The extremes of the Southern Oscillation are partly responsible for large portions of climate variability at interannual scales in Latin America. Therefore, some of the variations in the foregoing elements could be associated with manifestations of climate variability, such as the El Niño phenomenon, which represents the low phase of the Southern Oscillation; the positive phase is referred to as La Niña. Atmospheric circulation patterns are more perturbed during El Niño than during La Niña years (Salles and Compagnucci, 1995, 1997).
In Mexico and parts of the Caribbean, the ENSO signal corresponds to more winter precipitation and less summer precipitation (Magaña and Quintanar, 1997). Some of the most severe droughts in Mexico in recent decades have occurred during ENSO summers (Magaña et al., 1998). The signal of La Niña is almost opposite to the ENSO signal. In Central America, orographic effects play an important role in understanding regional ENSO effects in precipitation. During El Niño years, the Pacific side of Central America suffers an important reduction in precipitation, whereas some parts of the Caribbean side experience more rain than usual.
Over Colombia, ENSO events are associated with reductions in precipitation, river streamflows, and soil moisture, whereas La Niña is associated with heavier precipitation and floods (Poveda and Mesa, 1997). There also is a very high positive correlation between the Southern Oscillation Index (SOI) and river discharge in Colombia. This relationship is stronger during the December-January period and weaker during April-May. The influence of ENSO is stronger at river stations located in western Colombia and weaker for stations located in eastern Colombia. Over the eastern part of the Andes, Ecuador, and northern Peru, large positive anomalies in precipitation typically are observed during the warm episode.
Dry anomalous conditions affect the Amazon region of Brazil northward to the Caribbean through the latter half of the year (Ropelewski and Halpert, 1987, 1989, 1996; Díaz and Kiladis, 1992). In northern Amazonia and northeast Brazil, deficient rainy seasons have been observed during ENSO years (Aceituno, 1988; Marengo, 1992; Uvo, 1998). Droughts that led to forest fires were detected during the very strong ENSO events of 1911-1912, 1925-1926, 1982-1983, and 1997-1998. Extreme droughts also occurred during these years in northeast Brazil. In contrast, the ENSO signal in southern Brazil is opposite to that in northeast Brazil and northern Amazonia, with positive and sometimes extremely large anomalies of rainfall during the rainy season of ENSO years, whereas drought can occur during the positive Southern Oscillation phase (Ropelewski and Halpert, 1989; Grimm et al., 1996, 2000).
Through northern and central Chile and at high altitudes of the Andes in Argentina, between 30°S and 40°S, most precipitation is recorded during the winter, with positive anomalies registered during early stages of the warm phase of ENSO. Because of the semi-arid conditions of this area, their economy is strongly affected (Quinn and Neal, 1982; Compagnucci, 1991; Ruttland and Fuenzalida, 1991; Canziani et al., 1997; Compagnucci and Vargas, 1998). At the same time, strong rainfall events occur in low altitudes of Chile, triggering debris flows during the winter such as those in Santiago and its surrounding areas in 1991-1993 (Garreaud and Ruttland, 1996) and 1997.
At high altitudes of the Andes, large amounts of snow are recorded consistently. Melting of this accumulated snow is the main cause of river runoff during the summer. In Chile and central-western Argentina, north of 40°S, streamflows were normal or above normal during El Niño years (Waylen and Caviedes, 1990; Compagnucci and Vargas, 1998; Compagnucci, 2000). On the other hand, during cold events (La Niña), negative anomalies of rainfall and snowfall are presentwith opposite consequences, including below-normal summer streamflow. For this region, the likelihood of dry conditions during La Niña is higher than that of wet conditions during El Niño (Compagnucci and Vargas, 1998)
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