62
-----7 JOms-J
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l
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-----7 3 ms-J
Figure 21: As in Fig. 19, but for rainfall and surface wind. Rainfall estimates are based
on the microwave sounding unit, as described in Spencer (1993). The threshold for the
shading is 15 cm of rainfall per month in the composites and +4 and +8 cm per month
per standard deviation of the cold tongue index in the regression map.
1987). This positive feedback loop contributes to the large temporal variability of the climate of the equatorial Pacific. The slackening of the trade
winds in the central Pacific during the warm phase of the ENSO cycle is
dynamically consistent with the weakening of the east-west pressure gradient in that region. It induces a relaxation of the east-west slope of the
equatorial thermocline, leading to a warming of the surface waters on the
eastern side of the basin, where the thermocline deepens.
The dramatic increase in rainfall over the cold tongue region during
the warm phase of the ENSO cycle can be understood as follows. Convective rainfall tends to be suppressed over regions of the tropics where
SST is colder than 27 C because the air within the atmospheric planetary