Spatial Coherence and Predictability of Indonesian Wet Season Rainfall

Abstract

Rainfall from 63 stations across Indonesia is examined for the period 1950–98 to determine the spatial coherence of wet season anomalies. An example of almost unrelated anomalies at two neighboring stations is presented. Principal component analysis is used to quantify the spatial coherence across the entire region. The significant components show high loadings over only a small region, suggesting that rainfall in only this small region varies coherently on an interannual timescale. Correlation with the Southern Oscillation index (SOI) shows that rainfall over only this same region is being largely governed by the El Niño–Southern Oscillation (ENSO) phenomenon. In contrast, a similar analysis for the transition season (Sep–Nov) rainfall shows 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 percentile ranking of seasonal rainfal... Abstract Rainfall from 63 stations across Indonesia is examined for the period 1950–98 to determine the spatial coherence of wet season anomalies. An example of almost unrelated anomalies at two neighboring stations is presented. Principal component analysis is used to quantify the spatial coherence across the entire region. The significant components show high loadings over only a small region, suggesting that rainfall in only this small region varies coherently on an interannual timescale. Correlation with the Southern Oscillation index (SOI) shows that rainfall over only this same region is being largely governed by the El Niño–Southern Oscillation (ENSO) phenomenon. In contrast, a similar analysis for the transition season (Sep–Nov) rainfall shows 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 percentile ranking of seasonal rainfal...