Various earlier studies have demonstrated that rainfall in the maritime continent - Indonesia region is strongly related to the El Nino-Southern Oscillation (ENSO) during the dry half of the year but has a very weak association with ENSO during the summer-wet season months. This relationship is investigated over a wider domain through the use of outgoing longwave radiation (OLR) data as a proxy for rainfall.

Consistent with the hypothesis of Haylock and McBride (2001), it is found that the large scale structure of the low-order Empirical Orthogonal Functions (EOFs) of OLR have a strong resemblance to the patterns of correlation between OLR and the Southern Oscillation Index. This supports the hypothesis that the predictable component of rainfall is determined by the component that is spatially coherent, as quantified through EOF analysis.

As was found earlier with rainfall, the region of largest correlation between interannual OLR anomalies and the SOI lies in the winter hemisphere. The predictable component of OLR (or rainfall) remains in the region of the maritime continent throughout the year; and so does not accompany the minimum OLR (maximum rainfall) during its annual inter-hemispheric progression as the major monsoon heat source.

The sign of the OLR-SOI relationship is such that the maritime continent has increased rainfall during a La NiÁa or cold event. Patterns of correlation between sea surface temperature and the SOI show the existence of a region to the east of the maritime continent whereby sea surface temperature anomalies are positive during these (La NiÁa) conditions. This is in the sense of a direct relationship: i.e positive sea surface temperature anomalies corresponding to increased rainfall.

The annual cycle of the sea surface temperature structure of ENSO is represented by the first EOF of the interannual sea surface temperature series for each separate calendar month. The region of sea surface temperature anomaly giving the direct relationship with maritime continent rainfall is part of the "boomerang-shaped" pattern which lies between and has the opposite sign from the anomalies in the eastern-central Pacific and the Indian Oceans. Besides being a fundamental component of the large scale sea surface temperature structure of ENSO, the boomerang pattern goes through an annual cycle such that it has maximum amplitude in the winter hemisphere. This is suggestive that interannual variations of maritime continent rainfall are in direct response to upstream sea surface temperature anomalies in the ENSO boomerang pattern.