chapter - Atmospheric and Oceanic Science

Low-frequency variability
Fig. 14.9. Monthly El Niño-related mean SLP anomalies for the WPDO regime for the indicated months.
Contour interval is 0.5 hPa, with negative (positive) contours being dashed (continuous). Light to dark blue
(yellow to red) shading encompasses negative (positive) values which are significant at the 95% confidence
level. The zero contours have been omitted.
On the other hand, EN-related SLP patterns for the CPDO regime show dominant
positive anomalies in the Australasian region and quite small negative anomalies
in the central and eastern Tropical Pacific (Fig. 14.10). So, the associated
anomalous Walker circulation is relatively weaker than for the previous case.
Similarly, LN-related SLP anomaly patterns for the WPDO regime feature positive
anomalies in the central and eastern equatorial Pacific and negative anomalies in
the Australasian region (Fig.14.11). These anomalies show quite small magnitudes,
that indicates almost normal Walker circulation.
LN-related SLP composites for the CPDO show significant anomalies in the
tropics with the positive values in the central and eastern Tropical Pacific and the
negative values over the Australasian and/or the Atlantic/African regions (Fig.
14.12). It is interesting to note that the areas with the largest anomalies in the
Australasian-Pacific sector which take part in the East-West pressure balance are
neither centred at the equator nor located at the same latitude. In fact, the negative
area in the Australasian region has a variable latitudinal location. It is between the
equator and 20°S from October (0) to December (0) and in March (+) , approximately in
the equator in January (+) , and between the equator and 20°N in February (+) . On the
other hand, the positive area in the central eastern Pacific is located between 20°
and 40°S approximately in the southeastern side of the South Pacific Convergence
Zone (SPCZ) in most months of the period from October (0) to March (+) . This result
implies that considerable variations might occur in the convective activity along the
SPCZ during LN years of the CPDO regime.
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