Handout (3.2 MB)
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.