Ocean currents and what they do to climate

Ocean currents are large, continuous movements of seawater in a definite direction — rivers within the ocean. They carry heat from the tropics towards the poles, shape the climate of coastlines, create some of the world's richest fisheries and some of its driest deserts. For UPSC, they appear in Prelims map questions and in Mains questions on climate and resources.

What drives ocean currents?

Warm and cold currents

• Warm currents move from low latitudes towards higher latitudes, generally along the eastern coasts of continents in low and middle latitudes.
• Cold currents move from high latitudes towards lower latitudes, generally along the western coasts of continents in low and middle latitudes.

Together with the winds and the Coriolis force, this produces large circular systems called gyres — moving clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

The major currents

Atlantic Ocean

Pacific Ocean

Indian Ocean

The northern Indian Ocean is unusual: its currents reverse with the seasons under the influence of the monsoon winds. In summer the flow is broadly clockwise (the south-west monsoon current); in winter it reverses (the north-east monsoon current). The Southern Indian Ocean has a more regular anticlockwise gyre, with the warm Agulhas Current off south-east Africa and the cold West Australian Current.

How currents shape climate

Warming coasts. The North Atlantic Drift keeps the coasts of north-western Europe much milder in winter than places at similar latitudes in North America or Asia, and keeps some northern ports ice-free.

Deserts beside the sea. Cold currents cool the air above them, which stabilises it and reduces rainfall. Several of the world's great coastal deserts lie along cold currents:

• the Atacama beside the Humboldt Current;
• the Namib beside the Benguela Current;
• the western Sahara beside the Canary Current.

Fog. Where warm and cold currents meet — as the Gulf Stream meets the Labrador Current off Newfoundland — warm moist air cools rapidly and dense fog forms.

Currents and fisheries

The world's richest fishing grounds are associated with currents:

• Where warm and cold currents meet — the Grand Banks off Newfoundland and the waters off Japan — mixing brings up nutrients and supports plankton.
• Upwelling zones along cold currents, such as off Peru, where winds push surface water away from the coast and cold, nutrient-rich water rises from below.

When El Niño weakens upwelling off Peru, fish catches there can fall sharply — one of the clearest links between ocean currents and the global economy.

The deep ocean: thermohaline circulation

Beneath the wind-driven surface currents lies a slower, global circulation driven by differences in temperature (thermo) and salinity (haline). In the North Atlantic, cold, salty water sinks and flows south along the ocean floor, eventually rising elsewhere. This "global conveyor belt" redistributes heat across the planet over timescales of centuries.

The Atlantic Meridional Overturning Circulation (AMOC), of which the Gulf Stream system is part, is closely watched by scientists. Several studies suggest it may weaken as climate change adds freshwater from melting ice to the North Atlantic, with potentially large effects on climate.

Practice questions

1. Prelims-type: Which of these are cold currents — Benguela, Kuroshio, Canary, Humboldt? Why are the Grand Banks rich in fish?
2. Mains-type (150 words): Explain how ocean currents influence the climate of coastal regions, with examples.
3. Mains-type (150 words): Why do the currents of the northern Indian Ocean reverse with the seasons?