14.1.2.1.4. Variability and impacts from El Niño and the Southern Oscillation
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
present—with 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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