INTERACTION OF CO-OCCURING EL NIÑO, DIPOLE MODE PHENOMENA AND INDIAN SUMMER MONSOON WITH THE RAINFALL VARIABILITY OVER THE NORTHERN PART OF SUMATERA

• Main Author: WORO BUDIATI HARIJONO, SRI
• Format: Dissertations
• Language: Indonesia
• Subjects: Ilmu kebumian
• Online Access: https://digilib.itb.ac.id/gdl/view/11438
• Institution: Institut Teknologi Bandung

Equatorial atmospheric dynamics over the Indonesia Maritime Continent is strongly influenced by the zonal and meridional circulations, in addition to local circulations. The zonal circulations is closely related to EN dan DM events, while the meridional circulation is closely related to lateral monsoon especially the Indian Monsoon (MI) and the Australian Monsoon. Meanwhile the availability of water vapor as well as local circulations play important roles in the local convection process, function as the major mechanism for the tropical rain formation. Contrary to common understanding from previous researchers on the impact of El Nino (EN) and the impact of the Indian Dipole Mode (DM), which both influence in reducing rainfall, the observed data show normal rainfall over the north region of western Indonesia amount during simultaneous EN and positive DM occurrences. Results of previous studies indicate that the impact of EN will be reduced to the western part and the impact of DM will reduce the rainfall over south Indonesia, lead to the selection of the least studied area of northern part of the western Indonesia. Behaviors and roles of EN, DM and IM concurrently are not well understood yet. Based on this fact the focus of this study will be on the concurrent interaction of all three phenomena against rainfall in the northern part of the western Indonesia. Results of previous study identified several impacts of EN and DM against the Indonesian rainfall that reach their maximum during JJA, will be the basis of study's focus over that same period. The availability of the water vapor mass as well as zonal and meridional circulations as the ascending motion toward the northern part of Sumatera are described on the basis of NCEP (National Centers for Environmental Prediction), model re-analysis data and displayed spatially. Spatial distribution maps of correlation between EN and rainfall as well as between DM and rainfall shows inhomogeneity in correlations among the study area especially northern Sumatera and other areas of Indonesia. From the precipitable water, cloud cover and outgoing longwave radiation distribution as well as wind direction profile, the water vapor mass in the surrounding daerah of the northern part of Sumatera on JJA is available. On the other hand, the zonal-meridional circulation clearly indicates ascending motion toward the study area of northern part of Sumatera. During the normal years (without EN and DM), the Indian Summer monsoon circulation includes a number of dynamic structural elements, among which are: the monsoon trough over northern India, the Mascarene anticyclone system, the low level cross equatorial jet, the Tibetan high pressure system, the tropical easterly jet and monsoon cloudiness. Results of wavelet transformation show that the prominent amplitudes of both EN and DM phenomena in their specific frequencies are not in accordance to those of rainfall in the northern part of Sumatera especially in Aceh and Medan. The multivariable analyses are used to investigate the contributions of the three phenomena, i.e. EN, DM and IM, with an assumption that they are independent to each other. The independencies among them are studied through wavelet analyses. The results of multivariable analyses highlights that the EN impact brings increase of rainfall homogenously over the northern part of the western Indonesia by about 12.49%, the positive DM increase rainfall by about 21.76% and IM increase by about 10.52%. Synthesis of analyses results of the atmospheric dynamics based on reanalyzes data and statistical analyses through wavelet transformation and multivariable analyses provides understanding that interaction among EN, DM and IM is the principal cause of rainfall normality in the northern part of the western Indonesia during JJA even through EN and positive DM events occur concurrently. Compare to EN, DM contributes dominantly to the increase of rainfall in the study area.