Orographic Rainfall Mechanism During MJO Over Sumatra Island

Orographic enhancement affects the formation and distribution of rainfall in mountainous regions. Moreover, large-scale phenomena, such as the Madden–Julian Oscillation (MJO), influence the rainfall variability in mountainous areas. In this study, we investigated the interaction between those multiscale prosecutors, orography, and MJO, in modulating diurnal rainfall variability over the mountainous region of West Sumatra. We utilized MJO indices, the Convective Available Potential Energy (CAPE), and Convective Inhibition (CINH) data spanning 21 years (2001–2022) at three locations surrounding the mountains to explore their role in rainfall variability. These three locations vary in elevation: the windward side (L1), the mountaintop (L2), and the leeward side (L3). Our findings indicate that elevation influences rainfall, leading to varying rainfall intensities across the three locations. Rainfall was higher over L2 than over L1 and L2, except during the night when it was higher over L1 than L2. Furthermore, we observed that enhanced rainfall over L2 coincided with lower CAPE and CINH values, while reduced rainfall over L3 occurred when CAPE and CINH values were higher. Moreover, the results also showed that the effect of the MJO on rainfall depended on the phase of the MJO. MJO-3 has tended to increase precipitation over all locations, while MJO-4 reduced precipitation, particularly during the day. Analyzing the correlation between the MJO and the CAPE, we discovered that the MJO Phase 3 (MJO-3) reduced CAPE, and this reduction was more pronounced in Phase 4 (MJO-4), except over L3. Similarly, investigating the correlation between the MJO and the CINH, we observed that MJO-3 reduced CINH, while MJO-4 increased CINH. Hence, the reduction (increase) of CINH during MJO-3 (MJO-4) might explain the increase (decrease) of rainfall. The results could be important in understanding the effects of MJO-topography interaction on rainfall variability over the region.