Surface currents of the North Atlantic, with the Gulf Stream in orange, from NASA's Scientific Visualization Studio. The streaks are water on the move; the colours are the model's current field. Image: NASA/Goddard.
Ocean currents are ranked by volume transport, measured in sverdrups: one sverdrup (Sv) is one million cubic metres of water per second, about five times the flow of the Amazon. Speed and volume are different things — the fastest surface currents are narrow, while the largest mover of water on Earth is slow, deep and 21,000 km long.
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Antarctic Circumpolar Current — 173.3 Sv. The only current that circles the planet, driven eastward around Antarctica by the prevailing westerlies and unimpeded by any continent. Four years of mooring data in Drake Passage, published in 2016, measured 173.3 Sv; the long-used textbook figure is 134 Sv. It is the largest of all ocean currents and the main connection between the Atlantic, Pacific and Indian oceans, which is why it matters for climate as well as for shipping.
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Agulhas Current — 70 Sv, up to 135 Sv. The western boundary current of the south Indian Ocean, flowing south along the east coast of Africa at up to 2.5 metres per second. The American Meteorological Society's glossary gives 70 Sv near 31°S and up to 135 Sv near 35°S, among the largest transports of any current. South of the Cape it turns back on itself, and the rings it sheds carry warm, salty water into the Atlantic.
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Kuroshio — 42 Sv. The western boundary current of the north Pacific, flowing north past Japan. Its net eastward transport has been estimated at about 42 Sv, and the recirculation gyre south-east of Japan can push the current's total to roughly 130 Sv. It carries warm water to Japan's reefs at latitudes where the surrounding sea would otherwise be too cold for coral.
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Gulf Stream — about 34 Sv. The most famous current in the world, and only the fourth largest by volume. It carries about 30 Sv through the Florida Straits and rather more as it leaves the coast of the Carolinas, where it is joined by recirculating water. Its importance is heat, not volume: it moves tropical warmth north-east across the Atlantic and keeps north-west Europe several degrees warmer than its latitude would suggest.
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East Australian Current — 22 Sv. The western boundary current of the south Pacific, running south along Australia's east coast. An 18-month mooring array at 27°S measured a mean poleward transport of 22.1 Sv ± 7.5 Sv; further south, before the current separates from the coast, the mean rises to about 26.8 Sv. It is the weakest of the subtropical western boundary currents, and the one that carries tropical larvae as far south as Tasmania.
The numbers are averages, and every one of them varies by season and by year. A current has no fixed banks: its transport varies with season, wind and the shape of the seafloor, and surveys of the same current can differ by 30 per cent. The Antarctic Circumpolar Current's own figure moved from the textbook 134 Sv to 173.3 Sv in the 2016 mooring study.