The Effect of Local Forcing on Anomalously High Rainfall during Dry Season in Java, Indonesia

Abstract

During the dry season (May to October) in Java, Indonesia, anomalously high rainfall is investigated using 37-year rainfall data from the Climate Hazards Group InfraRed Precipitation with Station data. The analysis focuses on the years having high rainfall during the dry season between 1982 and 2019. It is conducted using a combination of the presence and absence of La Niña, negative Indian Ocean Dipole Mode events, and other atmospheric/oceanic parameters, such as 2-m temperature, sea surface temperature, outgoing longwave radiation, 200 mb and 850 mb wind. The results show that the presence of both La Niña and negative Indian Ocean Dipole Mode events contributes around 39% to the high rainfall during the dry season, the presence of negative Indian Ocean Dipole Mode - 22%, the absence of both events - 22%, and the presence of La Niña - 17%. The dynamics of monsoon circulation anomaly (200 mb and 850 mb) in the southern Indian Ocean off the coast of Sumatra and Java also plays a role in the increased rainfall during the dry season in Java. This anomaly occurs due to a vortex in the southern equatorial Indian Ocean around 10⁰S, triggering the formation of double Inter-tropical Convergence Zones over the area north of the equator and the southern waters of Java. The increase in rainfall due to this local factor reaches a maximum and extends in June and October, which is associated with the strengthening of circulation anomalies in southern Java, both spatially and vertically (850 and 200 mb).