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| Currents and Circulation Patterns in the Oceans |
The oceans are in constant motion. Ocean currents are the horizontal and vertical circulation of ocean waters that produce a steady flow of water in a prevailing direction. Currents of ocean water distribute heat around the globe and help regulate Earth’s climate, even on land. Currents carry and recycle nutrients that nourish marine (ocean) and coastal plants and animals.
Human navigators depend on currents to carry their ships across the oceans. Winds drive currents of surface water. Differences in temperature and salinity (saltiness) cause water to circulate in the deep ocean. The rotation of the Earth, the shape of the seafloor, and the shapes of coastlines also determine the complex pattern of surface and deep ocean currents.
Ocean water is layered. The shallowest water, called surface water, is warmer, fresher, and lighter than deep water, which is colder, saltier, and denser. The boundary between surface and deep water is a thin layer marked by an abrupt change of temperature and salinity. This layer, called the thermocline, exists in most places in the oceans. Surface and deep water only mix in regions where specific conditions allow deep water to rise or surface water to sink.
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Many organisms swim freely across the thermocline, and the remains of plants and animals continuously rain down through the deep water to the seafloor. However, most organisms live, or at least feed, close to the ocean surface where microscopic plants called phytoplankton float freely and absorb the sunlight they need to live. Very little light penetrates the surface water.
Surface currents
Earth’s atmosphere (mass of air surrounding Earth) and oceans together form a “coupled system.” Winds drive circulation of the oceans’ thin upper layer of surface water, and temperature differences in the oceans help to generate atmospheric winds. Friction (resistance to the motion of one surface over another) between the moving air and the water surface pushes water in the direction of the blowing winds.
Earth’s eastward rotation causes currents to deflect (bend) to the right in the northern hemisphere and to the left in the southern hemisphere, a phenomenon called the Coriolis effect. Coriolis deflection causes clockwise circulation of wind-driven surface ocean currents in the northern hemisphere and counterclockwise circulation in the southern hemisphere.
Warm surface currents
The subtropical trade winds, or trades, are strong, steady winds that blow warm water from west to east on either side of the equator (an imaginary line around Earth halfway between the North and South Poles), thereby creating west-flowing equatorial currents in the major oceans.
The trades and equatorial currents push piles of warm water into the western halves of the Pacific, Indian, and Atlantic Oceans. Water flows down and away from the centers of the mounds, not unlike pancake batter spreading out on a griddle.
The trades continue to push the water to the west, and Coriolis deflection guides it northward in the northern hemisphere and southward in the southern hemisphere. Warm, fast currents, called western boundary currents, flow away from the tropical warm pools toward the poles in the western halves of the ocean basins.
The Gulf Stream is the western boundary current in the North Atlantic. It flows north along the southeastern coast of the United States and then crosses the Atlantic on a diagonal path. Warm Gulf Stream waters create unusually mild climates in northern locations. Gulf Stream waters keep Bermuda balmy, Ireland green, and England foggy. The Gulf Stream is the major shipping route from North America to Europe.
The western boundary current in the North Pacific, called Kuroshio, likewise warms the islands of Japan and carries ships toward the Pacific Northwest. Western boundary currents in the southern hemisphere, the Brazil Current in the South Atlantic, East Australian Current in the South Pacific, and Aguellas Current in the Indian Ocean flow south from the equator.
Cool surface currents
Cold surface currents carry cool water from the poles toward the equator along the west coasts of the continents. The cool eastern boundary currents are generally shallower and weaker than the western boundary currents. Cold water flowing from the Arctic Ocean at the North Pole feeds the eastern boundary currents in the northern hemisphere, namely the California Current in the Pacific and the Canary Current in the Atlantic.
The Antarctic Circumpolar Current (ACC) that encircles the ice-covered Antarctic continent supplies the cool water to the eastern boundary currents of the southern hemisphere—the Peru, Benguela, and West Antarctic Currents. The ACC is an exception to the general rule that surface currents are shallow. It extends from the sea surface to the seafloor in several places.
Gyres
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| Gyres |
Surface water circulates in oceans in massive circular patterns called gyres. The major surface currents (eastern boundary, western boundary, and equatorial current) in each ocean link to form a circle. Gyres are clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
For example, a rubber duck dropped into the ocean near San Diego might float south on the California Current to the North Equatorial Current, west across the Pacific Ocean to the Kuroshio western boundary current, and then back across the northern Pacific to British Columbia.
In a few years, the California Current might return the duck to San Diego. Ocean researches have actually conducted many such experiments. One important study tracked 29,000 plastic bathtub toys that spilled from a cargo ship in the North Pacific.
The infamous “triangle trade” between Europe, Africa, and North America in the eighteenth and nineteenth centuries relied on the North Atlantic Gyre. European slave ships arrived in Africa via the Canary Current, then carried slaves to the sugar plantations of the Caribbean on the North Equatorial Current.
Having left off slaves and picked up sugar, they rode the Gulf Stream north to the rum distilleries of New England and the liquor shops in Europe. Hurricanes that form in the tropical Atlantic ride the North Equatorial Current toward the Caribbean Islands and then follow the Gulf Stream toward the southeast coast of the United States.
Deep ocean currents
Deep ocean currents are driven by differences in temperature and salinity. They are generally unaffected by surface currents. Deep water is colder, saltier, and denser than surface water. Deep water forms in polar regions where warmer surface water cools and sinks beneath the Arctic ice cap (permanent ice covering) or Antarctic ice shelves (permanent ice large enough to cover most of a land mass).
Salinity increases near the ice caps because seawater forms freshwater ice when it freezes. The salt stays behind and the remaining liquid water becomes saltier. A “global conveyor belt” carries deep water south through the Atlantic, around Antarctica, and north into the Pacific, Indian, and Atlantic Oceans. It could take the molecules in a drop of water more than a thousand years to make a complete circuit of this global deep ocean current.
Upwellings and downwellings
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| Upwellings and downwellings |
Upwellings are common along the west coasts of the continents, particularly in regions beneath the easterly (west-blowing) trade winds. Because they bring important minerals and nutrients from the deep ocean, upwellings typically support abundant marine and coastal life. Upwellings nourish waters rich with life off Peru, California, and southwestern Africa. A divergence between wind patterns creates a zone of intense upwelling that completely surrounds Antarctica.
Downwellings are ocean zones where surface water sinks into the deep ocean. Downwellings can occur at places were winds meet or blow toward shore. However, warm water does not sink, and warm surface currents are more likely to pile water up against obstacles like coastlines and opposing currents than to force it into the deep ocean. Most deep water forms at intense Arctic and Antarctic downwellings where ice cools the seawater and freezing increases its salinity.