A Comparison Study of Monsoon Onset and Retreat Definition in the Indonesian Region

Abstract:

Indonesian climate strongly affected by monsoon activity with the principal rainy season centred on December-February (DJF). Agricultural planning, practice and irrigation management relied on rainfall, largely affecting national food supply. Thus, the appropriate onset definition of Indonesian monsoon onset, therefore, coupled with effective dissemination of information, may allow farmers to plant crops in a timely manner, and facilitate strategic irrigation practice. The Maritime Continent of Indonesia (MCI) is a region of strong seasonal variation in wind and rainfall regimes, which consists of prevailing easterly wind and dry conditions during the boreal summer and prevailing westerly winds and wet conditions during the boreal winter. Thus, it is advantageous to use an objective and effective circulation index to define monsoon onset. Operationally, BMKG Indonesia uses dry-season onset which is defined by rainfall threshold of three consecutive dekad less than 50 mm, and wet-season onset is defined by rainfall threshold of three consecutive dekad with rainfall totals exceeding 50 mm. With this definition, a month-long period is required to declare that the monsoon has advanced over the region which is less beneficial for farmers to be on time for planting. This study will evaluate the onset of monsoon using several definitions. Several current methods of defining the onset criteria of monsoon in the region will be compared in this study. Finally, we use circulation index defined by using 850 hPa zonal wind averaged over the area (10 °S–0 °S, 75 °E–110 °E). Our study shows that the index can be used to depict the seasonal cycle (for instance the onset) and can detect the onset of the monsoon earlier in conjunction with the first dekad of the rainy season.