Owing to the wide latitudinal coverage of the continent, a large variety of climates occurs in South America. Although much of the continental mass is located within the intertropical belt, large regions of Chile, Argentina, and Uruguay lay in the Southern Temperate Zone, while the southern tip of the continent actually extends into sub-Antarctic latitudes. The near-surface wind circulation, which determines the general water vapor transport, is dominated by the following:
1. The low pressure belt at the Intertropical Convergence Zone (ITCZ).
2. The semi-permanent anticyclones of the South Atlantic and South Pacific Oceans (approximately at 30°S).
3. The sub polar low pressure belt at approximately 60°S.
4. The low pressure center in the central-western part of Argentina and southern Bolivia.
As the position of the ITCZ oscillates seasonally, there is a north-south displacement of the South Atlantic and Pacific highs, which conditions the annual cycle of precipitations over the whole region.
In the Equatorial belt (roughly between 10N and 10S), the trade winds from both hemispheres (NE-SW or SE-NW, in the North and South Hemisphere, respectively) converge in the ITCZ and generate an upward motion of the air. This process becomes locally intensified in tropical storms resulting in heavy rains. The rainfall regime is driven by convection with a bimodal distribution and maximum values coincident with the equinoxes (i.e., March-April; September-October). Constant high temperatures and humidity, and the absence of a dry season are also characteristic of this region.
Below about 10°S appears a wide belt of arid land, the so-called arid diagonal of South America. This arid region runs in a roughly NW-SE direction from the Peruvian and northern Chilean Pacific coast, down to the Patagonian Atlantic coast in southern Argentina (yellow area in the precipitation maps of Figure 1(b)). Lack of moisture, the most salient feature of this area, results from several factors, including the cold Humboldt Current in the Pacific coast and mountain rain shadows.
Above the arid diagonal, the sources of water vapor originate in the Atlantic Ocean and the tropical rainforest. In general, the amount of precipitation decreases along an east-west gradient, although precipitation associated with mountain ranges is responsible for local rain maxima in the inner continent (i.e., parts of Bolivia and northern Argentina). The seasonal precipitation pattern has typical monsoon features, with maximum precipitation during the austral summer.
Below the arid diagonal, the predominant winds blow from the Pacific Ocean. The westerly winds discharge most of their moisture on the western slope of the Andes (Chile), causing a sharp precipitation gradient from about 4000mmyear_1 on the western slope of the Patagonian Andes to less than 250 mmyear-1 in the Atlantic Patagonian coast. Precipitation peaks during winter due to the northward movement of the Pacific high, the intensification of the subpolar low, and the thermal difference between oceanic and land masses. This seasonal pattern is opposite to the one described for the region above the arid diagonal.
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