Ocean currents: rivers in the sea — Earth science, 11–13 years
Ocean currents move seawater over long distances. Wind drives many surface currents, while differences in temperature and saltiness help move deep water.
What an ocean current is
An ocean current is a broad flow of seawater, not a single wave. Surface currents are pushed mainly by winds, while deep currents form when water becomes colder or saltier and therefore denser. Currents can carry warm, cold, salty or nutrient-rich water, linking distant parts of the ocean.
Why currents matter
The ocean absorbs and redistributes heat, so currents help move energy from warm regions towards cooler ones. They also carry nutrients that support ocean food webs and affect where fish are found. Sailors noticed these routes long ago because a current could speed up or slow down a journey even when the wind stayed similar.
A current changes a journey
A boat travels 120 kilometres east through water moving east at 10 kilometres per hour. If the boat’s speed through still water is 20 kilometres per hour, its speed over the seabed is 30 kilometres per hour, so the trip takes 120 ÷ 30 = 4 hours. Against the same current, the speed is 20 − 10 = 10 kilometres per hour, so the return takes 12 hours.
The trap: currents are only waves
It is reasonable to confuse them because both are seen on the sea surface. A wave mainly carries energy while water moves in a local rise-and-fall pattern; a current carries water in a more lasting direction over a distance. A floating object can bob with waves but still drift steadily with a current.
Currents outside school
Ship planners use current maps to estimate fuel and travel time. Fishers and marine scientists track currents because they move larvae, plankton, pollution and heat. Beach safety teams also care about narrow rip currents: these can pull swimmers away from shore, so swimming parallel to the beach is usually safer than fighting straight against the flow.
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