The austral summer monsoon onset and post-onset rainfall and their associated low-level winds are analyzed during the August-February season over Indonesia from 1979 to 2006 using surface and satellite products as well as reanalyses and regional climate model simulations. Onset date is defined using a local agronomic definition. Its leading empirical orthogonal function is found to exhibit a regional-scale spatially-coherent signal across "monsoonal" Indonesia, i.e. mostly south of the Equator, with an asymmetric temporal behavior, such that delayed onsets are more intense than early ones. Associated anomalies in rainfall tend to weaken quickly after mid-to-late November or early December, especially over islands, while they tend to persist over ocean. This weakening is shown to be associated with the evolution of distinct weather types revealed by a kmeans cluster analysis. In particular, late onsets—usually related to warm El Niño Southern Oscillation (ENSO) events—are found to be accompanied by an increased prevalence of a weather type characterized by weak low-level daily-averaged winds across monsoonal Indonesia and increased (decreased) rainfall over most of the island orography and southern and western coasts (seas). The regional model simulations provide evidence that this land-sea rainfall contrast could be associated with an enhanced diurnal sea-land breeze circulation.
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