Ocean currents

The ocean never stands still.

Its water moves in big streams called currents. Currents are like rivers in the sea. They can carry things a very long way.

The ocean is never still. Currents are movements of water from one place to another, and some are so big and steady that they have names, like the Gulf Stream. NOAA, the U.S. ocean agency, explains that currents are driven by three main things: the rise and fall of the tides, the wind, and differences in how warm and how salty the water is.

Currents don't just move water. They carry heat, tiny living things, people's boats and goods, and sadly pollution and litter as well.

NOAA defines ocean currents as the movement of water from one location to another, usually measured in meters per second or knots (1 knot is 1.85 km/h). It identifies three main drivers. Tides create tidal currents, strongest near shore and in bays and estuaries, regular enough to predict and in places faster than eight knots. Winds drive currents at or near the surface, locally along coasts and across whole basins in the open ocean. Thermohaline circulation, driven by density differences from temperature and salinity, moves water at deep and shallow levels, much more slowly.

Currents carry more than water: NOAA notes they move people and goods as well as pollution and debris, which is why it collects data on tides and currents for navigation and search and rescue.

Wind blows over the sea. It pushes the top of the water.

The moving water starts a current. Big winds that always blow the same way make big currents.

When wind blows across the sea, it drags the surface water along with it. NOAA explains that winds drive currents in the top 100 meters or so of the ocean. Steady winds that blow in the same direction for much of the year, such as the trade winds near the equator, push water across whole oceans.

The trade winds got their name because sailing ships used them to travel between Europe and the Americas. Farther from the equator, winds called the westerlies blow the other way.

Wind drives currents in roughly the upper 100 meters of the ocean. NOAA traces the global wind pattern to atmospheric circulation: warm air rises at the equator, flows poleward, is deflected, and descends near 30 degrees latitude, returning toward the equator as the trade winds, which meet in the calm Intertropical Convergence Zone (the doldrums) between about 5 degrees north and 5 degrees south. Air that continues poleward forms the westerlies.

The transfer of motion downward is not simple. In the Ekman spiral, first theorized by Vagn Walfrid Ekman in 1902, each layer of water is dragged by the one above, moving more slowly and deflected further, until motion ceases at about 100 meters (330 feet).

Earth is spinning. That makes moving air and water curve.

In the north, they curve to the right. In the south, they curve to the left.

If Earth did not spin, air would simply flow back and forth between the poles and the equator. But Earth does spin, so moving air and water are bent off course. NOAA explains that they curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect.

It is named after Gaspard Gustave de Coriolis, a French mathematician who studied energy in spinning machines such as waterwheels. The Coriolis effect is one reason currents travel in curves instead of straight lines.

NOAA's explanation of the Coriolis effect starts from a thought experiment: on a non-rotating Earth, the atmosphere would circulate in a simple back-and-forth pattern between high pressure at the poles and low pressure at the equator. Because Earth rotates, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, producing curved paths.

The effect is named after Gaspard Gustave de Coriolis (1792–1843), who studied the transfer of energy in rotating systems like waterwheels. It shapes both the prevailing winds that drive surface currents and the water those winds set moving, which is deflected in the same sense, as the Ekman spiral shows.

In the big oceans, currents go round and round in giant loops.

These loops are called gyres. There are five big ones.

Put the winds and the Coriolis effect together, and the big surface currents join up into huge circling systems called gyres. NOAA lists five major gyres: the North Pacific, South Pacific, North Atlantic, South Atlantic and Indian Ocean gyres.

Each gyre has a strong, narrow current on its western side and a weaker, broader current on its eastern side. Sometimes people use the word gyre to mean the patches where plastic litter collects, but NOAA notes that the word really means the big, rotating currents themselves.

NOAA identifies five major subtropical gyres: North and South Pacific, North and South Atlantic, and Indian Ocean. Each is flanked by a strong, narrow western boundary current and a weak, broad eastern boundary current. The larger, sustained currents within them, such as the Gulf Stream, have proper names.

NOAA cautions that 'gyre' is increasingly used for the concentrations of plastic and other debris found in parts of the ocean, but the term traditionally refers to the large rotating current systems. NASA's 'Perpetual Ocean' visualization, built from the MIT/JPL ECCO2 ocean model, shows the surface currents and swirling eddies of June 2005 to December 2007.

The Gulf Stream is a warm current in the Atlantic Ocean.

It carries warm water from near Florida all the way across the ocean toward Europe.

The Gulf Stream is a strong, warm current in the North Atlantic. NOAA explains that it flows north along the coast of Florida, turns east off North Carolina and heads northeast across the Atlantic. Its average speed is about 6.4 kilometers per hour, and it carries more water than all the world's rivers put together.

It carries warmth too. NOAA says the Gulf Stream brings milder winter weather to Bergen, in Norway, than to New York, even though New York is much farther south.

The Gulf Stream is the western boundary current of the North Atlantic gyre, paired with the eastern boundary Canary Current. NOAA describes it flowing north along Florida, turning east off North Carolina and crossing the Atlantic to the northeast, the main conveyor of heat from south to north in the Atlantic. Its maximum speed, near the surface, is typically about 9 km/h; its average is about 6.4 km/h (4 mph), slowing to about 1.6 km/h as it widens northward. It transports more water than all the world's rivers combined.

NOAA credits it with milder winters in Bergen, Norway, than in New York, much farther south, and with warmer winters and cooler summers on Florida's east coast than in other southeastern states.

Deep down, there are slow currents too.

Near the poles, very cold, salty water sinks to the bottom. Then it creeps slowly all around the world.

Not all currents are near the surface. Near the poles, seawater gets very cold, and when sea ice forms, the salt is left behind in the water. Cold, salty water is heavy, or dense, so it sinks toward the bottom.

NOAA explains that in the far North Atlantic, cooled water sinks and flows slowly south along the deep ocean floor, past the equator all the way to Antarctica, and eventually rises back to the surface. Together with the surface currents, it makes a giant loop around the planet called the global ocean conveyor belt.

Below the wind-driven layer, NOAA explains, currents are driven by density, controlled by temperature (thermo) and salinity (haline): thermohaline circulation. In polar regions, sea ice forms and leaves salt behind, so the surrounding water becomes saltier and denser and sinks; surface water flows in to replace it and in turn sinks, setting the deep currents moving.

NOAA describes the global ocean conveyor belt as starting in the Norwegian Sea, where warm Gulf Stream water loses heat to the cold atmosphere, cools, becomes denser and sinks. The cold bottom water flows south past the equator to Antarctica and eventually returns to the surface through mixing and wind-driven upwelling, completing a loop that encircles the globe.