush polluted water from river estuaries.
- Power: tides are used to generate electricity in Canada, France, Russia and China. NCERT says a 3 MW tidal project at Durgaduani in the Sundarbans (West Bengal) "is under way". This page could not verify that project's current status, so treat NCERT's line as the textbook position only.
Beyond NCERT, the Parliamentary Standing Committee on Energy (2020-21; report tabled August 2021) recorded India's estimated potential as 12,455 MW for tidal power and 41,300 MW for wave power. It recommended a pilot tidal project at a cost-effective site such as the Gulf of Kutch (Lok Sabha Secretariat report, summarised by India Environment Portal).
Ocean currents
NCERT describes currents as "like river flow in oceans". It separates the primary forces that start the water moving from the secondary forces that influence the flow (Table 4).
From forces to gyres
- Solar heating makes equatorial water expand, so the sea surface near the equator stands about 8 cm higher than in middle latitudes.
- Gravity pulls water down that slight slope.
- Wind drags the surface water. The trade winds drive the equatorial currents westward, and the westerlies drive water eastward in middle latitudes.
- The Coriolis force turns moving water to the right in the NH and to the left in the SH.
The result is a set of large circular flows, the gyres, in each ocean basin. NCERT notes that ocean circulation roughly follows the atmospheric circulation. In the middle latitudes the air circulation over the oceans is mainly anticyclonic, more so in the Southern Hemisphere, and the gyres follow it. At higher latitudes, where the wind flow is mostly cyclonic, the ocean follows that pattern. In monsoon regions, the monsoon winds steer the currents.
Density-driven (deep) circulation. Cold water and salty water are dense and sink. NCERT says deep waters sink into the ocean basins at high latitudes and that cold-water currents form when cold polar water sinks and moves slowly towards the equator, while warm surface currents flow poleward to replace it.
The global conveyor belt and the AMOC
NOAA describes a global "conveyor belt" formed by deep thermohaline currents (driven by temperature and salinity) together with wind-driven surface currents. It begins in the North Atlantic near the pole. NOAA's ocean facts page places the start in the Norwegian Sea, where water cools after giving up its heat to the atmosphere. The water also becomes saltier, because salt is left behind when sea ice forms. The cold, salty, dense water sinks and flows south to Antarctica, where it is "recharged". Branches then turn north into the Indian and Pacific Oceans, warm, rise and loop back to the North Atlantic. NOAA estimates that a parcel of water can take 1,000 years to complete the circuit, and the conveyor moves at a few centimetres per second.
The AMOC is the Atlantic part of this circulation; it is not another name for the whole conveyor. IPCC AR6 (WGI SPM, C.3.4, 2021) says the AMOC is very likely to weaken over the 21st century under all emissions scenarios. It has high confidence in that decline but low confidence in its size, and medium confidence that there will be no abrupt collapse before 2100.
Sources: NOAA National Ocean Service, "What is the global ocean conveyor belt?" and Currents Tutorial; IPCC AR6 WGI SPM.
Effects of ocean currents
NCERT draws these effects:
- West coasts in tropical and subtropical latitudes (except close to the equator) are bordered by cool water. Average temperatures are relatively low, diurnal and annual ranges are narrow, and there is fog, but the areas are generally arid. Examples are the Atacama coast beside the Peru Current and the Namib coast beside the Benguela Current. Mains GS1 2013 (gs1-pyq-2013-21) asked why the major hot deserts of the NH lie on the western sides of continents.
- West coasts in middle and higher latitudes are bordered by warm water and have a marine climate: cool summers, relatively mild winters and a narrow annual range. North-west Europe, warmed by the Gulf Stream and North Atlantic Drift, is NCERT's example (exercise 3.i).
- East coasts in tropical and subtropical latitudes have warm currents alongside, giving warm and rainy climates. These coasts lie on the western margins of the subtropical anticyclones.
- Fishing: where warm and cold currents mix, oxygen is replenished and plankton, the primary food of fish, grows well. The best fishing grounds of the world are mainly in these mixing zones. The Grand Banks off Newfoundland, where the Gulf Stream meets the Labrador Current, is the standard example.
Upwelling, fishing and El Niño (NCERT plus NOAA)
- Upwelling: NCERT explains that offshore winds drive warm surface water away from the coast and cold water rises from below. NOAA adds that this water is cold and nutrient-rich, so good fishing grounds are typically found where upwelling is common. The cold current does not cause the upwelling; the wind does.
