Movements of Ocean Water: Waves, Tides and Currents

1. Waves: energy on the move

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• Ocean water moves in two ways (Class 11, Movements of Ocean Water). It moves horizontally as currents and waves. It moves vertically as tides, upwelling and sinking.
• A wave is energy moving across the sea, not water. Wind supplies the energy, and gravity pulls the crests back down.
• Orbital motion of water particles: under a wave, water moves in small circles. It goes up and forward as the crest comes, then down and back as it passes. A floating object only bobs in place. The motion dies out with depth, so the deep water stays still.
• How waves grow: a breeze of 2 knots or less makes ripples. Stronger wind builds bigger waves until white caps form.
• Wave height (trough to crest) depends on wind strength, how long it blows, and fetch (the stretch of open water the wind blows over in one direction).

• So the biggest waves form in open oceans such as the Southern Ocean.

• Swell: long, slow, regular waves from distant storms, sometimes in another hemisphere. Steep, short waves are young waves raised by local wind.

• Wave parts: crest (top) and trough (bottom); amplitude (half the height); wavelength (crest to crest); period (time between two crests passing one point); speed (in knots); frequency (waves passing a point per second).
• Breaking of waves: near the shore, friction with the floor slows the wave. When the depth falls below about half the wavelength, the wave steepens and breaks as surf.
• Beyond NCERT:
• Longshore drift: waves that hit the beach at an angle carry sand along the coast. The swash runs up at an angle and the backwash runs straight down. Breakwaters trap this sand: Chennai harbour built up Marina beach and caused erosion to its north.
• Kallakkadal ("sea thief" in Malayalam) is sudden flooding of the Kerala coast. Long swells from far-off Southern Ocean storms cause it, with no local warning. INCOIS started its Swell Surge Forecast System in 2020.
• Tsunamis are covered in natural-hazards-disasters.

2. Tides: why the sea rises and falls

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• A tide is the regular rise and fall of sea level, once or twice a day. It is caused mainly by the Moon's pull and, to a lesser extent, the Sun's. Surges driven by wind and air pressure are irregular, so they are not tides.
• Tide-generating force: the difference between the Moon's pull and centrifugal force (the outward force that balances gravity).
• On the side facing the Moon, the Moon's pull is stronger, so water bulges towards the Moon.
• On the far side, the pull is weaker and centrifugal force wins, so a second bulge forms away from the Moon.

• The horizontal part of this force does more to pile up the water than the vertical part.

• Tidal bulge: as the Earth turns, each place passes through both bulges, so it gets two high and two low tides. Beyond NCERT, the full cycle takes about 24 hours 50 minutes, so high tide comes about 50 minutes later each day.

• Tidal range (the gap between high and low water): it is larger over wide continental shelves and in funnel-shaped bays, and small at mid-ocean islands.
• Tidal current: the flow of a tide squeezed between islands or into bays and estuaries.
• Bay of Fundy (between Nova Scotia and New Brunswick, Canada): the world's highest tides, a 15–16 m bulge. The water rises about 240 cm an hour.
• India (beyond NCERT): the Gulf of Khambhat has the largest range, about 10–11 m at its head (verify current). The Gulf of Kachchh comes next. East-coast ranges are small except in the Hooghly–Sundarbans.
• Tidal bore: a wall of water rushing upriver when a strong tide is forced into a narrowing estuary. Examples: the Hooghly (India), the Qiantang (China, the largest), the Severn (UK) and the Amazon's pororoca.

3. Types of tides: by frequency and by the sun-moon-earth positions

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By frequency

• Semi-diurnal tide: two highs and two lows a day, of about equal height. This is the most common pattern, and it is the pattern along most of India's coast.
• Diurnal tide: one high and one low a day, as in parts of the Gulf of Mexico.
• Mixed tide: successive tides differ sharply in height. Found along the west coast of North America and on many Pacific islands.

