Regional variation of the influence of cross-equatorial northerly surge towards diurnal cycle of rainfall over Java Island

Abstract

Cross-equatorial northerly surge (CENS) is known to cause torrential rainfall over Java Island in the Maritime Continent. Some studies indicated that the rainfall increase is attributed to changes in the diurnal cycle of rainfall (DCR) in northwestern Java. It is not well understood whether the DCR changes are present in the other parts of Java Island and whether the changes are similar to those in northwestern Java. This study performs climatological analyses to investigate the regional variation of the influence of the CENS on the DCR over Java Island using IMERG and ERA5 data sets of 20 years of boreal winter. We find that there are differences in intensity, timing, and coverage of DCR responses over western, central, and eastern Java. CENS modifies the timing of coastal rainfall on the northern coast, resulting in early morning rainfall enhancement owing to the convergence between the incoming northerlies and land breeze. A small increase in early morning rainfall is observed in eastern Java, possibly due to the northerly flow gaining more zonal components on the eastern coast, which then results in relatively weaker convergence than the western coast. The timing of early morning rainfall appears to be slightly different between the three areas. CENS is also found to suppress afternoon rainfall inland because of increasing static stability due to cold air advection from the north. The suppression varies from western to eastern Java, with the largest suppression seen in western Java. Relatively narrower and wider suppression areas are found in central and eastern Java, respectively. These differences are attributed to the topography characteristics of Java.