Warm Currents and Cold Currents — How the Ocean Carries Heat Around the World

It comes down to which way they flow. On the western side of each ocean basin, strong narrow currents carry warm tropical water toward the poles — the Gulf Stream, the Kuroshio off Japan, the Agulhas off southern Africa, the Brazil and East Australian currents. These are the warm 'western boundary currents.' On the eastern side, broader, slower currents bring cool water back toward the equator — the California, Canary, Benguela and Humboldt currents. Many of these cold currents also pull deep, cold water up to the surface (upwelling), which makes them even cooler and extraordinarily rich in life.

The most famous current on Earth: a warm, fast western-boundary current that runs north up the US east coast and then bends out across the Atlantic. It transports about 30 sverdrups of water past Florida, growing to around 150 Sv downstream (1 Sv = one million cubic metres per second) — more water than all the world's rivers combined, with surface speeds up to about 2.5 m/s (NOAA Ocean Service, 2024). The heat it carries toward Europe is a big part of why northwest Europe is so mild for its latitude.

It is the unit oceanographers use for how much water a current moves: one sverdrup (Sv) equals one million cubic metres per second. For scale, the entire flow of all the world's rivers combined is roughly one sverdrup. The Gulf Stream runs at tens of sverdrups; the Antarctic Circumpolar Current, the largest of all, moves well over a hundred. The unit is named after the Norwegian oceanographer Harald Sverdrup.

The Antarctic Circumpolar Current, the only current that flows all the way around the globe. Driven by the relentless westerly winds of the Southern Ocean, it links the Atlantic, Pacific and Indian Oceans into one connected system and rings Antarctica in cold water. Its transport through Drake Passage was long quoted at about 134 Sv (Whitworth & Peterson, 1985); a 2007–2011 mooring array measured a larger 173 Sv (Donohue et al., 2016). Either way, it dwarfs every other current on the planet.

Enormously. They set regional climates — warm currents make coasts mild and wet, cold currents make them cool and often foggy and dry (the cold Benguela current is why the Namib coast is a desert). Their upwelling feeds the world's biggest fisheries: the cold Humboldt Current off Peru supports the single largest fishery on Earth. They move heat that drives monsoons and steers storms, and they are a key part of the planet's climate system. The currents you see here are the surface face of that circulation; deeper down, the same water feeds the slow global loop of the great ocean conveyor.

The currents are drawn along their real routes, but as clean schematic arrows rather than exact measured streamlines — a real current wanders, splits and shifts with the seasons. The figures attached to each one (its transport in sverdrups, its speed) are sourced, dated values from published oceanography, shown when you tap it. For the actual moving ocean, the live particle-current layer animates real surface-current data, and the Argo floats show measured temperature and salinity with depth.