| As a finer classification of El Niños, ENSOW were
defined as years when El Niño (EN) existed on the Peru coast, Southern
Oscillation Index SOI (Tahiti minus Darwin pressure) was negative (SO), and
Pacific SST anomalies were positive (W). Further, Unambiguous ENSOW were
defined as years when SO and W occurred in the middle of the calendar year,
while Ambiguous ENSOW were defined as years when SO and W occurred in the
earlier or later part of the calendar year (not in the middle). In contrast
with India and some other regions where Unambiguous ENSOW were associated
predominantly with droughts, in the case of South America, the association
was mixed. In Chile on the western coast and Uruguay etc. on the eastern
coast, the major effect was of excessive rains. In Argentina and central
Brazil, the effects were unclear. In Amazon, the effects were not at all
uniform, and were different (droughts or excess rains) or even absent in
regions only a few hundred kilometers away from each other. Even in
Peru-Ecuador, the effects were clear only in the coastal regions. In the
interior and in the Andes, the effects were obscure. In NE Brazil, El
Niños have been popularly known to be causing severe droughts. The fact
is that during 1871–1998, there were 52 El Niño events, out of which 31
were associated with droughts in NE Brazil, while 21 had no association. The
reason is that besides El Niños, another major factor affecting NE
Brazil is the influx of moisture from the Atlantic. In some years, warmer
Atlantic in conjunction with westward winds can bring moisture to NE Brazil,
nullifying the drought effects of El Niños. A curious feature at almost
all locations is the occurrence of extreme events (high floods or severe
droughts) in some years, apparently without any El Niño or La Niña
events. This possibility should always be borne in mind. |