Several studies have reported that Indonesian rainfall is poorly correlated with El Nino-Southern Oscillation (ENSO) events during the northern winter wet monsoon season. This work studies the relationship between the Nino-3 (5 deg S-5 deg N, 150 deg- 90 deg W) sea surface temperature (SST) and the Maritime Continent monsoon rainfall during 1979-2002. The study indicates that the correlations are mostly negative except in the vicinity of Sumatra and Malay Peninsula (SMP, including the western sections of Java and Borneo), where the correlations range from zero to weakly positive. The monsoon rainfall during ENSO events is influenced by a pair of anomalous Walker cells and a low-level closed circulation centered near the Philippines. East of SMP therainfall is negatively correlated with Nino-3 SST. The anomalous low-level wind over the Indian Ocean west of SMP causes rainfall to also be correlated negatively with Nino-3 SST, but rainfall over SMP is sheltered from this effect because of the high mountains along its western coast. The anomalous cross-equatorial flow associated with ENSO also affects the rainfall over SMP and the area to its east differently. A variation of the cross-equatorial flow may also contribute to the SMP rainfall anomaly.The result suggests that the previously reported low correlations between Indonesian monsoon rainfall and ENSO are due in part to the averaging of rainfall in two regions with opposite characteristics. The correlation is positive for Indonesia west of 112 deg E and negative to the east. There is also an inlcrdecadal trend of increasingly negative correlations from 1950-78 to 1979-97. The correlation changes from significantly positive (at 1%) to insignificant in western Indonesia and from insignificantto significantly negative in eastern Indonesia.
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