- El Niño: in normal years the trade winds push warm water west along the equator, away from South America, and cold water upwells off the Americas. During El Niño the trade winds weaken, warm water moves back east, and upwelling weakens or stops. With fewer nutrients there is less phytoplankton, which affects the fish that feed on it (NOAA, "What are El Niño and La Niña?").
- Monsoon reversal: NCERT notes that in regions of pronounced monsoonal flow, the monsoon winds influence current movements. In the northern Indian Ocean, currents such as the Somali Current reverse direction between the south-west and north-east monsoons.
PART 3 — UPSC Integration
PYQ pattern (from the question banks)
- Mains GS1 2015 (gs1-pyq-2015-14, 12.5 marks): "Explain the factors responsible for the origin of ocean currents. How do they influence regional climates, fishing and navigation?"
- Mains GS1 2022 (gs1-pyq-2022-16, 15 marks): "What forces influence ocean currents? Describe their role in world fishing industry."
- Mains GS1 2019 (gs1-pyq-2019-04, 15 marks): "How do ocean currents and water masses differ in their impacts on marine life and coastal environment? Give suitable examples."
- Mains GS1 2013 (gs1-pyq-2013-21, 10 marks): why the major hot deserts of the NH lie between 20° and 30°N on the western sides of continents. Cold currents are one part of the answer.
- Mains GS3 2017 (gs3-pyq-2017-38, 15 marks): the December 2004 tsunami, its causes, its effects, and preparedness under the NDMA guidelines (2010).
- Prelims 2015 (bank id geo-016): what explains the eastward flow of the equatorial counter-current. Answer: the belt of calm near the equator.
- Prelims 2013 (bank id geo-028): important fishing grounds are found where warm and cold ocean currents meet.
The two 2015 and 2022 Mains questions are answered almost directly by NCERT's primary forces (Table 4) and its effects section.
Mains frameworks
- "Forces influencing currents": primary (solar heating with the 8 cm slope, wind, gravity, Coriolis) → density and the deep circulation → modifiers (coastline shape, monsoon reversal) → gyres as the result.
- "Role in climate, fishing and navigation": climate (arid tropical west coasts, mild marine climates on mid-latitude west coasts, rainy tropical east coasts) → fishing (mixing zones, upwelling, El Niño collapse) → navigation (currents and tides used for routes and harbour access; fog where warm and cold water meet).
- Tides: causes (gravity, centrifugal force) → types → importance (navigation, desilting, power) → India (Durgaduani in NCERT; the Standing Committee's potential estimate).
Cross-paper relevance
- GS1 (Physical geography): waves, tides, currents and their effects (2013, 2015, 2019, 2022).
- GS3 (Disaster management): tsunami and storm surge, including the 2004 tsunami (GS3 2017). GS3 (Environment, Energy): AMOC weakening under climate change; tidal and wave energy potential.
Exam Strategy
Prelims fact-traps:
- Amplitude is half the wave height, not equal to it.
- Wave speed is measured in knots, and wave frequency is waves per second.
- Tides are vertical motion; currents and waves are horizontal (NCERT exercise 1.i: "upward and downward movement" is the tide).
- Spring tides happen twice a month (full moon and new moon), and neap tides about seven days later. "Spring" has nothing to do with the season.
- Surges come from wind and pressure, and are not regular like tides.
- Mixed tides: west coast of North America and many Pacific islands.
- Perigee (Moon closest) vs perihelion (Earth closest to the Sun, ~3 January).
- Primary forces are solar heating, wind, gravity and Coriolis. Density mainly affects vertical movement.
- Surface currents are 10% of ocean water, in the upper 400 m.
- Cold currents lie on west coasts in low and middle latitudes and on east coasts in high NH latitudes, so the Labrador Current runs along the north-east coast of North America. A rule that puts all cold currents on west coasts is wrong.
- East coasts with warm currents are warm and rainy in tropical and subtropical latitudes (NCERT's scope). Do not extend the rule to all latitudes.
- Equatorial counter-current flows east because of the belt of calm (Prelims 2015).
Mains patterns: "What forces…", "Explain the factors… How do they influence…" and "How do X and Y differ…". Answer with NCERT's forces and effects, illustrated with two or three named currents and one Indian-Ocean example.
