IDENTIFICATION OF ANOMALOUSLY WET-DRY SEASONS IN INDONESIA AND THEIR RELATION TO MOISTURE TRANSPORT ANOMALIES

• Main Author: Gammamerdianti
• Format: Theses
• Language: Indonesia
• Subjects: Ilmu kebumian
• Online Access: https://digilib.itb.ac.id/gdl/view/86304
• Institution: Institut Teknologi Bandung

As a tropical country, Indonesia tends to have highly variable rainfall intensity. Based on this rainfall intensity, the seasons in Indonesia are determined. The dry season in Indonesia occurs from June to September (JJAS). This period coincides with the Indian Summer Monsoon (ISM), during which the Indian subcontinent and surrounding areas primarily receive moisture transport from the Indo-Pacific Ocean region. However, there are specific years when the dry season experiences wet anomalies, commonly referred to as the "wet-dry season," which is widely discussed in Indonesian media. On the other hand, the Meteorology, Climatology, and Geophysics Agency (BMKG) of Indonesia officially defines seasonal rainfall variations into three categories: above normal (AN), normal (N), and below normal (BN). However, there is no clear definition of wet-dry season events based on the AN criteria for dry-season rainfall. This study aims to clearly define wet-dry season events by applying the AN, N, and BN criteria to seasonal rainfall variations using grid-based rainfall data. Moisture convergence anomalies associated with years identified as wet-dry seasons are also analyzed. The variability of ISM rainfall is associated with the negative phase of the El Niño Southern Oscillation (ENSO), namely La Niña. However, the relationship between La Niña and dry season rainfall anomalies in Indonesia has not been further studied. Additionally, La Niña events are known to be classified into two main types: Central Pacific (CP) and Eastern Pacific (EP). However, another type of La Niña, known as La Niña Mix, is a combination of EP and CP types in the NINO 3.4 area. These types of La Niña have different relationships with climate anomalies in each monsoon region. The research results show that over 30 years (1993 to 2022), eight wet-dry season events were identified, occurring in 1995, 1996, 1998, 2010, 2013, 2016, 2021, and 2022. Identification of wet-dry season events based on the negative phase of ENSO (La Niña), as generally defined using the standard index (NINO 3.4), explained only 37.5% or just three out of the eight wet-dry season events. However, when the definitions of Eastern Pacific (EP), Central Pacific (CP), and mixed (Mix) types of La Niña were used, 62.5% of the events could be linked to ENSO in its negative phase or La Niña, namely five out of the eight wet-dry season events in 1995, 1998, 2010, 2013, and 2022. The wet-dry season events are characterized by a distinctive dipole moisture convergence pattern, particularly over Indonesia and South Asia. The Moisture Convergence Dipole Indices (MCDI) developed for these regions show the evolution of anomalies that may indicate early signs of wet-dry season events in Indonesia. The MCDI indicates that moisture convergence in Indonesia and India is in the same phase when the negative phase of ENSO or La Niña influences the wet-dry season in Indonesia. This also suggeststhat the wet-dry season in Indonesia coincides with the Indian Summer Monsoon (ISM), which occurs from June to September (JJAS). More research is needed to understand the wet-dry season events that cannot be directly linked to La Niña.