By position

	Spring tide	Neap tide
Alignment	Sun, Moon and Earth in a straight line	Sun and Moon at right angles to the Earth
Moon phase	New moon and full moon	First and last quarter
Effect	Pulls combine: highest highs, lowest lows	Pulls partly cancel: smallest range
Timing	Twice a month	About 7 days after each spring tide

• "Spring" has nothing to do with the season. The Moon's pull is more than twice the Sun's, so tides still occur at neap.
• Perigee: the Moon at its closest to Earth (about 363,000 km), once a month. Ranges are larger, and a full moon at perigee is a "supermoon". Apogee: two weeks later the Moon is farthest away (about 405,000 km) and ranges are smaller.
• Perihelion: the Earth closest to the Sun (about 3 January), with larger ranges. Aphelion: farthest from the Sun (about 4 July), with smaller ranges.
• A perigee that falls on a spring tide gives "king tides", which flood low coasts.
• Ebb and flood tide: ebb is the falling tide (high to low water). Flow or flood is the rising tide (low to high water).

4. The importance of tides and tidal energy

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• Importance of tides: tides can be predicted well in advance because the Sun–Moon–Earth positions are known. Navigators and fishermen plan around them (Class 11, Movements of Ocean Water).
• Navigation: estuary ports with shallow bars at the entrance can be entered only at high tide. Example: Kolkata (Syama Prasad Mookerjee Port) on the Hooghly, with Haldia downstream for larger ships.
• Desilting: tides flush silt and polluted water out of estuaries.

• Ecology: mangroves and other intertidal life depend on the tides.

• Tidal power:

• Floodgate dams (barrages) across an inlet trap water at high tide.
• The water is released through turbines as the tide falls.

• Plants exist in Canada, France, Russia and China (NCERT). Beyond NCERT: La Rance, France (240 MW, 1966) was the first large plant. Sihwa Lake, South Korea (254 MW, 2011) is the largest.

• India: Class 10, Minerals and Energy Resources names the Gulf of Khambhat, the Gulf of Kachchh and the Sundarbans (Gangetic delta) as ideal sites.

• Durgaduani tidal power project: a 3 MW plant at Durgaduani creek in the Sundarbans, West Bengal (NCERT: "under way"; now: dropped as too costly, verify current). The Kachchh (Mandvi) and Kalpasar (Khambhat) proposals have also not gone ahead.
• India's identified tidal potential is about 12,000 MW, mostly in Khambhat and Kachchh (verify current).
• Barriers: high capital cost, silting, damage to mudflats and mangroves, and few suitable sites. Wave energy and OTEC are covered in energy-resources-india.

5. Ocean currents: forces, types and gyres

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• An ocean current is a river in the sea: a steady flow of water along a set path and direction. Its speed is called its "drift". Currents are fastest at the surface (often up to 5 knots) and slower than 0.5 knots at depth.
• Forces driving ocean currents:
• Primary forces start the flow:
  • Solar heating makes equatorial water expand, so it stands about 8 cm higher than in mid-latitudes and flows down that slope.
  • Wind drags the surface water.
  • Gravity pulls the piled-up water down.
  • The Coriolis force (the turning effect of Earth's rotation) bends moving water to the right in the north and to the left in the south.

• Secondary forces are density differences from temperature and salinity, which drive vertical movement.

• Gyre: a large circular current system around a pile of water in each ocean basin. Gyres turn clockwise in the Northern Hemisphere and anticlockwise in the Southern. Beyond NCERT, there are five subtropical gyres (North Pacific, South Pacific, North Atlantic, South Atlantic, Indian), and they trap floating plastic.

• Ekman transport (beyond NCERT; V.W. Ekman, 1905): the Coriolis effect turns each deeper layer a little more. As a result, the wind-driven layer moves at right angles to the wind (to the right in the north). This pushes water off west coasts and causes upwelling.
• Surface currents: the upper 400 m, about 10% of ocean water. They follow the planetary winds.
• Deep-water currents: the other 90%. Cold, dense water sinks at high latitudes and moves under density and gravity.
• Cold current: carries cold water into warm areas. Found on west coasts in low and middle latitudes, and on east coasts at high northern latitudes.
• Warm current: carries warm water into cold areas. Found on east coasts in low and middle latitudes, and on west coasts at high northern latitudes.

