chapter - Atmospheric and Oceanic Science

egime occurred during the periods of 1925-1946 and 1977-1999 and the CPDO
regime occurred during the periods of 1912-1924 and 1947-1976.
ENSO-related monthly anomaly patterns for the rainfall, SST and SLP are
obtained using the composite technique. The SST composites are done in the 60ºN-
30ºS band and the SLP composites are done in the 20ºN-80ºS band. The anomalies
for all variables are relative to monthly means of the years that exclude (from the
1912-1999 period) the onset years of the ENSO extremes for the months from July
to December and the following years for the months from January to June. So,
monthly mans are obtained from 49 years of the 1912-1999 period with close to normal
conditions relative to ENSO phenomenon. Monthly anomalies are computed for
the SST and SLP time series at each grid point. Monthly precipitation anomaly time
series are standardized by the standard deviation of the anomaly time series.
Monthly mean anomalies of precipitation, SST and SLP for EN and LN years
stratified according to the PDO phases are calculated separately. These composites
are obtained for each month of the period from September (0) to August (+) for rainfall
anomalies, every other month of this same period for the SST anomalies and for each
month of the period from October (0) to March (+) for SLP. The symbols ' (0) ' and ' (+) ' after
the month refer to the onset and the following years of the ENSO extremes, respectively.
The statistical significance of the composites is assessed assuming that the
number of degrees of freedom is the number of events and using the Student-t tests
(Press et al. 1986). The confidence level of 95% is used.
14.3. Precipitation patterns
Low-frequency variability
The sequential EN-related monthly mean precipitation anomaly patterns for
the WPDO regime shows a robust feature in SSA during the period from October (0)
to February (+) (Fig. 14.1). Significant anomalies are observed in southern Brazil and
in central Chile (positive) and in small areas to the North (negative) in October (0) .
Gradually, the positive anomalies intensify and expand, occupying most of SSA in
December (0) . At this time, the negative anomalies intensify and extend over southern
Peru and over eastern Brazil. Significant positive anomalies remain over northeastern
Argentina and Paraguay, whereas the rest of the study domain shows nonsignificant
anomalies in January (+) . This configuration is modified in such a way
that significant positive anomalies are noted in central and eastern Argentina, and
opposite sign anomalies are found in southern Peru, Bolivia, northern Chile, and
northern and northwester Argentina during February (+) . Although the magnitude of
the anomalies is quite small in March (+) , significant positive anomalies are reestablished
in southern Brazil and in part of southeaster Brazil in April (+) . These anomalies
weaken and remain in a small area of southern Brazil in May (+) . The precipitation
anomaly patterns for the subsequent months are quite disorganized without a
robust structure.
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