chapter - Atmospheric and Oceanic Science

Low-frequency variability
Fig. 14.1. Monthly El Niño-related mean standardized rainfall anomalies for the WPDO regime.
Contour interval is 0.2 standard deviation, with negative (positive) contours being dashed (continuous).
Shading encompasses values which are significant at the 90% confidence level. Red contours encompass
values which are significant at the 95% confidence level.
The sequence of the monthly mean rainfall anomaly patterns of EN composites
for the CPDO regime (Fig. 14.2) shows less robust feature than that of EN composites
for the WPDO regime (Fig. 14.1). In fact, the areas with significant positive anomalies
remain to the South of 30°S and for only three months of the sequence (November (0) ,
December (0) and March (+) . It seems that the significant negative anomalies in southern
Peru, in northern Chile and in northern Bolivia show consistent evolving features during
the period from October (0) to April (+) . EN-related precipitation anomalies for the
CPDO regime show quite small magnitudes in the subsequent months.
The monthly mean rainfall anomaly patterns of LN composite for the WPDO
regime feature well organized structures, in particular in SSA, during the period
from October (0) to February (+) (Fig. 14.3). Consistent with LN events, negative rainfall
anomalies are established in most of the study domain by October (0) . A robust
structure with significant negative rainfall anomalies occupying a large area in the
central and eastern SSA is conspicuous in November (0) . These anomalies weaken
but remain well organized during the following three months being significant in
most of SSA in December (0) , in a small area centred at (60°W; 25°S) in January (+) ,
and in most of SSA area South of 30°S in February (+) . LN-related rainfall anomalies
in SSA present small magnitudes during the period from March (+) to August (+) .
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