Precipitation anomalies in southern South America associated with a finer classification of El Nino and La Nina events

The relationship between rainfall in sub-regions of Uruguay and South Brazil and a finer classification of El Ninos (ENs), was studied. ENSOWs were defined as years when an EN existed on the Peruvian coast, the southern oscillation index (SOI = Tahiti minus Darwin pressure) was negative (SO), and Pacific sea-surface temperature SST) anomalies were positive (W). Further, unambiguous ENSOWs were defined as years when SO and W occurred in the middle of the calendar year, and ambiguous ENSOWs 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 ENSOWs were associated predominantly with droughts, in the case of southern South America the association was with excess rains. Among the ambiguous ENSOWs, some were associated with floods in southern South America, but some had normal or mixed rainfalls (floods in some sub-regions, droughts in others) and a few even had droughts. C events (La Ninas, i.e. no EN, SOI positive, and SST negative) were associated mostly with droughts, but occasionally with floods in southern South America. Many non-events were associated with floods or droughts, indicating that factors other than EN/La Nina could also be important. Copyright (C) 2002 Royal Meteorological Society.