Ocean Circulation and Currents

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Ocean Circulation and Currents

Introduction

Ocean circulation is the large, connected system of water movements in the oceans. It involves both surface and deep currents. Surface currents are generated by wind and an uneven distribution of heat between equatorial regions and regions farther north and south of the tropics. The deeper currents are produced by the sinking of colder water and the upwelling of warmer water. The global ocean circulation system transfers heat from low to higher latitudes, making the oceans responsible for about 40% of the global heat transport. This is a vital determinant of the global climate. Disruptions in the global ocean currents, which could occur as polar ice melts and the freshwater is added to the ocean, could markedly change Earth's climate.

Historical Background and Scientific Foundations

Ocean currents make up about 10% of the volume of ocean waters. The currents are essentially rivers of water flowing within the surrounding ocean. These movements of water form a continuing pattern that is global in scale.

The Gulf Stream is one example of a warm ocean current. This current, which flows past the eastern coast of North America before arcing over to flow in a southward direction past the United Kingdom and Europe, originates as a current in the Pacific Ocean. Britain experiences a much more temperate climate than regions at a similar northern latitude, such as parts of the United States and Canada, because of these warm waters. Ultimately, this current flows back to the Pacific Ocean, and the cyclical global route of the current is repeated.

The Gulf Stream is part of the global system of ocean currents that is known as the Ocean Conveyor. The flow of water in the conveyor mixes colder waters nearer to the polar regions with the warmer waters in the mid-latitudes and equatorial regions. The warmer water from equatorial regions rises to the surface, while the colder polar water sinks to the bottom. The colder water then moves to more tropical latitudes, where it warms and rises upward. It is this continuous cycling of water that drives the formation of currents.

Currents also develop closer to the surface of the ocean because of prevailing winds—winds that blow predictably. The friction between the moving air and the water molecules pushes the water in the direction of the wind, piling the water up similar to the piling up of water as waves form. Gravity acts to pull the piled water downward. If Earth did not rotate, the water would flow in a straight line, but as Earth rotates on its axis, a force called the Coriolis force rotates the water to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere. The results are gyres—large mounds of water with a flow of water around them.

Gyres produce currents in the Northern and Southern Hemispheres of the Atlantic and Pacific Oceans. The Gulf Stream is part of the North Atlantic Gyre.

The clockwise rotating gyre in the northern part of the Atlantic Ocean consists of the warm North Equatorial Current flowing near the equator, joining the western boundary of the warm Gulf Stream, which turns toward the east and becomes the warm North Atlantic Drift, while the cold Labrador Current, West Greenland Drift, and East Greenland Drift return cold water from the North. The eastern boundary, the southward Canary Current close to West Africa, also brings cold water toward the equator, closing the North Atlantic Ocean gyre. In the southern part of the Atlantic Ocean, in the counterclockwise gyre, the warm South Equatorial Current joins the warm Brazil Current going south toward Antarctica at the east coast of South America, meeting the cold Falkland Current; the cold, West Wind Drift heads east around Antarctica. The gyre is closed by the cold Benguela Current going north at the southwestern

coast of Africa, while the eastward flowing warm Equatorial Countercurrent connects the gyres from the North and South Atlantic Oceans.

The system of currents in the Pacific Ocean is very similar to the Atlantic currents. The clockwise rotating gyre in the northern part of the Pacific Ocean consists of the warm North Equatorial Current flowing westward north of the equator, joining the western boundary warm Kuroshio Current, which turns toward the East and becomes the warm North Pacific Drift, while the cold Oyashio Current returns cold water from the North. At the coast of Alaska loops the warm Alaska Current. The eastern boundary California Current, close to the coast of California, also brings cold water toward the equator, closing the North Atlantic Ocean gyre. This is where upwelling, the rising of cold water replacing surface water that drifts away due to the winds, occurs. Although it brings cold water, low clouds, and sometimes fog in summer, the nutrient-rich, cold water helps the fishing industry. South of this gyre, but still in the Northern Hemisphere, the eastward flowing warm North Equatorial Countercurrent can be found. In the southern part of the Pacific Ocean, the warm, westward South Equatorial Current flows opposite of the eastward warm South Equatorial Countercurrent, and continues toward the South at the east coast of Australia. While the West Wind Drift is heading east around Antarctica, the cold Peru or Humboldt Current going northward at the southwestern coast of South America closes this gyre.

The system of surface ocean currents is simpler in the Indian Ocean, because instead of two figure eight-shaped gyres, only one is present. The gyre is influenced by seasonally changing winds. It consists of the warm North Equatorial Current, the South Equatorial Countercurrent, the South Equatorial Current, and the eastward, circumpolar current around Antarctica, the cold West Wind Drift.

Impacts and Issues

The global movement of ocean water is crucial to establishing and maintaining Earth's climate. Put another way, disruption in the global pattern of ocean circulation could cause marked climate changes.

A number of computer models that simulate the ocean and climate have predicted that the addition of large amounts of freshwater to the North Atlantic Ocean would disrupt the northward flow of the warm Gulf Stream water. The melting of the ice caps in the Arctic and Greenland, which has been documented through the 1990s and which is continuing into 2007, is adding freshwater to this region of the ocean.

In their Fourth Assessment Report published in 2007, the Intergovernmental Panel on Climate Change (IPCC) wrote that “warming of the climate system is unequivocal.” Part of the evidence for the group's conclusion is the widespread melting of snow and ice (including the polar ice) and rising sea level.

WORDS TO KNOW

CORIOLIS FORCE: The apparent tendency of a freely moving particle to swing to one side when its motion is referred to a set of axes that is itself rotating in space, such as Earth. The acceleration is perpendicular to the direction of the speed of the article relative to Earth's surface and is directed to the right in the Northern Hemisphere. Winds are affected by rotation of Earth so that instead of a wind blowing in the direction it starts, it turns to the right of that direction in the Northern Hemisphere; left in the Southern Hemisphere.

GULF STREAM: A warm, swift ocean current that flows along the coast of the Eastern United States and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.

GYRE: A zone of spirally circulating oceanic water that tends to retain floating materials, as in the Sargasso Sea of the Atlantic Ocean.

UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Sub-surface water of greater density and lower temperature replaces the surface water and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling.

IN CONTEXT: WARMING AND SEA LEVEL RISE TO CONTINUE

“Anthropogenic warming and sea level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.”

“Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere.”

Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

A scientific paper published in 2005 in the journal Nature reported a 30% decrease in the northern flow of the warm Gulf Stream as compared to surveys done in the late 1950s, early 1980s, and early 1990s. The significance of these results is being debated as of 2007. Some climate scientists argue that the change was temporary and not related to climate change. Other scientists argue that, even if temporary, the current alteration is an early sign of a climate shift.

BIBLIOGRAPHY

Books

Bigg, Grant R. The Oceans and Climate. Cambridge: Cambridge University Press, 2004.

Desonie, Dana. Oceans: How We Use the Seas. New York: Chelsea House Publications, 2007.

Fujita, Rodney. Heal the Ocean: Solutions for Saving Our Seas. Gabriola Island, British Columbia, Canada: New Society Publishers, 1998.

Roberts, Callum. The Unnatural History of the Sea. Washington, DC: Island Press, 2007.

Web Sites

Barry, Patrick L. “A Chilling Possibility.” National Aeronautics and Space Administration (NASA), March 5, 2004. <http://science.nasa.gov/headlines/y2004/05mar_arctic.htm> (accessed December 1, 2007).

Agnes Galambosi

Climate Change: In Context