Spatial Coherence and Seasonal Predictability of Monsoon Onset Over Indonesia

Abstract

The seasonal potential predictability of monsoon onset during the August–December season over Indonesia is 20 studied through analysis of the spatial coherence of daily station rainfall and gridded pentad precipitation data from 21 1979 to 2005. Onset date, defined using a local agronomic definition, exhibits a seasonal northwest-to-southeast 22 progression from northern and central Sumatera (late August) to Timor (mid December). South of the equator, 23 interannual variability of the onset date is shown to consist of a spatially-coherent large-scale component, together 24 with local-scale noise. The high spatial coherence of onset is similar to that of the September–December seasonal 25 total, while post-onset amounts averaged over 15–90 days and September–December amount residuals from large26 scale onset show much less spatial coherence, especially across the main islands of monsoonal Indonesia. The 27 cumulative rainfall anomalies exhibit also their largest amplitudes before or near the onset date. This implies that 28 seasonal potential predictability over monsoonal Indonesia during the first part of the austral summer monsoon 29 season is largely associated with monsoon onset, and that there is much less predictability within the rainy season 30 itself. A cross-validated canonical correlation analysis using July sea surface temperatures over Tropical Pacific and 31 Indian Oceans (80°–280°E, 20°S–20°N) as predictors of local-scale onset dates exhibits promising hindcast skill 32 (anomaly correlation of ~0.80 for the spatial average of standardized rain gauges and ~0.70 for standardized gridded 33 pentad precipitation data).