How ocean currents are formed
Put a drop of ink in a glass of water on a warm stove and watch it swirl and spread. The sea does the same on a huge scale: warm and cold water keep moving in fixed paths.
Ocean currents (सागरी प्रवाह) are movements of ocean water in a definite direction and with a certain velocity, caused by differences in temperature, density and planetary winds.
Rule 1: Tray activity and cause — Fill a large tray with water, drop plastic sequins and heat one corner with a spirit lamp: the sequins start moving, after some time in a circle. Heated water becomes lighter (lower density) and cooler, heavier water replaces it. In the ocean the heat source is the sun. Surface movements depend on temperature and salinity; planetary winds give them speed.
In the figure : In the figure tray activity: a spirit lamp heats one corner of a tray of water, and the sequins start moving, after some time in a circular motion, because the water is flowing.
- Easy : Easy: Heated water becomes lighter.
- Medium : Medium: Cool heavy water sinks and replaces it.
- Exam-type : Exam: What is the ocean's source of heat?
In the figure : In the figure heating makes the water lighter (lower density); the cooler, heavier water replaces it and the water starts to flow. In the ocean the source of heat is the sun.
- Easy : Easy: Sun heats the sea.
- Medium : Medium: Warm → light → rises; cold → heavy → sinks.
- Exam-type : Exam: Why does water start to flow?
Rule 2: Speed and the toy journey — Currents move at about 2 to 10 km per hour; at 4 km/h water travels 4 × 24 = 96 km a day. In 1992 a container with about 28,000 rubber toys fell into the Pacific near Hawaii (10 January). After about 10 months toys reached Alaska; some crossed the Bering Strait to the Arctic by 2000, then the Atlantic: the eastern coast of America in 2003 and Europe by 2007; some went from Hawaii to Australia. Currents carried them thousands of kilometres. Without currents the water would be still.
In the figure : In the figure about 28,000 rubber toys fell near Hawaii in 1992: they reached Alaska after about 10 months, the Arctic Ocean by 2000, the eastern coast of America in 2003 and Europe by 2007, carried by ocean currents.
- Easy : Easy: Toys fell near Hawaii.
- Medium : Medium: 1992 → 2007 is 15 years.
- Exam-type : Exam: What did the toys prove?
Currents: temperature + density + winds; 2–10 km/h.
Thinking currents are waves. Currents are slow, steady flows.
Memory clue: "गरम हलके, थंड जड = प्रवाह".
Spot the difference
| Cause | Effect |
|---|---|
| Heating | Water lighter |
| Cold water | Heavier, sinks |
| Planetary winds | Give speed |
Practice questions
- 1What are ocean currents?
- 2Name the three causes.
- 3What is the heat source of the ocean?
- 4Speed of currents?
- 5Where did the 1992 toys fall?
- 6Which year did toys reach Europe?
- 7A current moves at 5 km/h. Distance in one day?
- 8Why does heated water rise?
Show answers (try them yourself first)
- 1Movement of ocean water in a definite direction.
- 2Temperature, density, planetary winds.
- 3The sun.
- 42 to 10 km per hour.
- 5Near Hawaii in the Pacific.
- 6By 2007.
- 75 × 24 = 120 km.
- 8Its density falls.
Surface currents, volume and gyres
Rivers flow on land in beds. In the same way, a vast river of sea water flows through the ocean, but only in its upper part.
Surface currents (पृष्ठीय प्रवाह) flow in the upper 500 m as warm or cold currents; their volume is measured in Sverdrups.
Rule 1: Surface and deep water — Ocean water has surface water from sea level to 500 m (sunlight reaches here) and deep water below. The horizontal flow moves only 10% of ocean water. Currents from the equator towards the poles are warm; from the poles towards the equator are cold. Winds push them over long distances.
In the figure : In the figure the surface water extends from the sea level up to 500 m and is moved by the horizontal flow, which carries only 10% of the oceanic water; below 500 m is the deep water.
- Easy : Easy: Surface water is up to 500 m.
- Medium : Medium: If the ocean has 100 units, 10 units move horizontally.
