Ocean Currents

Ocean Currents

Earth’s oceans have patterns of movement known as ocean currents. There are several major factors that contribute to the direction of ocean currents: Earth’s rotation; wind friction at the surface of the water; variations in water density resulting from differences in temperature and salinity; and the effect of landmasses. Surface currents flow in several basic circulatory patterns known as gyres. There are numerous regional exceptions, with many branch and feeder currents helping circulate waters. Surface currents are classified by intensity as “streams,” the fastest at two to four miles per hour; the slower “drifts”; or the barely perceptible “creeps.” Tidal currents, a type of surface current, are shaped by tide, the alternate rise and fall of waters caused by the gravitational effect of the Moon, Sun, and, to a lesser extent, other celestial bodies. There are also flow patterns in the deep ocean, including immense underwater cascades, or cataracts, that develop when cold water near the ocean floor spills over the edge of one ocean basin into another.

The TRADE WINDS are the basic wind systems in the Atlantic, the Pacific, and the Indian Oceans in both the Northern Hemisphere and Southern Hemisphere. In the Northern Hemisphere, they circulate in a clockwise direction, while in the Southern Hemisphere they move counterclockwise. They contribute significantly to the circular motion of the oceans. Another significant factor is the Coriolis effect (or, Coriolis force or Coriolis acceleration), named for its discoverer Gaspar Coriolis, a 19th-century French civil engineer. This is an apparent force on winds, cloud, and aircraft caused by the rotation of Earth under them, such that their motion is deflected clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, although speed is unaffected. The Coriolis effect arises from the fact that, because of the higher pressures at higher altitudes of the warm air masses near the EQUATOR relative to those at the same altitudes to the north, an airflow toward the NORTH POLE and the SOUTH POLE occurs. In the Northern Hemisphere this flow is deflected by friction to the right or eastward as Earth’s surface rotates from west to east beneath it. The effect is zero at the equator and increases with increasing latitude; it is responsible for the prevailing westerly winds over North America. When winds and the Coriolis effect interact with the continents, the major circular current systems of the ocean, or gyres, are created. It is useful to remember that meteorologists name winds for the direction from which they come, and oceanographers cite the direction in which currents go.

The surface waters of the Atlantic have two large gyres, one in the North Atlantic and one in the South Atlantic. The currents of the North Atlantic, including the North Equatorial Current, the Canaries Current, and the GULF STREAM, flow in a clockwise direction, again caused by heating at the equator, which causes ocean water to move northward at the same time as it is being carried eastward by Earth’s rotation. The Gulf Stream is thought to have been slowed in the past by warming of North Atlantic waters, perhaps contributing to the ice ages. The currents of the South Atlantic, including the South Equatorial Current, Brazil Current, and Benguela Current, flow in a counterclockwise direction.

The North Pacific has a clockwise current system divided into four main sections. In the north is the North Pacific Current. This feeds the California Current, which becomes the North Equatorial Current, and then the Japan (Kuroshio) Current. The South Pacific has currents rotating in a counterclockwise direction, from the Antarctic Drift, to the Peru Current, and the South Equatorial Current, which is split into three parts by Australia and New Guinea. In its southern regions, the Indian Ocean has counterclockwise currents similar to those of the Atlantic and Pacific. These include the South Equatorial Current, the Mozambique Current, and the West Australian Current. In the north, the currents are dominated by the seasonal monsoon winds.

In the higher latitudes of the Northern and Southern Hemispheres are counter-rotating gyres, one rotating counterclockwise in Arctic regions and one rotating clockwise in Antarctic regions. The Antarctic Circumpolar Current (West Wind Drift) encircles the globe, merging the waters of the Atlantic, Pacific, and Indian Oceans without interference of land.

Just to the north of the equator, in all three oceans, there runs an eastward-flowing equatorial countercurrent. When the two westward-flowing currents from each hemisphere meet land near the equator, water pressure builds up. Some of the water flows north and some flows south. A third current is formed in the middle, encouraged by the band of upwelling air in the region known as the DOLDRUMS. The doldrums are relatively calm and, combined with the slowmoving equatorial countercurrents, have been feared by sailors for centuries.

Once a current system was understood, it could be used to great advantage by the skilled navigator. By taking a route in harmony with a strong current, the sailor can add several knots to the speed of his ship. One of Italian mariner CHRISTOPHER COLUMBUS’s accomplishments on his first transatlantic journey was using the Gulf Stream to return to Europe in 1493. The first charts to show ocean currents were drafted in about 1665.

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