Practice Questions
- Mains GS1 2022 (gs1-pyq-2022-16): What forces influence ocean currents? Describe their role in world fishing industry. (15 marks)
- Mains GS1 2015 (gs1-pyq-2015-14): Explain the factors responsible for the origin of ocean currents. How do they influence regional climates, fishing and navigation? (12.5 marks)
- Prelims 2015 (bank id geo-016): What explains the eastward flow of the equatorial counter-current? (a) The Earth's rotation on its axis (b) Convergence of the two equatorial currents (c) Difference in salinity of water (d) Occurrence of the belt of calm near the equator. Answer: (d).
- Prelims 2013 (bank id geo-028): The important fishing grounds are found in the regions where: (a) Warm and cold ocean currents meet (b) Rivers drain into the sea (c) The continental shelf is deep (d) Ocean floor is flat and even. Answer: (a).
- Practice (UPSC-pattern, not a past paper): Consider the following statements: (1) Wave amplitude is one-half of the wave height. (2) Tidal ranges are greater than average when the Earth is at aphelion. (3) Surges are regular, like tides. Which of the statements given above is/are correct? (a) 1 only (b) 1 and 2 only (c) 2 and 3 only (d) 1, 2 and 3. Answer: (a). Ranges are greater at perihelion (~3 January), and surges are irregular.
📦 Revision Capsule
Hard Facts
- Horizontal motion: currents and waves; vertical motion: tides (also upwelling and sinking)
- Wave amplitude is half the wave height; wave speed in knots; frequency in waves per second
- A wave breaks when water depth is less than half its wavelength (NCERT)
- Spring tides twice a month (full and new moon); neap tides ~7 days later (Sun and Moon at right angles)
- Perigee: unusually high and low tides; perihelion (~3 January): greater range; aphelion (~4 July): smaller range
- Bay of Fundy: highest tides, 15–16 m bulge (NCERT)
- Mixed tides: west coast of North America and Pacific islands
- Primary forces of currents: solar heating (equator ~8 cm higher), wind, gravity, Coriolis
- Surface currents: ~10% of ocean water, upper 400 m; deep currents: 90%
- Cold currents: west coasts in low and middle latitudes, east coasts in high NH latitudes
- Best fishing grounds: mixing zones of warm and cold currents (NCERT)
- Conveyor circuit ~1,000 years (NOAA); AMOC very likely to weaken this century (IPCC AR6, 2021)
- NCERT's tidal example for India: 3 MW Durgaduani project, Sundarbans
Core Concepts
- Waves move energy, not water; particles move in circles
- Two tidal bulges come from the balance of the Moon's gravity and centrifugal force
- Solar heating, wind, gravity and Coriolis together make the basin-wide gyres
- Currents move heat from low to high latitudes, much as the atmosphere does
- Upwelling is driven by offshore winds and makes cold, nutrient-rich, productive water; El Niño weakens it
Confused Pairs
- Wave height (trough to crest) vs amplitude (half of height)
- Perigee/apogee (Moon–Earth) vs perihelion/aphelion (Earth–Sun)
- Tide (regular, astronomical) vs surge (irregular, meteorological)
- Spring tide (aligned, larger range) vs neap tide (right angles, smaller range)
PYQ Pattern
- Mains GS1: forces and effects of currents (2015, 2022); currents vs water masses (2019); west-coast deserts (2013); GS3 2017 tsunami
- Prelims: equatorial counter-current (2015); fishing grounds at current meeting zones (2013)
Sources
Sources
- NCERT, Fundamentals of Physical Geography, Class XI, Chapter 13 "Movements of Ocean Water", Reprint 2026-27: kegy213.pdf; Chapter 12 for the temperature factors: kegy212.pdf
- NOAA National Ocean Service, "What is the global ocean conveyor belt?": oceanservice.noaa.gov; Currents Tutorial, "The Global Conveyor Belt": oceanservice.noaa.gov
- NOAA National Ocean Service, "What is upwelling?": oceanservice.noaa.gov; "What are El Niño and La Niña?": oceanservice.noaa.gov
- NOAA JetStream, "Tsunami Propagation": noaa.gov
- USGS Pacific Coastal and Marine Science Center, "Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake": usgs.gov
- PIB, Home Minister's statement in Parliament on tsunami relief and rehabilitation (2005): pib.gov.in
- IPCC AR6 WGI Summary for Policymakers (2021), C.3.4: ipcc.ch
- Lok Sabha Secretariat, Standing Committee on Energy (2020-21), "Tidal power development in India", summary on India Environment Portal (4 August 2021): indiaenvironmentportal.org.in