- Exam-type : Exam: Why do currents not flow along the coast?
In the figure : In the figure warm currents flow from the equator towards the poles and cold currents flow from the poles towards the equator.
- Easy : Easy: Warm: equator → poles.
- Medium : Medium: Cold: poles → equator.
- Exam-type : Exam: Define warm and cold currents.
Rule 2: Volume and gyres — 1 Sverdrup = 1 million m³ per second, so 25 Sverdrups = 25 million m³/s. Currents flow near the lower edge of the continental shelf, not close to the coast. Their speed is low but the water carried is immense. Circular patterns of currents make gyres (भोवरे). The Sargasso Sea in the Atlantic has no land boundary, is surrounded by currents, has still water, is 1100 km wide and 3200 km long, and is named after Sargassum seaweed.
In the figure : In the figure the Sargasso Sea (Atlantic Ocean) is a gyre: still water with no land boundary, surrounded by ocean currents; it is 1100 km wide and 3200 km long.
Surface currents, volume and gyres
In the app you can move this figure with your finger.
Open in the app- Easy : Easy: Sargasso Sea has no land boundary.
- Medium : Medium: 3200 − 1100 = 2100 km longer than wide.
- Exam-type : Exam: How did it get its name?
Surface 0–500 m, 10% of water. Warm: equator → poles. 1 Sverdrup = 1 million m³/s.
Thinking fast current means more water. Volume is huge though speed is low.
Memory clue: "उष्ण = बाहेर जाते, शीत = विषुववृत्ताकडे येते".
Spot the difference
| Type | Direction |
|---|---|
| Warm | Equator → poles |
| Cold | Poles → equator |
Practice questions
- 1Up to what depth is surface water?
- 2How much water moves in the horizontal flow?
- 3Which currents are warm?
- 4Unit of current discharge?
- 51 Sverdrup equals?
- 6What is a gyre?
- 7A current carries 40 Sverdrups. How many m³ per second?
- 8Why is the Sargasso Sea called still?
Show answers (try them yourself first)
- 1500 m.
- 2Only 10%.
- 3Those flowing from the equator towards the poles.
- 4Sverdrup.
- 51 million m³ per second.
- 6A circular pattern of ocean currents.
- 740 million m³.
- 8It lies at the centre of the gyre.
Direction of currents: rotation, coastline and the Indian Ocean
When you stir tea in a cup, it turns around. The Earth also turns under the oceans, and the currents bend because of it.
The direction of currents is set by Earth's rotation, the coastline, the Westerlies and, in the Indian Ocean, the monsoon winds.
Rule 1: Rotation, coastline, winds — Because of rotation, currents move clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The alignment of the coastline changes the direction. Under the Westerlies currents flow west to east in mid-latitudes and east to west near the equator, forming a circular pattern.
In the figure : In the figure because of the rotation of the earth, ocean currents move clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
- Easy : Easy: North: clockwise.
- Medium : Medium: South: anticlockwise.
- Exam-type : Exam: Why do currents turn?
In the figure : In the figure under the Westerlies the currents flow from west to east in the mid-latitudes and from east to west near the equator; the coastlines turn them, so a circular pattern forms.
- Easy : Easy: Mid-latitudes: west → east.
- Medium : Medium: Near equator: east → west.
- Exam-type : Exam: How does a circle form?
Rule 2: The Indian Ocean — Pacific and Atlantic currents follow similar patterns, but the Indian Ocean is different: it is land-locked in the north and the equator divides it into two parts. Monsoon winds change direction with the season, so in the northern part currents flow clockwise in summer and anticlockwise in winter.
In the figure : In the figure in the northern Indian Ocean the currents flow clockwise in summer and anticlockwise in winter, because the monsoon winds reverse their direction.
- Easy : Easy: Indian Ocean currents reverse.
- Medium : Medium: Summer: clockwise; winter: anticlockwise.
- Exam-type : Exam: Why is the Indian Ocean different?
North clockwise, south anticlockwise. Indian Ocean reverses with monsoon.
