Impacto de Episódios El Niño e La Niña sobre a Freqüência de ...

ABSTRACT
This study examines how El Niño (EN) e La Niña (LN) episodes modify the frequency
and the intensity of extreme precipitation events in South America, and the reason for
this modification. Extreme events are those with a three-day average percentile
above 90. The difference between the mean frequencies of extreme events for EN
and normal years, and for LN and normal years show that EN and LN episodes
influence significantly the frequency of extreme events in several regions in South
America during certain periods. The relationships between large-scale atmospheric
perturbations and variations in the frequency of extreme events are analyzed for
seven regions, where significant differences are found. The general features of the
anomalous atmospheric fields during extreme events are similar, no matter if they
occur during EN or LN episodes or in normal years. They are compared with
composites of monthly atmospheric anomalous fields during those episodes. The
frequency of extreme events increases (decreases) when large-scale perturbations
associated with EN and LN favor (hamper) the circulation anomalies associated with
these events. The correlation analysis of sea surface temperature with monthly total
precipitation and with the frequency of extreme events in the focused areas shows
that the variations in monthly rainfall and extreme events are associated with ENSO.
The frequency distributions of daily rainfall and extreme events for the different
categories of years are compared, in order to verify changes in these distributions
during EN and LN with respect to neutral years. ENSO influence seems to be
stronger on the categories of more intense daily precipitation. This is an important
aspect, since the most dramatic consequences of climate variability are brought
about by its influence on extreme events. The analysis of the EN and LN influence on
the frequency of extreme streamflow events in Brazil shows a picture that is coherent
with the influence on extreme rainfall events and with the delay time between rainfall
and streamflow.
Key words: extremes events of precipitation and streamflow, El Niño and La Niña.