Your turn
Each lab is one part of the video. Tap and slide, and see what happens.
Each lab isolates one mechanism. They are simplified teaching sketches: sped up and not to scale.
Tap the buttons and slide the sliders.
Drop a box of toys where the ship lost them. Where do they wash up?
Release drifters at the spill site (near the date line, ~45°N) and watch the North Pacific circulation carry them.
Tap to drop the toys in the sea. Where do they go?
Toy model: speeds and shapes are simplified.
Turn Earth's spin off and on. What happens to the loop?
Wind stress alone vs. wind stress plus rotation: see the basin-wide gyre (with a fast western boundary current) form.
Stop the Earth spinning. Does the water still go round?
Freeze sea ice, then add meltwater. When does the water sink best?
Brine rejection makes dense water that sinks; a freshwater cap weakens the overturning.
Make ice. Then add melted water. What sinks?
Toy model, not to scale.
Switch the currents off. Where does the heat pile up?
Remove the circulation and compare the pole-to-equator temperature contrast (a rule-of-thumb picture, not data).
A simplified picture, not measured temperatures.
Slide the offshore wind up. What happens to the plankton?
Offshore wind drives coastal upwelling of nutrient-rich water; watch the bloom respond.
Slide the wind. Watch the tiny plankton and fish come.
Tap anywhere on the ocean to drop a toy, then follow it.
Seed drifters anywhere and trace the gyres, the North Atlantic Drift and the Antarctic Circumpolar Current.
Tap the sea to drop a toy. Follow it!
Toy model: speeds and shapes are simplified.
The sea is always moving. Big rivers of water flow all around the world. We call them currents.
One stormy day, a ship lost a box of toys. Toy ducks, frogs and turtles floated away. The currents carried them far, far away, to beaches on the other side of the sea!
The wind pushes the top of the sea, like when you blow on your soup. Our Earth spins, and the spin bends the water, so it goes round and round in a giant circle. Wind makes air move, too: see what makes the wind.
Near the North and South Poles, the sea is icy cold and salty. Cold, salty water is heavy, so it sinks. Then it crawls along the sea floor all around the world. (Why is the sea salty? Find out in why the ocean is salty.)
Ocean currents are huge, steady flows of seawater that follow set paths, a bit like rivers inside the ocean.1011 Three main things drive them:12
Earth's rotation and the shape of the continents and seafloor steer all of these currents.114 Storms and undersea earthquakes can also set off short, powerful currents.1
On 10 January 1992, a storm in the North Pacific knocked shipping containers off a cargo ship sailing from Asia to the United States. One container burst open and spilled 28,800 plastic bath toys: yellow ducks, blue turtles, red beavers and green frogs.2526 In November 1992, about ten months later, the first toys washed up near Sitka, Alaska, about 3,200 km (2,000 miles) from the spill.2527
Oceanographers Curtis Ebbesmeyer and James Ingraham used the toys as free science tools. Every toy a beachcomber reported was a data point they could use to test a computer model of the North Pacific's surface currents.2528 Some toys circled the North Pacific. Others drifted north through the Bering Strait and froze into Arctic sea ice. Years later, toys were reported on the Atlantic side, in Maine and the UK.2527 (Not every sighting was a real 1992 toy, but the big picture held up: the oceans are all connected.)
When wind blows across the sea, friction drags the top layer of water along.12 Winds drive currents in roughly the top 100 metres (330 feet) of the ocean. Below that, the wind's push fades away.59
The winds that matter most are the steady ones that blow almost all year. The trade winds blow toward the equator from the east, and sailing ships have used them for centuries to cross between Europe and the Americas. Farther from the equator, the westerlies blow the other way.7 Read more in what causes wind.
