If you've ever lost your hat or a pair of sunglasses in the ocean, then you know that the sea doesn't stay still. If you didn't retrieve the lost item immediately, it was likely well on its way to the other side of the world, carried away by ocean currents.
When speaking of water, the word current refers to the motion of the water. Currents are found in rivers, ponds, marshes and even swimming pools. Few bodies of water have the intricate system of currents that oceans do, though. Ranging from predictable tidal currents to fickle rip currents, ocean currents may be driven by tides, winds or differences in density. They profoundly affect the weather, marine transportation and the cycling of nutrients.
How exactly? Among other things, ocean currents are responsible for the warmer temperatures in Western Europe, they enable the Antarctic to support vast amounts of plant and animal life and their disruption likely caused a mass extinction of 95 percent of all marine life 250 million years ago [source: NOAA: "Ocean"]. One type of ocean current even continually empties oceans into one another and essentially flips the water in them upside down every 1,000 years [source: NOAA: "Ocean"].
Knowledge of ocean currents is essential to the shipping and fishing industries and is helpful for search-and-rescue operations, hazardous material cleanups and recreational swimming and boating. Using a combination of predicted and real-time measurements of current patterns, boaters can safely dock and undock boats, rescuers can determine where a missing person may drift, cleanup crews can anticipate where spills might go and surfers can position themselves to catch the perfect wave.
Whether you want to learn more about local currents, like the ones that pull you out to sea when you visit the beach, or the global currents that circumnavigate the globe, this article will answer all of your basic questions about ocean currents. What causes them? What forms do they take? How do they affect ecosystems? On the next page, you'll learn about currents that take place at the ocean's surface.
Ocean Current Types: Surface Currents
Ocean currents that occur at 328 feet (100 meters) deep or above usually are classified as surface currents. Surface currents, which include coastal currents and surface ocean currents, are driven primarily by winds.
You're likely familiar with coastal currents if you've ever gone to the beach. These surface currents also affect wave and land formations. In order to better understand coastal currents, it helps to first understand waves.
As winds blow across the ocean, they pull on the water's surface, and the buildup of energy forms waves. The speed of the wind, the distance it blows and the length of time that it blows all affect the size of waves. If the wind blows fast, for a long time and for a long distance in the same direction, large waves form. Waves break when their bases encounter the sea floor and they become unstable, toppling over onto the shore.
The energy released when waves break on the beach creates longshore currents. When waves approach the beach at an angle rather than head on, part of the wave's energy is directed perpendicular to the shore and part of it is directed parallel to the shore. The parallel energy generates the longshore current, which runs along the shoreline. If you've ever been swimming in the ocean and felt the ocean tugging you farther down the shore, then you've felt the impact of a longshore current.
As these currents travel, they pick up sediment and transport it down the beach in a process known as longshore drift. Longshore drift can form long, narrow outcroppings of land called spits, as well as barrier islands, long islands located parallel to the coast. Barrier islands constantly change as longshore currents keep picking up, moving and redepositing sand.
Rip currents are another type of coastal current that form where underwater land formations prevent waves from flowing straight back out to sea. You've probably seen signs posted at the beach, warning of rip currents. They result from spent waves (or waves that have already crashed) funneling out of a narrow opening, like a break in a sandbar, with great force. Imagine the great volume of water that rushes out of the tub when you open the small drain, and you get the general idea of a rip current. You can learn all about rip currents in "How Rip Currents Work."
Yet another type of coastal current called upwelling occurs when winds displace surface water by blowing it away and deeper water rises up to replace it. The opposite process, downwelling, occurs when wind blows surface water towards a barrier, like the coastline, and the resulting accumulation of water forces the water on top to sink. Both of these processes can occur in the open ocean as well.
Upwelling and downwelling are crucial to the cycling of nutrients in the ocean. The cold, deeper layers of water are rich in nutrients and carbon dioxide, while the warmer surface waters are rich in oxygen. When the layers trade places, the nutrients and gases do too.
Downwelling prevents dissolved oxygen from being used for the decay of organic matter at the surface, which could lead to a bloom of anaerobic bacteria and a buildup of toxic hydrogen sulfide. Upwelling, meanwhile, enables ecosystems to flourish where they otherwise would not. The influx of nutrients from deeper colder waters nourishes a wide variety of life in unlikely places, such as Antarctica.
While coastal currents are caused by local winds, surface currents in the open ocean originate from global wind patterns. On the next page, you'll learn about these currents.
More Surface Currents
As you've probably gathered by now, wind and water are inseparable. To understand surface ocean currents, which, as their name suggests, occur in the open ocean, you should know a little about the winds that fuel them.
Some of these wind patterns are caused by the Coriolis force. If the earth didn't rotate, wind would travel the globe in straight lines. Instead, the spin of the earth causes winds to seemingly curve to the right in the Northern Hemisphere and the left in the Southern Hemisphere. This curvature of the winds is known as the Coriolis effect.
In the Northern Hemisphere, that means that the strong trade winds that originate in the northeast and blow westward pull the surface of the ocean along with them near the equator. Thanks to the coastline and the Coriolis effect, the warm-water current then heads north, turning at about 30 degrees north latitude. The westerlies take over then, completing the circuit. Blowing from the west, these winds guide the current eastward and south after they hit land. The two wind patterns create a continual circular pattern of wind flowing clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
These circular wind patterns create spiral ocean currents called gyres. Five major gyres flow both north and south of the equator: the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean gyres. Smaller gyres also exist at the poles, and one circulates around Antarctica. Short-lasting, smaller currents often spin off both small and large gyres.
