Impact of IOD Phases on Rainfall Intensity Along Sumatra and Java Coasts: Analysis of 2023 Patterns and Correlations

The Indian Ocean Dipole (IOD) is a climate phenomenon that significantly influences weather patterns, particularly rainfall, in the Indian Ocean. Characterized by differences in sea surface temperatures (SST) between the western and eastern Indian Ocean, the IOD has two phases: positive and negative. During a positive IOD phase, the western Indian Ocean becomes warmer while the eastern part, near Sumatra and Java Island, cools down. This temperature gradient affects atmospheric circulation and leads to variations in weather patterns, including rainfall intensity (Saji et al., 1999).

The impact of the IOD on rainfall has been studied widely. Studies have shown that positive IOD phases generally lead to increased rainfall in East Africa and reduced rainfall in parts of Indonesia, including Sumatra and Java (Wainwright et al., 2020). Conversely, negative IOD phases are associated with higher rainfall in Indonesia and decreased rainfall in East Africa (Nur’utami & Hidayat, 2016). These variations can potentially lead to droughts or floods in the affected regions.

In Indonesia, the IOD’s influence on rainfall variability is particularly significant due to the country's dependence on rainfall for agriculture and water supply. Previous research by Hermawan (2018) highlighted the connection between IOD phases and rainfall patterns in Java, emphasizing the need for detailed regional studies to improve climate prediction models. Additionally, Firda (2019) demonstrated the strong correlation between IOD phases and monthly rainfall anomalies in northern coastal regions of Java, reinforcing the importance of understanding these relationships for effective climate risk management.

Despite the extensive research on IOD impacts, there remains a need for updated and region-specific analyses. This study aims to address this problem by analyzing the impact of IOD phases on rainfall intensity along the coasts of Sumatra and Java for the year 2023. By comparing observed monthly rainfall data for 2023 with the normal average rainfall from 1991 to 2020, as reported by the Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) (2021), we aim to identify significant deviations and correlations. Furthermore, we will analyze the correlation between the IOD index and rainfall during positive IOD phases at ten specific locations: Meulaboh (MBO), Sibolga (SBG), Kualatanjung (TBT), Jambi (KLT), Palembang (PLG), Panjang (BDL), Patimban (SNG), Sadeng (WNO), Rembang (RBG), and Malang (MLG).

In 2023, the IOD exhibited a predominantly positive phase from April and from June to December, while remaining neutral in January, February, March, and May. This study found that, in comparison to the normal average rainfall during IOD neutral events, rainfall intensity generally increased or remained stable along the coast of PLG, SNG, WNO, RBG, and MLG. However, a decrease in rainfall intensity was observed in WNO and MLG in March, and in BDL in January.

During the positive IOD phase, rainfall intensity generally decreased during the rainy season along the coasts of PLG, SNG, WNO, RBG, and MLG. Conversely, during the dry season, rainfall intensity in these regions tended to either remain stable or increase. Furthermore, a significant negative correlation (correlation coefficient greater than 0.5) was identified between the IOD index and rainfall intensity in KLT, PLG, SNG, WNO, and MLG. This indicates that positive IOD phases were associated with decreased rainfall intensity, which could potentially lead to drought conditions.

In contrast, the coasts of MBO and TBT experienced an increase in rainfall intensity during positive IOD phases. Additionally, the coasts of SBG, BDL, and RBG revealed a low correlation between the IOD index and rainfall intensity, indicating the influence of other factors. Lastly, the increased rainfall intensity observed during the rainy season under IOD neutral conditions is attributed to the active Madden Julian Oscillation.