6. The equatorial currents and the West Wind Drift: the system's backbone

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• North Equatorial Current: warm and westward, between about 10° and 20° N. The north-east trades drive it, and it forms the southern limb of the northern gyres in the Pacific and Atlantic.
• South Equatorial Current: warm and westward, just south of the equator in all three oceans. The south-east trades drive it, and it feeds the Brazil, East Australian and Agulhas currents.
• Equatorial Counter Current: flows east between the two equatorial currents, near the doldrums. It carries back the water that piles up on the western sides of the oceans.
• Beyond NCERT:
• The Cromwell Current (Equatorial Undercurrent) flows east beneath the Pacific's South Equatorial Current.

• The western Pacific "warm pool" shifts during El Niño (see pressure-winds-circulation).

• West Wind Drift (Antarctic Circumpolar Current): a cold current flowing east right round Antarctica.

• It is driven by the Roaring Forties, Furious Fifties and Shrieking Sixties, with no land in its way.
• It is the largest current by volume and links the Pacific, Atlantic and Indian Oceans.

• Beyond NCERT, it squeezes through the Drake Passage and keeps Antarctica cold. Its branches turn north as the Humboldt, Benguela, Falkland and West Australian currents.

• Rule of thumb: water flows west along the equator, then poleward as warm currents along continents' east coasts. It turns east with the westerlies at 40–50°, then returns towards the equator as cold currents along west coasts. Every gyre can be rebuilt from this rule.

7. Currents of the Atlantic Ocean

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North Atlantic gyre (clockwise)

• Gulf Stream (warm): leaves the Gulf of Mexico through the Florida Strait and runs north along the US east coast past Cape Hatteras to Newfoundland.
• North Atlantic Drift (warm): the Gulf Stream's extension across the Atlantic.
• It keeps Britain and Norway mild and their ports ice-free (Hammerfest, Narvik, and Murmansk in Russia).

• It bends isotherms (lines of equal temperature) northward and raises North Sea salinity.

• Canary Current (cold): flows south along north-west Africa, from Spain to Senegal. It adds to the dryness of the coastal Sahara and feeds upwelling fisheries.

• Sargasso Sea: a calm, warm sea with no land boundary. It is ringed by the Gulf Stream, North Atlantic Drift, Canary and North Equatorial currents, and covered with sargassum seaweed. Beyond NCERT, eels breed there.
• Labrador Current (cold): flows south from the Arctic past Labrador and Newfoundland. It chills north-east North America and carries icebergs (the Titanic, 1912). Where it meets the Gulf Stream, it causes fog and the Grand Banks fishery.

South Atlantic gyre (anticlockwise)

• Brazil Current (warm): flows south along Brazil as a branch of the South Equatorial Current.
• Falkland Current (cold): flows north along Argentina from the West Wind Drift. It meets the Brazil Current off the Río de la Plata.
• Benguela Current (cold): flows north along south-west Africa. It makes the Namib a foggy coastal desert, and its upwelling feeds fisheries off Namibia and South Africa.

8. Currents of the Pacific Ocean

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North Pacific gyre (clockwise)

• Kuroshio Current ("black current", warm): flows north-east past Taiwan and Japan. It is the Pacific's Gulf Stream.
• North Pacific Drift (warm, slow): carries the Kuroshio east. Near North America it splits into:
• the California Current (cold, southward), which brings San Francisco's summer fog, cool summers, and dryness in Baja California;

• the Alaska Current (warm, northward), which keeps southern Alaskan ports ice-free (beyond NCERT).

• Oyashio Current (cold, nutrient-rich): flows south from the Bering Sea past the Kuril Islands. It meets the Kuroshio near Hokkaido, giving fog and one of the world's richest fisheries.

South Pacific gyre (anticlockwise)

• Humboldt Current (Peru Current, cold): flows north along Chile and Peru.
• Offshore winds cause upwelling, which feeds the anchoveta fishery (among the world's largest single-species catches) and the guano birds.
• It helps make the Atacama the driest non-polar desert.

