Interactions between ENSO, Monsoon and Diurnal Cycle in Rainfall Variability over Java, Indonesia

Using a high-resolution regional climate model RegCM3, station and satellite observations, we have studied the spatial heterogeneity of climate variability over Java Island, Indonesia. Besides the well-known anomalous dry conditions that characterize the dry and transition seasons during an El Niño year, analysis of regional model output reveals a wet mountainous south versus dry northern plains in precipitation anomalies associated with El Niño over Java during the peak rainy season. Modeling experiments indicate that this mountains-plains contrast is caused by the interaction of the El Niñoinduced monsoonal wind anomalies and the island/mountain induced local diurnal cycle of winds and precipitation. During the wet season of El Niño years, anomalous southeasterly winds over the Indonesian region oppose the climatological northwesterly monsoon, thus reducing the strength of the monsoon winds over Java. This weakening is found to amplify the local diurnal cycle of land-sea breezes and mountain-valley winds, producing more rainfall over the mountains, which are located closer to the south coast than to the north coast. Therefore, the variability of the diurnal cycle associated with this local spatial asymmetry of topography is the underlying cause for the heterogeneous pattern of wet south/dry north rainfall anomalies in El Niño years. It is further shown that the mean southeasterly wind anomalies in December to February of El Niño years result from more frequent occurrence of a quiescent monsoon weather type, during which the strengthened sea-breeze and valley-breeze convergence leads to above normal rainfall over the mountains.