Understanding Interannual Variations of the Local Rainy Season over the Southwest Indian Ocean

Located at the southern boundary of the tropical rainfall belt within the South Africa monsoon regime, Rodrigues Island, ~2500 km east of East Africa, is ideally located to investigate climatic changes over the southwest Indian Ocean (SWIO). In this study, we investigate the climatic controls of its modern interannual rainfall variability in terms of teleconnection and local effects. We find that increased rainfall over the SWIO tends to occur in association with anomalously warm (cold) SSTs over the equatorial central Pacific (Maritime Continent), resembling the central Pacific El Niño, closely linked with the Victoria mode in the North Pacific. Our analyses show that the low-level convergence induced by warm SST over the equatorial central Pacific leads to anomalous low-level divergence over the Maritime Continent and convergence over a large area surrounding the Rodrigues Island, which leads to increased rainfall over the SWIO during the rainy season. Meanwhile, the excited Rossby wave along the tropical Indian Ocean transports more water vapor from the tropical convergence zone into the SWIO via intensified northwest wind. Furthermore, positive feedback induced by the Rossby wave response to the increased rainfall in the region contributes to the large interannual variations over the SWIO.