Earth's rotation bends anything moving over long distances, including water, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect.612 Put it together: the trade winds push water west, the spin turns it toward the pole, the westerlies push it back east, and the continents block it. The water ends up going round and round an ocean basin in a gyre.11
Deep currents flow thousands of metres below the surface, far from any wind. They are driven by density, meaning how heavy a given amount of water is. Colder water and saltier water are both heavier.516
This worldwide loop is called the global conveyor belt. It crawls along at just a few centimetres per second, and a single cubic metre of water takes about 1,000 years to go all the way round.1315 Even so, it moves more than 100 times the flow of the Amazon River.13 Salt is a big part of the story: find out why the ocean is salty.
| Type | Driven by | Where | Typical speed |
|---|---|---|---|
| Surface currents | Wind, steered by Earth's spin and the continents | Top ~100 m, across whole oceans | Tens to hundreds of cm/s; Gulf Stream averages ~6.4 km/h (4 mph)413 |
| Deep (thermohaline) currents | Differences in temperature and salt | From the surface to the seafloor, worldwide | A few cm/s13 |
| Tidal currents | The rise and fall of the tides | Coasts, bays and estuaries | Can reach 8 knots or more in places2 |
| Upwelling | Wind pushing surface water away | Some coasts and the open ocean | Slow vertical flow22 |
Currents are different from waves. A wave moves energy through the water while the water itself mostly goes round in circles. A current actually carries the water, and anything floating in it, from place to place. See what causes waves.
The tropics get about twice as much sunlight as the poles.20 Ocean currents, together with winds, move a huge amount of that heat toward the poles: warm water flows poleward and cold water flows back toward the equator.220
The heat map in the explainer is a simplified picture that shows the idea, not measured temperatures. Speeds and shapes in all the toys are simplified too.
Upwelling feeds the ocean. Along some coasts, the wind pushes surface water out to sea, and colder water rises from below to replace it. That deep water is rich in nutrients, which "fertilize" the sunlit surface.22 Tiny drifting plants (phytoplankton) bloom, and they are the base of a food chain that runs through small fish to bigger fish, seabirds and whales. That's why good fishing grounds are usually found where upwelling is common.2213
When upwelling stops, life suffers. During El Niño, upwelling off South America weakens or stops, fewer phytoplankton grow, and that affects the fish that eat them and everything that eats fish.21 Places where cold and warm currents meet are often crowded with life, and fishing fleets use current maps to find schools of fish.14
Animals ride currents, too. Currents carry food to animals that stay in one place, and carry eggs and young animals to new homes.11 Young loggerhead sea turtles spend their first 7 to 15 years out in the open ocean, and some cross the whole Pacific.23 Baby eels hatch in the Sargasso Sea and drift on the Gulf Stream to the coasts of America and Europe.24 Find out more in how do eels reproduce.
Three main things: wind, which drives currents at and near the surface; differences in water density caused by temperature and salt (thermohaline circulation), which drive deep currents; and tides, which make tidal currents near coasts. Earth's spin and the shape of the continents steer where the currents go.
Differences in density. Near Greenland and Antarctica, seawater gets very cold, and when sea ice forms it leaves its salt behind. The cold, salty water is heavier than the water around it, so it sinks and spreads along the seafloor. This drives the slow global conveyor belt.
Surface currents form when steady winds drag the top layer of the sea along. Earth's rotation bends the moving water (the Coriolis effect), and continents block it, so the water loops around whole ocean basins in giant circles called gyres. Deep currents form where cold, salty water sinks.
They carry heat. Warm currents move heat from the tropics toward the poles and cold currents flow back. The Gulf Stream helps give northwest Europe milder winters than places at the same latitude across the Atlantic (scientists still debate how much of that is the ocean and how much is the atmosphere). Changes like El Niño shift rain and storms around the world.
Currents bring food. Where wind pushes surface water away from a coast, cold, nutrient-rich water rises from below (upwelling). The nutrients feed plankton, which feed fish, seabirds and whales, so many of the best fishing grounds are in upwelling zones. Currents also carry fish eggs, baby eels and young sea turtles to new places.
Heat would stay where the Sun delivers it, so the tropics would be hotter and the high latitudes colder. Deep nutrients would rise far less often, so there would be less plankton and fewer fish. Scientists study even a partial slowdown of the Atlantic currents because it could change weather patterns.
The fastest non-tidal currents are the narrow 'western boundary currents' on the west side of the big gyres, such as the Gulf Stream in the North Atlantic, the Kuroshio off Japan and the Agulhas off southern Africa. They can flow faster than five miles per hour. Some tidal currents near coasts are even faster.