South America extends over a wide latitudinal range, thus encompassing a great variety of climates. South America’s broadest extent is in the equatorial zone, so that tropical conditions prevail over more than half of the continent. Elevation, particularly in the Andes, is another important climatic control.
Three principal factors control the features of South America’s climate. The first and most important of these are the subtropical high-pressure air masses over the South Atlantic and South Pacific and their seasonal shifts in position, which determine both large-scale patterns of wind circulation and the location of the rain-bearing intertropical convergence zone(ITCZ). The second is the presence of cold ocean currents along the continent’s western side, which affect both air temperatures and precipitation along the Pacific coast; on the Atlantic coast, warm currents are predominant. Finally, the orographic barrier of the Andes produces a vast rain shadow over much of the southern tier of the continent.
The South Atlantic and South Pacific high-pressure cells take the form of great semipermanent anticyclones (centres of high atmospheric pressure around which winds circulate), the positions and mean intensities of which change with the seasonal north-south migration of the Sun. The eastern part of the South Pacific anticyclone influences the climate of most of South America’s west coast, causing stable, subsiding air conditions that yield minimal precipitation. The cold Peru (Humboldt) Current, flowing northward along the coast from southern Chile to the Equator, cools and further stabilizes the Pacific air that drifts over the continent. One of the world’s driest regions, the Atacama Desert along the northern coast of Chile, results from these conditions. The east coast (north of Patagonia), by contrast, receives greater amounts of precipitation from the winds emanating from the South Atlantic, and the humidifying action of the warm air currents forms a belt of low pressure.
The ITCZ is responsible for the seasonal character of precipitation in South America’s extensive tropical wet-dry climatic zone. The trade winds of both hemispheres converge between the subtropical anticyclones of the Northern and Southern hemispheres in this low-pressure region. A migrating zone of unstable atmospheric conditions results, bringing periods of prolonged, abundant precipitation. The ITCZ follows the annual migration of the Sun and reaches its most northerly positionduring the Southern Hemisphere’s winter, which is the driest period for most of tropical South America. The southern tier of the continent is unaffected by the ITCZ and falls instead under the influence of cold fronts and the mid-latitude westerlies, which are particularly strong in the Southern Hemisphere because of the large extent of ocean area there and the unimpeded air flow this allows. As the westerlies rise over the Andes, most of their moisture is released in orographic precipitation, while on the lee side a typical rain shadow develops over the vast desert and semidesert region of Patagonia.
Vertical climatic zones are particularly well-defined in the Andes, ranging from humid to warm (tierra caliente) at the base or foothills; temperate and semi-humid (tierra templada) at mid-level; cool and dry (tierra fría) in the highlands; and cold to freezing (tierra helada) in the glacial summit areas. "South America" © Emmanuel BUCHOT, Encarta, Wikipedia
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