The Gulf Stream, a particularly strong current that is part of the North Atlantic gyre, carries warm water north from the Gulf of Mexico up the coast of the eastern United States and over to western Europe. As a result, lucky Floridians living on the state's east coast are cooler in summer and warmer in winter than surrounding areas, and Western Europe is much warmer than other areas at the same latitude.
If winds affect only the upper 100 meters (328 feet) of water, how are deeper ocean currents formed? Find out on the next page.
Deep Ocean Currents (Global Conveyor Belt)
Invisible to us terrestrial creatures, an underwater current circles the globe with a force 16 times as strong as all the world's rivers combined [source: NOAA: "Ocean"]. This deep-water current is known as the global conveyor belt and is driven by density differences in the water. Water movements driven by differences in density are also known as thermohaline circulation because water density depends on its temperature (thermo) and salinity (haline).
Density refers to an object's mass per unit volume, or how compact it is. A heavy, compact bowling ball is obviously going to be denser than an air-filled beach ball. With water, colder and saltier equals denser.
At the earth's poles, when water freezes, the salt doesn't necessarily freeze with it, so a large volume of dense cold, salt water is left behind. When this dense water sinks to the ocean floor, more water moves in to replace it, creating a current. The new water also gets cold and sinks, continuing the cycle. Incredibly, this process drives a current of water around the globe.
The global conveyor belt begins with the cold water near the North Pole and heads south between South America and Africa toward Antarctica, partly directed by the landmasses it encounters. In Antarctica, it gets recharged with more cold water and then splits in two directions -- one section heads to the Indian Ocean and the other to the Pacific Ocean. As the two sections near the equator, they warm up and rise to the surface in what you may remember as upwelling. When they can't go any farther, the two sections loop back to the South Atlantic Ocean and finally back to the North Atlantic Ocean, where the cycle starts again.
The global conveyor belt moves much more slowly than surface currents -- a few centimeters per second, compared to tens or hundreds of centimeters per second. Scientists estimate that it takes one section of the belt 1,000 years to complete one full circuit of the globe. However slow it is, though, it moves a vast amount of water -- more than 100 times the flow of the Amazon River. [source: NOAA: "Currents"].
The global conveyor belt is crucial to the base of the world's food chain. As it transports water around the globe, it enriches carbon dioxide-poor, nutrient-depleted surface waters by carrying them through the ocean's deeper layers where those elements are abundant. The nutrients and carbon dioxide from the bottom layers that are distributed through the upper layers enable the growth of algae and seaweed that ultimately support all forms of life. The belt also helps to regulate temperatures.
Read on to learn about a current that isn't caused by winds or density differences but by forces that are out of this world.
Tidal currents, as their name suggests, are generated by tides. Tides are essentially long, slow waves created by the gravitational pull of the moon, and to a lesser degree, the sun, on the earth's surface. Since the moon is so much closer to the earth than the sun, its pull has more influence on the tides.
The moon's gravitational pull forces the ocean to bulge outwards on opposite sides of the earth, which causes a rise in the water level in places that are aligned with the moon and a decrease in water levels halfway between those two places. This rise in water level is accompanied by a horizontal movement of water called the tidal current.
Tidal currents differ from the currents previously mentioned in that they don't quite flow as a continuous stream. They also switch directions every time the tide transitions between high and low. Although tides and tidal currents don't have much impact in the open oceans, they can create a rapid current of up to 15.5 miles (25 kilometers) per hour when they flow in and out of narrower areas like bays, estuaries and harbors [source: Skinner]. Fast tidal currents toss sediment around and affect plant and animal life. Currents may, for example, transfer a fish's eggs from an estuary out into the open sea or carry nutrients that the fish needs from the sea into the estuary.
The strongest tidal currents occur at or around the peak of high and low tides. When the tide is rising and the flow of the current is directed towards the shore, the tidal current is called the flood current, and when the tide is receding and the current is directed back out to sea, it is called the ebb current. Because the relative positions of the moon, sun and earth change at a known rate, tidal currents are predictable.
Currents, whether tidal, surface or deep ocean, profoundly affect the world as we know it. To learn more about the complex systems that drive ocean currents, dive into the links on the next page.
Related HowStuffWorks Articles
More Great Links
- MSN Encarta Online Encyclopedia. "Amazon (river)." 2007. (April 22, 2008) http://encarta.msn.com/encyclopedia_761571466/Amazon_(river).html
- NOAA's National Ocean Service. "Currents." March 25, 2008. (April 15, 2008) http://oceanservice.noaa.gov/education/kits/currents/welcome.html
- NOAA's National Ocean Service. "Ocean Currents." March 25, 2008. (April 15, 2008)http://learningdemo.com/noaa/
- NOAA's National Ocean Service. "Tides and Water Levels." March 25, 2008. (April 18, 2008)http://oceanservice.noaa.gov/education/kits/tides/
- Osher Map Library. "The Gulf Stream." University Southern Maine. 2001. (April 18, 2008) http://www.usm.maine.edu/maps/lessons/nr05b.htm
- Skinner, Brian J. and Stephen C. Porter. "The Dynamic Earth." John Wiley & Sons. 1989.