COMPARATIVE STUDY OF MONSOON AND IOD EFFECTS ON RAINFALL VARIABILITY IN NORTH AND SOUTH SUMATRA

Joko - Wiratmo, Grace Fidelia Situmorang, Rifda Amara Aulia, Abyaz - Nawfaldhiyaa

Sumatra Island exhibits distinct rainfall characteristics between its northern and southern regions due to its equatorial position. Generally, Northern Sumatra displays a bimodal rainfall pattern driven by the movement of the Intertropical Convergence Zone (ITCZ), whereas Southern Sumatra follows a monomodal pattern influenced by the Asian-Australian Monsoon system. This study aims to evaluate the respective roles of the monsoon and the Indian Ocean Dipole (IOD) on rainfall variability in both regions from 1991 to 2020. Monthly rainfall data were analyzed using Continuous Wavelet Transform (CWT) to identify dominant periodicities. Subsequently, a Butterworth Bandpass Filter was applied to isolate monsoon-related (0.85-1.15 years) and IOD-related (3-5 years) signals, while Cross Wavelet Transform (XWT) was utilized to examine the coherence between rainfall variability and the climate indices (DMI and AUSMI). Furthermore, multiple linear regression was employed to assess the relative contributions of these indices. The results indicate that the annual monsoon cycle is the primary driver of rainfall variability in both regions. Meanwhile, the IOD's influence is more pronounced at inter-annual scales. Regression analysis reveals that DMI and AUSMI contributions are negligible in Northern Sumatra (R² = 0.012) but significant in Southern Sumatra (R² = 0.356). These findings highlight a clear divergence in climatic response, with Northern Sumatra predominantly governed by equatorial dynamics, while Southern Sumatra is highly sensitive to the combined variability of the IOD and the Australian monsoon.