Thinking the Indian Ocean copies the Atlantic. Monsoon makes it different.
Memory clue: "उत्तर = घड्याळ, दक्षिण = उलट".
Spot the difference
| Factor | Effect |
|---|---|
| Rotation | Clockwise north, anticlockwise south |
| Coastline | Changes direction |
| Westerlies | West → east in mid-latitudes |
| Monsoon | Reverse Indian Ocean currents |
Practice questions
- 1How do currents move in the Northern Hemisphere?
- 2And in the Southern?
- 3What changes direction according to coastline?
- 4Westerlies move currents in which direction in mid-latitudes?
- 5Which ocean is land-locked in the north?
- 6Which winds change the Indian Ocean currents?
- 7In which season are the northern Indian Ocean currents clockwise?
- 8Why is the Pacific pattern more regular than the Indian?
Show answers (try them yourself first)
- 1Clockwise.
- 2Anticlockwise.
- 3Ocean currents.
- 4West to east.
- 5The Indian Ocean.
- 6Monsoon winds.
- 7Summer.
- 8The Pacific is open to the north; the Indian Ocean is closed.
Effects of ocean currents on human life
Sailors in a sailing ship choose a route that rides the current, as a swimmer rides the river. And fishermen go where two currents meet, because fish gather there.
Ocean currents affect the climate, fishing, shipping, fog and icebergs of coastal regions.
Rule 1: Climate — Warm currents make cold regions warmer and may add rain. Warm currents near Western Europe, Southern Alaska and Japan soften winters, so ports do not freeze. Cold currents bring low rain, as in Peru, Chile and the desert of South-West Africa; the cold Humboldt current flows along South America's west coast. The ports of eastern Canada freeze in winter because the cold Labrador current flows there.
In the figure : In the figure the warm Gulf Stream and the cold Labrador current meet near Newfoundland, causing dense fog; the cold current also freezes the ports of eastern Canada.
- Easy : Easy: Warm currents keep ports ice-free.
- Medium : Medium: Cold currents → low rain.
- Exam-type : Exam: Why does fog form near Newfoundland?
Rule 2: Fishing, shipping, hazards — Where cold and warm currents meet, plankton and algae grow, so fish gather and breed: Grand Bank (North America), Dogger Bank (Europe). Ships sailing with a current go faster and save fuel. Fog forms where currents meet (Gulf Stream + Labrador near Newfoundland). Cold currents carry icebergs from polar areas, hazardous to ships. Without currents, water would be still and marine life would be limited.
In the figure : In the figure warm currents keep ports ice-free; cold currents bring low rainfall and icebergs; meeting currents give fishing grounds and fog; ships save fuel by sailing along the flow.
- Easy : Easy: Plankton feeds fish.
- Medium : Medium: Grand Bank is a fishing ground.
- Exam-type : Exam: Give two effects of cold currents.
Warm: mild winters, ice-free ports. Cold: dry, icebergs. Meeting: fish + fog.
Thinking all currents are good for ships. Fog and icebergs are dangers.
Memory clue: "उष्ण = उबदार; शीत = कोरडे; भेट = मासे + धुके".
Spot the difference
| Current | Effect | Example |
|---|---|---|
| Warm | Mild winter, ports ice-free | Western Europe, Japan |
| Cold | Low rain, icebergs | Peru, Chile |
| Meeting | Fish and fog | Newfoundland |
Practice questions
- 1How do warm currents affect climate?
- 2Name a cold current.
- 3Why do fishing grounds form where currents meet?
- 4Name two fishing grounds.
- 5Which currents meet near Newfoundland?
- 6Why are icebergs dangerous?
- 7A ship uses 60 tonnes of fuel per trip. With the current it saves 10%. How much is saved?
- 8Why do Peru and Chile coasts have little rain?
Show answers (try them yourself first)
- 1They make it warmer.
- 2Humboldt or Labrador.
- 3Plankton grows and fish gather.
- 4Grand Bank and Dogger Bank.
- 5Gulf Stream and Labrador.
- 6They can hit and damage ships.
