Maarg Manthan · Topic 11.1

Ocean Currents – Causes and Types

Geography › Ocean Movements, Temperature and Salinity · Topic 11.1

Ocean currents are continuous flows of water in a definite direction, driven by prevailing winds, the Coriolis force, differences in temperature and salinity, and the shape of the ocean basins. Surface currents form large circular gyres, clockwise in the northern hemisphere and anticlockwise in the southern. Currents are classified as warm or cold and as surface or deep, and they redistribute heat across the planet.

6 min read · UPSC Prelims · Geography · Ocean Movements, Temperature and Salinity

At a Glance

Main drivers Wind, Coriolis force, density, basin shape
Pattern Five subtropical gyres
North gyres Clockwise
South gyres Anticlockwise
Deep flow Thermohaline circulation
Exam link UPSC Prelims, Geography: Ocean Movements, Temperature and Salinity

Where Does It Fit?

This is the first topic of the category on ocean movements, temperature and salinity. The previous macro described the shape of the oceans and seas, and this one begins the study of how ocean water moves. The next topic describes the currents of the Pacific and Atlantic oceans. Previous topic: Bays, Gulfs, Straits and Isthmuses. Next topic: Pacific and Atlantic Ocean Currents.

What Are Ocean Currents and What Causes Them?

In short: currents are rivers within the ocean, pushed by winds and shaped by the Earth’s rotation, temperature and salinity.

An ocean current is a large-scale movement of water in a fairly fixed direction. A drift is a slow, broad, wind-driven movement, and a stream is a fast, narrow, well-defined flow such as the Gulf Stream. The forces that produce currents fall into three groups.

Group	Force	Effect
Forces that start the motion	Heating by the Sun	Water expands when warm, so the equatorial sea level is slightly higher, by about 8 cm in the Pacific, and water flows down the slope toward the poles
Forces that start the motion	Wind	Prevailing winds drag the surface water along by friction; the trade winds and the westerlies are the main drivers of the surface gyres, as in General Circulation of the Atmosphere
Forces that start the motion	Gravity	Piled-up water tends to flow downhill and form a slope
Forces that modify the flow	Coriolis force	Deflects moving water to the right in the northern hemisphere and to the left in the southern, as in Forces Affecting Wind Movement; produces the circular gyres
Forces that modify the flow	Shape of the coast and sea floor	Continents block the flow and direct it; the Gulf of Guinea and the Caribbean turn the flow
Density differences	Temperature and salinity	Cold, salty water is dense and sinks, and warm or fresh water is lighter; this drives the vertical movement and the deep currents

Wind drags the top layer, which drags the layer below, and the Coriolis force turns each layer a little more, forming a spiral called the Ekman spiral. The net movement of water over the whole layer is at about 90 degrees to the right of the wind in the northern hemisphere, called Ekman transport. It explains coastal upwelling, such as off Peru, as in Walker Circulation and Normal Conditions in the Pacific.

What Are the Gyres and Why Are Currents Stronger in the West?

Under the combined action of winds and the Coriolis force, the surface currents form five large circular systems called subtropical gyres, in the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian oceans. They turn clockwise in the northern hemisphere and anticlockwise in the southern. The north Atlantic gyre, for example, consists of the North Equatorial Current flowing west, the Gulf Stream flowing north along the American coast, the North Atlantic Drift flowing east, and the Canary Current flowing south along Africa. A gyre has a calm centre, such as the Sargasso Sea, where floating debris collects, as in the Great Pacific Garbage Patch. Smaller gyres occur at high latitudes.

The currents on the western side of the oceans, such as the Gulf Stream and the Kuroshio, are narrow, deep and fast, up to about 2 m per second, whereas those on the eastern side, such as the Canary and California currents, are broad, shallow and slow. This is called western intensification, and it arises from the variation of the Coriolis force with latitude. Western boundary currents carry huge volumes of water: the Gulf Stream moves around 30 million cubic metres per second, which is 30 sverdrups, many times the flow of all rivers combined.

How Are Ocean Currents Classified?