• During El Niño, warm water replaces it, and plankton and fish collapse.

• East Australian Current (warm): flows south along eastern Australia as a branch of the South Equatorial Current. It bathes the Great Barrier Reef.

Map practice: label both Pacific gyres, the Kuroshio–Oyashio meeting zone off Japan, and the Humboldt upwelling coast. The North Pacific Gyre holds the Great Pacific Garbage Patch.

9. Currents of the Indian Ocean: the monsoon-reversing north and the southern gyre

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• Monsoon currents of the north Indian Ocean: Asia closes the ocean's northern side. So monsoon winds, not steady trades, steer the currents here (Class 11, Movements of Ocean Water).
• Summer (south-west monsoon): the circulation is clockwise, and the South-west Monsoon Current flows east south of India and Sri Lanka.
• Winter (north-east monsoon): the circulation flips to anticlockwise, and the North-east Monsoon Current flows west.

• Beyond NCERT, the East India Coastal Current also reverses.

• Somali Current: flows north in summer as one of the fastest currents and south in winter.

• In summer, strong upwelling off Somalia and Oman cools the Arabian Sea and feeds fisheries.

• Beyond NCERT, monsoon upwelling off Kerala and Karnataka supports the oil sardine and mackerel catch. The catch crashes in warm years.

• Southern gyre (anticlockwise):

• The South Equatorial Current flows west, and part of it turns south through the Mozambique Channel.
• The Agulhas Current (fast, warm) flows south along the east coast of South Africa. It is fed by the Mozambique and South Equatorial currents. Beyond NCERT, "Agulhas leakage" passes warm, salty water into the Atlantic.
• The West Wind Drift carries water east.
• The West Australian Current (cold) flows north along Western Australia. It adds to the dryness of the Great Sandy and Gibson deserts.

10. Effects of ocean currents: climate, fog, upwelling and fishing grounds

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• Effects of ocean currents on climate (Class 11, Movements of Ocean Water):
• Tropical and subtropical west coasts (except near the equator) have cool water offshore. They get low temperatures with narrow ranges, plus fog and dryness. Examples are the coastal deserts: Atacama, Namib, coastal Sahara, Baja California and Western Australia.
• Mid- and high-latitude west coasts have warm water offshore. They get a marine climate of cool summers and mild winters (north-west Europe, British Columbia).
• Tropical and subtropical east coasts have warm currents and a warm, rainy climate. They lie on the western edges of the subtropical highs.

• Like the winds, currents carry heat towards the poles.

• Upwelling: cold, nutrient-rich water rises from below.

• Cause: offshore winds or Ekman transport push surface water away from west coasts (Peru, California, Canary, Benguela), and the monsoon does the same off Somalia–Oman and Kerala.
• Effect: the coast becomes cool, the air stable and foggy, and plankton thrive.

• Upwelling zones cover about 1% of the ocean but give roughly a quarter of the world's fish catch (verify current).

• Plankton: tiny drifting plants (phytoplankton) and animals (zooplankton). They are the main food of fish.

• Fishing grounds at current convergence: where warm and cold currents meet, mixing adds oxygen and helps plankton grow. Examples are the Grand Banks (Gulf Stream and Labrador) and the waters off Japan (Kuroshio and Oyashio). Beyond NCERT, the Dogger Bank in the North Sea.
• Other effects:
• Fog at meeting zones hinders shipping.
• Icebergs drift south on cold currents.
• Old sailing routes followed currents and winds.
• Currents spread oil spills, plastic and larvae.

11. The thermohaline circulation: the deep-ocean conveyor

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• NCERT's starting point: denser water (colder or saltier) sinks and lighter water rises. Cold polar water sinks and creeps towards the equator. Warm surface water flows poleward to replace it.