- 760 × 10 ÷ 100 = 6 tonnes.
- 8The cold Humboldt current cools the air, so less moisture.
Deep ocean currents
Hot soup in a pot has cooler soup sinking at the edges and hot soup rising in the middle. The deep sea does this slowly over centuries.
Deep ocean currents (खोल सागरी प्रवाह) flow below 500 m, driven by differences in temperature and density: the thermohaline circulation (थर्मोहेलाइन अभिसरण).
Rule 1: The conveyor belt — Deep currents flow up to the sea-bed like rivers. Warm water with lower salinity is lighter and comes up; cold water with high density goes down. Surface water near Greenland and Europe sinks, moves at depth to Antarctica and later rises. This redistribution takes about 500 years and is called the conveyor belt: in 2000 years there are 2000 ÷ 500 = 4 cycles.
In the figure : In the figure deep ocean currents (below 500 m) form the conveyor belt: water sinks near Greenland and Europe, flows at depth to Antarctica and moves to the surface again; one cycle takes about 500 years.
- Easy : Easy: One cycle is about 500 years.
- Medium : Medium: 1500 years = 3 cycles.
- Exam-type : Exam: What is the conveyor belt?
Rule 2: Importance — By thermohaline circulation sea water moves from the surface to the bottom and back. Warm water is carried down and nutrient-rich cold water is brought up, which feeds plankton and fish.
In the figure : In the figure because of the thermohaline circulation, warm water is carried down and nutrient-rich cold water is circulated to the surface, which feeds plankton and fish.
- Easy : Easy: Cold water brings nutrients up.
- Medium : Medium: This feeds plankton and then fish.
- Exam-type : Exam: Why is deep circulation important?
Deep currents: below 500 m; thermohaline; conveyor belt 500 years.
Thinking deep currents are driven by wind. Temperature and density drive them.
Memory clue: "थंड + जड = खाली; गरम + हलके = वर".
Spot the difference
| Feature | Surface current | Deep current |
|---|---|---|
| Depth | 0–500 m | Below 500 m |
| Driver | Winds | Temperature, density |
| Speed | Faster | Very slow |
Practice questions
- 1What are deep ocean currents?
- 2What drives them?
- 3What is this circulation called?
- 4Which water sinks?
- 5Near which places does surface water sink?
- 6How long does one conveyor-belt cycle take?
- 7How many cycles occur in 3000 years?
- 8Why is deep circulation good for fish?
Show answers (try them yourself first)
- 1Currents below 500 m.
- 2Differences in temperature and density.
- 3Thermohaline circulation.
- 4Cold, dense water.
- 5Greenland and Europe.
- 6About 500 years.
- 73000 ÷ 500 = 6.
- 8It brings nutrient-rich water to the surface.
Quick revision
- Currents are formed by differences in temperature, density and planetary winds. Heating → water lighter → heavier cold water replaces it. Speed ≈ 2–10 km per hour.
- 1992 toys: about 28,000 toys fell near Hawaii; Alaska after about 10 months; Arctic by 2000; eastern America 2003; Europe 2007.
- Surface water: 0–500 m, moves only 10% of ocean water. Warm currents go equator → poles, cold currents poles → equator. 1 Sverdrup = 1 million m³ per second.
- Gyre = circular pattern of currents. Sargasso Sea (Atlantic): still water, 1100 km × 3200 km, named after Sargassum seaweed.
- Direction: clockwise in the north, anticlockwise in the south; coastline changes direction. Westerlies: west → east in mid-latitudes, east → west near the equator.
- Indian Ocean: land-locked in the north, monsoon winds; clockwise in summer, reverse in winter.
- Effects: warm currents keep ports ice-free (Western Europe, S. Alaska, Japan); cold currents → low rainfall (Peru, Chile, SW Africa), icebergs; meeting of currents → fog and fishing grounds (Grand Bank, Dogger Bank); ships save fuel.
- Deep currents: below 500 m, temperature + density (thermohaline), Greenland/Europe → Antarctica, conveyor belt ≈ 500 years.