Spatial coherence and predictability of Indonesian wet season rainfall

Rainfall from 63 stations across Indonesia is examined for the period 1950- 98 to determine the spatial coherence of wet season anomalies. An example o f almost unrelated anomalies at two neighboring stations is presented. Prin cipal component analysis is used to quantify the spatial coherence across t he entire region. The significant components show high loadings over only a small region, suggesting that rainfall in only this small region varies co herently on an interannual timescale. Correlation with the Southern Oscilla tion index (SOI) shows that rainfall over only this same region is being la rgely governed by the El Nino-Southern Oscillation (ENSO) phenomenon. In co ntrast, a similar analysis for the transition season (Sep-Nov) rainfall sho ws coherence across almost the entire region and a similarly large area of high correlation with the SOI. Results for all seasons are summarized with the use of an all-Indonesian rainfall index constructed from an averaged pe rcentile ranking of seasonal rainfall from each station across the region. At the times of the year when a large (small) percentage of the variance of rainfall is described by the lowest-order principal components, there is a large (small) correlation between the SOI and the all-Indonesian rainfall index. The implication is that wet season rainfall in Indonesia is inherent ly unpredictable.