MONSOON ONSET AND ITS RELATION WITH THE COMMENCE OF RAINY SEASON IN JAVA ISLAND

• Main Author: Azizah, Siti
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/71397
• Institution: Institut Teknologi Bandung

Monsoon onset is a part of monsoon circulation phase that marks the beginning of active monsoon period. The date of monsoon onset is not yet operationally defined in Indonesia. Instead, the Indonesian Meteorological Agency (BMKG) defines the commencement of rainy season as the first three decad in a row, each having 50 mm rainfall. This work attempted to find a practical but consistent definition of monsoon onset and its relationship with the commencement of rainy season as defined by BMKG. Herein, the monsoon onset date is defined by modified criteria of Australian Monsoon Rainfall Index (AMRI), which prescribes the occurrence of onset when the 5-day running mean of rainfall exceeds the value of 150% Mean Annual Cycle (MAC) and the difference between 150% MAC and MAC itself exceeds 1 mm. Using this definition, the onset date of monsoon over Java Island can be consistently determined in every year. The decads, in which monsoon onset date was defined, were then compared with the commencement of rainy season from BMKG’s definition. It is found that, climatologically, monsoon onset leads the commencement of rainy season by 2 decads in eastern, and 3 and 6 decads in the central and most of the western parts of Java Island. However, the 6-decad lead time of monsoon onset in some areas of the western part of Java Island seems to represent a pseudo-onset due to the strong effects of local process such as sea-breeze convergence. This could be the shortcoming of the aforementioned modified AMRI method, which should be carefully considered before using it for operational purposes.