Climate variability means the alternation between the "normal climate" and a different, but recurrent, set of climatic conditions over a given region of the world. In the Latin American region, climatic variability is related, inter alia, to the Southern Oscillation (SO) and the El Ni�o phenomenon (EN). Studies on the SO effects, using an SO Index (SOI), have shown its connection with pressure, temperature, and rainfall, as well as with hydrometeorological anomalies (e.g., record river discharges and lake levels) (Aceituno, 1987).
Prominent among SO-related anomalies in South America is the well-documented tendency for anomalously-wet conditions along the otherwise arid coast of northern Peru and southern Ecuador during El Ni�o episodes (Rasmuson and Carpenter, 1982). The relationship between the SO and rainfall anomalies in northeastern Brazil has long been recognized (Walker, 1928; Doberitz, 1969; Caviedes, 1973; Hastenrath, 1976; Kousky et al., 1984). Rainfall anomalies related to the SO in extratropical South America have been documented primarily for central Chile (Rubin, 1955). A tendency for precipitation in subtropical Chile to be exceptionally abundant during El Ni�o years has been noted. This relationship is consistent with the significant negative correlation between pressure differences (Tahiti minus Darwin) and annual precipitation in central Chile (Pittock, 1980) and snow accumulation in the southern Andes (Cerveny et al., 1987). Rainfall in the Central America-Caribbean domain also is related to the SO, as revealed by the tendency for drought conditions to occur during warm episodes off the Peruvian coast (Hastenrath, 1976).
The strong El Ni�o episode in 1982-83, which coincided with a marked negative SO phase, was associated with extreme climatic conditions in various parts of South America. Examples include the convection regime associated with flooding in northern Peru (Horel and Cornejo-Garrido, 1986), droughts in northeastern Brazil (Rao et al., 1986), and the remarkable precipitation and circulation anomalies over South America (Nobre and De Oliveira, 1986; Minetti and Vargas, 1995).
During recent decades, the influence of the ENSO phenomenon on the interannual variability of weather and climate in South America has been the research subject of other authors (Berlage, 1966; Burgos et al., 1991; Santiba�ez and Uribe, 1994; Vargas and Bishoff, 1995; IPCC 1996, WG I, Chapter 4). During ENSO years, precipitation in some areas of northern South America is lower (Aceituno, 1987), increasing the likelihood of drought. ENSO events also can lead to higher precipitation and air temperature, as has occurred in the coastal deserts of Peru and Chile (Caviedes, 1973). In addition, these events are related to massive fluctuations in marine ecosystems of the southern Pacific and Atlantic Oceans, with adverse socioeconomic consequences for commercial fishing and fishmeal production in Chile, Peru, Brazil, and Argentina (Pauly and Tsukayama, 1987; Pauly et al., 1987; Pauley et al., 1989; Ya�ez, 1991; Bakun, 1993; Sharp and McLain, 1993).
Records of precipitation in countries of the Central American isthmus show an important reduction in precipitation during the ENSO period, particularly along the Pacific watershed. This reduction has considerable effects on the most important economic activities and sectors in these countries (Campos et al., 1996).
It is unclear whether the ENSO phenomenon would change with long-term global warming-and what the consequences would be of overlapping sources of climatic variability on the Earth's systems. However, ENSO effects have been used by regional scientists to help define real scenarios that could be useful as analogs for climate change and for studies of potential responses of the countries and sectors affected (Campos et al., 1996).
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