Ocean Currents: Causes, Warm and Cold Currents of Each Ocean, Gyres, Thermohaline Circulation and Effects on Climate (UPSC Notes + PYQs)

What drives currents

• Primary forces: solar heating (water expands near the equator, so sea level is slightly higher there), wind (friction drags surface water; trade winds and westerlies set the big loops), gravity, and the Coriolis force (deflects water to the right in the northern hemisphere and to the left in the southern).
• Secondary forces: differences in water density due to temperature and salinity, which drive deep currents.
• Shape of coastlines and the ocean floor deflects and splits currents.

Rotation of the Earth, wind and density all influence currents; the Earth’s revolution does not, as Prelims 2012 tested.

Gyres: the big loops

In each ocean basin, trade winds push water west near the equator and westerlies push it east in mid-latitudes. With the Coriolis force and the continents, this forms large circular gyres: clockwise in the northern hemisphere, anticlockwise in the southern. The North Atlantic gyre encloses the calm, weed-covered Sargasso Sea. Garbage patches, like the Great Pacific Garbage Patch, collect in gyre centres.

Major currents

The Indian Ocean: currents that reverse

The northern Indian Ocean is closed off by Asia, so monsoon winds control its surface currents. In summer the South-West Monsoon Current flows clockwise and the Somali Current flows north along East Africa, bringing upwelling of cold water off Somalia and Oman. In winter the North-East Monsoon Current flows anticlockwise from east to west. South of the equator the pattern is a normal anticlockwise gyre: South Equatorial, Agulhas, West Australian.

Thermohaline circulation

In the North Atlantic near Greenland and in the Weddell Sea, sea water becomes cold and salty (salt is left behind as sea ice forms), sinks, and spreads along the ocean floor. Water rises elsewhere and returns at the surface. This slow “global conveyor belt” takes about a thousand years for a full loop. The Atlantic part, the AMOC (Atlantic Meridional Overturning Circulation), carries heat north and keeps western Europe mild; the IPCC says it is very likely to weaken this century as melting ice freshens the North Atlantic.

Effects

• Climate: the North Atlantic Drift keeps Norway’s ports ice-free; cold currents cool and dry coasts.
• Deserts: cold currents on western coasts stabilise air and reduce rain, helping form the Atacama (Humboldt), Namib (Benguela), and parts of the Sahara (Canary).
• Fog and fishing grounds: where warm and cold currents meet, plankton thrive and fog forms: Grand Banks off Newfoundland (Gulf Stream and Labrador), off Japan (Kuroshio and Oyashio).
• Upwelling along cold-current coasts (Peru) brings nutrients and rich fisheries; its weakening during El Niño hurts fish catch.
• Navigation and pollution: ships use currents; oil spills and plastics spread with them.

Ekman transport and upwelling, step by step

When wind blows over the sea, it drags the top layer of water, but the Coriolis force bends that layer to the right in the northern hemisphere (left in the southern). Each deeper layer is moved by the one above it and is bent a little more and moves a little slower, forming the Ekman spiral. Averaged over the whole wind-driven layer, the net movement of water, called Ekman transport, is at about 90 degrees to the wind. Along the coasts of Peru, California, Namibia and north-west Africa, winds blowing parallel to the shore push surface water offshore. Cold, nutrient-rich water rises from below to replace it. This coastal upwelling supports some of the richest fisheries on Earth: the Peruvian anchoveta fishery is among the largest single-species fisheries in the world. Upwelling also happens along the Somali and Arabian coasts during the summer monsoon and, more weakly, off Kerala, where it feeds the oil sardine shoals that local fishers depend on.

Currents, El Niño and the Indian Ocean Dipole

Normally the trade winds pile warm water in the western Pacific near Indonesia, while upwelling keeps the eastern Pacific off Peru cool. During El Niño, the trade winds weaken, warm water spreads east, upwelling off Peru weakens, and fish catches fall. Rain shifts east with the warm water, and India’s monsoon is often weaker; 2002, 2009 and 2015 were El Niño years with poor rainfall. La Niña is the opposite, with stronger trade winds and colder eastern Pacific water. The Indian Ocean Dipole is a similar see-saw in the Indian Ocean: in a positive IOD the western Indian Ocean is warmer than the eastern, which tends to help the Indian monsoon, as in 2019. Both depend on currents and winds working together, which is why the IMD watches them before issuing its monsoon forecast.

How currents are measured

Sailors once tracked currents by the drift of ships and floating bottles. Today satellites measure sea surface height and temperature, from which currents can be worked out. The international Argo programme, started in 2000, keeps nearly 4,000 floats drifting through the oceans; each dives to about 2,000 m and rises again, recording temperature and salinity. India contributes Argo floats and moored buoys through the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, under the Ministry of Earth Sciences. INCOIS uses this data for ocean state forecasts, potential fishing zone advisories sent to fishers, and tsunami warnings. The Deep Ocean Mission (2021) adds further capacity to study the seas around India.

Currents and fishing grounds

The world’s richest fishing grounds lie where warm and cold currents meet, such as the Grand Banks off Newfoundland (Gulf Stream and Labrador) and the waters off Japan (Kuroshio and Oyashio), because mixing brings nutrients to the surface and feeds plankton.

PYQ trend

Prelims asked the factors influencing currents (2012) and the reasons for desert belts (2011), where cold currents are a key point. Mains links currents to climate, fisheries and sea level; draw a world sketch with a few labelled currents.

Ocean Currents: UPSC angle

Ocean Currents UPSC PYQs (Prelims)

Questions UPSC has actually asked, from the official papers. Pick an option to check yourself.

Mains PYQs on this topic

No UPSC Mains question from 2023-2026 is directly on this topic. Older papers (2013-2022) will be added as the Mains bank grows.

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Image: https://commons.wikimedia.org/wiki/File:Gulf_Stream_Sea_Surface_Currents_and_Temperatures_NASA_SVS.jpg