By depth: surface currents make up about 10 per cent of the ocean water, in the upper 400 m, while deep water currents make up the other 90 per cent, and move by density differences.
By temperature: warm currents carry water from low to high latitudes, usually on the east coasts of continents in the low and mid-latitudes, and raise the temperature and rain of the coasts, as the Gulf Stream does. Cold currents bring water from high to low latitudes, usually on the west coasts of continents in the low and mid-latitudes, and lower the temperature and cause fog and deserts, as the Peru and Benguela currents do. In the high latitudes the opposite pattern occurs, and the warm currents flow on the west coasts of continents, such as the North Atlantic Drift near Europe.
By permanence: permanent currents such as the trade-wind drifts, and seasonal currents, such as those of the northern Indian Ocean, which reverse with the monsoon, as in Indian Ocean Currents.

The deep circulation is driven by density, so it is called the thermohaline circulation, from thermo for heat and haline for salt. Cold, salty surface water sinks in the North Atlantic and around Antarctica, flows slowly through the deep ocean, and returns to the surface in the Indian and Pacific oceans, in a loop called the global conveyor belt that takes a thousand years or more to complete. It transports heat and carbon, and its weakening in a warming world is a concern, as in The Atlantic Ocean. As an analytical point, the ocean currents are the reason why the climate of a place cannot be judged from its latitude alone, and why the study of currents is also the study of fisheries, shipping and climate risk.

Why Is This Topic Important?

Concepts Causes and gyres asked
Link Climate and fisheries
Climate change Circulation slowdown risk

What Came Next?

The next topic describes the main currents of the Pacific and Atlantic oceans. Next topic: Pacific and Atlantic Ocean Currents.

Key People and Terms

Gyre Large circular current system
Ekman transport Net water movement at 90 degrees to the wind
Western intensification Strong narrow currents on the western side
Thermohaline circulation Density-driven deep circulation
Sverdrup One million cubic metres per second
Drift Slow broad wind-driven current

Exam Corner

Points to Remember

Gyres turn clockwise in the north.
Western boundary currents are narrow and fast.
Warm currents flow poleward.
Thermohaline means temperature and salt.
Surface currents are about 10 per cent of the ocean water.

Do Not Confuse With

Drift and stream Slow and broad versus fast and narrow
Warm current and cold current Poleward versus equatorward flow
Wave and current Up-down motion versus horizontal flow

Memory Hook

Wind pushes, Coriolis bends, the continents turn it round: gyres in the ocean, density in the deep.

Mains Angle

Questions ask for causes and effects. These points add depth.

Causes Wind, rotation, density
Pattern Gyres and western intensification
Climate Heat redistribution

A Question You May Face

An original practice question, not a past paper question.

“Explain the factors that influence ocean currents and the formation of the oceanic gyres.”

How to Answer

Forces Primary and secondary.
Gyres Wind plus Coriolis.
Types Warm, cold, deep.
Importance Heat, fish, shipping.

GS Relevance

Prelims: causes, gyres, warm and cold currents, thermohaline circulation. GS Paper 1: oceanography and climatic effects of currents.

Frequently Asked Questions

What are ocean currents?

Ocean currents are large, continuous flows of sea water in a fairly definite direction. They are driven by winds, the rotation of the Earth, differences in water density caused by temperature and salinity, and the shape of the coastlines, and they carry heat across the world.

What causes ocean currents?

Ocean currents are caused by the prevailing winds that drag the surface water, the Coriolis force that deflects the moving water, heating by the Sun, differences in temperature and salinity that create density differences, and the shape of coasts and the sea floor that direct the flow.

What is a gyre?

A gyre is a large circular system of ocean currents formed by prevailing winds and the Coriolis force. There are five major subtropical gyres, which turn clockwise in the northern hemisphere and anticlockwise in the southern hemisphere, with a calm centre where floating material often accumulates.

What is the difference between warm and cold currents?

Warm currents carry warm water from low latitudes toward the poles, mostly along eastern coasts, and warm and moisten the climate. Cold currents carry cold water from high latitudes toward the equator, mostly along western coasts in the low and middle latitudes, and cool and dry the climate.

What is thermohaline circulation?

Thermohaline circulation is the deep ocean circulation driven by differences in density, which depends on temperature and salinity. Cold, salty water sinks in the North Atlantic and around Antarctica and slowly flows through the deep ocean, forming a global conveyor belt that takes about a thousand years for a full cycle.

PYQ Practice — Statement Analysis

1 Gyres in the northern hemisphere move clockwise.

True

The Coriolis force deflects water to the right.

2 Cold currents generally flow from the equator toward the poles.

False

They flow from the poles toward the equator.

3 The Gulf Stream is a western boundary current.