Indian Ocean Dipole Induces Rainfall Anomalies in the South American Monsoon

As many areas of the globe, South America is under a monsoon regime, known as the South American Monsoon System (SAMS). While it is established that the El Niño Southern Oscillation (ENSO) is the main driver of interannual rainfall variability over the SAMS, not much attention was given to other modes of climate variability, for example, the Indian Ocean Dipole. Nonetheless, we find in observations that the SAMS onset date has a greater correlation with the Indian Ocean Dipole (IOD) than with ENSO. The IOD spatial pattern is defined as the second empirical orthogonal function of monthly sea surface temperature anomalies over the tropical Indian Ocean. It reaches its peak over the austral spring, when SAMS has its onset.

In this project, we use observations, perturbation experiments in an atmospheric global climate model, and multiple realizations of a fully coupled global climate model to determine how the development of the Indian Ocean Dipole influences rainfall over South America. Using the CESM Large Ensemble Simulations (LENS), we composited the dates when IOD is active and ENSO is neutral. Finally, experiments were performed using Community Atmospheric Model, version 5 (CAM5). A control run was performed with climatological SST over the globe, and the perturbation experiments were forced by the composite of the observed monthly SST anomalies in the cases of IOD positive and IOD negative. One hundred ensemble members were run for each case, between April and December.

We find that IOD negative events lead to a displacement of the ITCZ over the Atlantic Ocean and South America, especially during the early stages of the IOD development. As a consequence, there is an increase in rainfall over the Amazon and Venezuela during IOD negative, especially from April to June. During austral spring, the anomalous convection over the Indian Ocean induces a Rossby wave source and leads to a Rossby wave train. This wave train propagates along the upper level jets, reaching South America and dislocating the South Atlantic Subtropical High. As a result, there is a reduction in rainfall over the area related to the South Atlantic Convergence Zone during IOD positive both in our perturbation experiments and in LENS. During IOD negative, the experiments show a positive rainfall anomaly over Southern Brazil.