• Thermohaline circulation (beyond NCERT; "thermo" means heat, "haline" means salt): Broecker's "global conveyor belt". 1. North Atlantic Deep Water forms when the Gulf Stream's salty water cools and sinks in the Greenland, Norwegian and Labrador Seas. 2. Antarctic Bottom Water, the densest water, forms in the Weddell and Ross Seas. When sea ice freezes, it leaves salty brine behind, which sinks (brine rejection). 3. This deep water spreads along the floor into the Indian and Pacific Oceans, rises slowly, and returns at the surface. One full loop takes about 1,000 years.

• Why it matters:

• It moves heat round the globe.
• It brings oxygen to the deep sea.
• It stores carbon in the deep ocean.

• It spreads nutrients.

• AMOC (its Atlantic limb) keeps north-west Europe several degrees warmer than its latitude would suggest. The concept is covered in ocean-relief-temperature-salinity.

• Risk: Greenland meltwater and more high-latitude rain make surface water fresher and lighter, so less of it sinks. This could weaken the circulation, a possible climate tipping point (IPCC AR6; climate-change). A past shutdown is linked to the Younger Dryas cold snap about 12,900 years ago.

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Exam angles

Prelims — high-yield facts and traps

• Waves: waves move energy, not water. They break when depth falls below half the wavelength. Height depends on wind speed, duration and fetch. Amplitude is half the height. Swell has travelled far.
• Tide strength: the Moon's pull is more than twice the Sun's. Spring tides come at new and full moon. Neap tides come at the quarters, about 7 days later.
• Distances: perigee and apogee refer to the Moon. Perihelion (about 3 January) and aphelion (about 4 July) refer to the Earth.

• Trap: "the Moon is closest at perihelion" is FALSE (it is perigee).

• Ebb and flood: ebb is the falling tide; flood (flow) is the rising tide.

• Tide types: semi-diurnal (most common; India), diurnal (Gulf of Mexico), mixed (west coast of North America, Pacific islands).
• Tide records and sites: highest tides at the Bay of Fundy (15–16 m). India's tidal sites are Khambhat, Kachchh and the Sundarbans.
• Warm currents: Gulf Stream, North Atlantic Drift, Kuroshio, Alaska, Brazil, East Australian, Agulhas.
• Cold currents: Labrador, Canary, Benguela, Falkland, California, Oyashio, Humboldt, West Australian, West Wind Drift.
• Meeting zones: Gulf Stream–Labrador (Grand Banks); Kuroshio–Oyashio (Japan); Brazil–Falkland (Río de la Plata).
• Gyre direction: clockwise in the north, anticlockwise in the south. The north Indian Ocean reverses with the monsoon.
• Sargasso Sea: has no land boundary.
• Volume: surface currents are about 10% of ocean water (upper 400 m); deep water is 90%. The West Wind Drift is the largest current by volume.
• Statement traps:
• "Cold currents flow on all west coasts" is FALSE: high northern west coasts have warm currents.
• "Spring tides occur in the spring season" is FALSE.

Mains — GS-I themes

1. How ocean currents shape coastal climate: north-west Europe (North Atlantic Drift), Peru–Chile (Humboldt) and Namibia (Benguela).
2. Why the richest fishing grounds lie at current meeting zones and upwelling coasts, with India's Kerala and Somali upwelling as examples.
3. Tidal energy in India: potential, sites (Khambhat, Kachchh, Sundarbans) and why projects such as Durgaduani and Kalpasar stalled.
4. The thermohaline circulation and the risk of its weakening under climate change.
5. The monsoon and the currents of the north Indian Ocean as one linked system.

Current-affairs hooks

• INCOIS Kallakkadal (swell surge) and high-wave alerts along Kerala and the west coast; Potential Fishing Zone advisories.
• Studies on AMOC weakening (verify current).
• El Niño and La Niña episodes, Peruvian anchoveta catches and the Indian monsoon (verify current).
• Longshore drift and coastal erosion at Indian ports and beaches (shoreline change assessments).
• India's ocean energy policy: tidal, wave and OTEC (verify current).
• Supermoon and king-tide flooding; fog and iceberg risks in shipping lanes.