Like the atmosphere, ocean currents transport heat. Instead of transporting it by means of vertical cells, in which air rises, moves horizontally, and subsides again, the oceans transport it by a system of surface and deep currents.
Ocean currents have names, many of which are familiar. Most people have heard of the Gulf Stream, for example, and perhaps of the California and Labrador Currents. There are also the Kuroshio and Oyashio Currents which affect the weather in Japan, and the Peru Current which flows northward parallel to the western coast of South America carrying nutrients that sustain a vast population of plankton, fish, seals, and seabirds. During an El Niño (see the sidebar "El Niño" on page 19), the South Equatorial Current which ordinarily carries warm water away from South America and toward Asia weakens or even reverses direction. The West
Wind Drift, also known as the Antarctic Circumpolar Current, is the only ocean current that flows all the way around the world. It moves through the Southern Ocean around the continent of Antarctica, where there is no large land mass to interrupt it.
There are many currents. Their names make them easier to remember, but also obscure an important fact. Despite having individual names, the major ocean currents are not separate. They are all linked into a global pattern of currents forming a closed loop that is known as the Great Conveyor. It is shown on the map.
At intervals of between two and seven years, the weather changes across much of the Tropics and especially in southern Asia and western South America. The weather is drier than usual in Indone sia, Papua New Guinea, eastern Australia, northeastern South America, the Horn of Africa, East Africa, Madagascar, and in the northern part of the Indian subcontinent. It is wetter than usual over the
El Niño. A reversal of pressure distribution allows warm water to flow eastward.
central and eastern tropical Pacific, parts of California and the southeastern United States, eastern Argentina, central Africa, southern India, and Sri Lanka. The phenomenon has been occurring for at least 5,000 years.
The change is greatest around Christmastime— midsummer in the Southern Hemisphere, of course. That is how it earned its name of El Niño, "the Christ child," in Peru, where its effects are most dramatic. Ordinarily, the western coastal regions of South America have one of the driest climates in the world, but El Niño brings heavy rain. Farm crops flourish, but many communities rely on fishing, and the fish disappear.
Most of the time the prevailing low-level winds on either side of the equator are the trade winds, blowing from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. At high level the winds flow in the opposite direction, from west to east. This is known as the Walker circulation, in memory of Sir Gilbert Walker (1868-1958), who discovered it in 1923. Walker also discovered that air pressure is usually low over the western side of the Pacific, near Indonesia, and high on the eastern side, near South America. This pressure distribution helps drive the trade winds, and the trade winds drive the Equatorial Current that flows from east to west, carrying warm surface water toward Indonesia. The warm water accumulates around Indonesia, in a warm pool.
In some years, however, the pressure distribution changes. Pressure rises over the western Pacific and weakens in the east. The trade winds then slacken. They may cease to blow altogether or even reverse direction, so they blow from west to east instead of east to west. This causes the Equatorial Current to weaken or reverse direction. Water then begins to flow out of the warm pool, moving eastward, and the depth of warm water increases off the South American coast. This suppresses upwelling cold water in the Peru Current and deprives fish and other marine life of the nutrients in the cold water. Air moving toward South America is warmed and carries a great deal of moisture. This brings heavy rain to the coastal region. This is an El Niño.
In other years the low pressure deepens in the west, and the high pressure in the east rises. This accelerates the trade winds and Equatorial Current, increasing the rainfall over southern Asia and the dry conditions along the South American coast. This is called La Niña. The periodical change in pressure distribution is known as the Southern Oscillation, and the complete cycle is an El Niño-Southern Oscillation (ENSO) event. The diagram illustrates how this happens.
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