How the Indian Ocean Dipole modulates ENSO effects over South America during austral spring

Abstract

This study assesses the individual and combined effects of the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on South America (SA) climate during austral spring (September to November) using a multi-method approach. By integrating partial regressions, composite analysis on reanalysis and a large ensemble of simulations from the NCEP CFSv2 model, we find strong agreement across methodologies for positive phases. Pure positive IOD events are associated with positive precipitation anomalies over Southeastern SA (SESA) and negative anomalies to the north, along with strong positive temperature anomalies in both regions. When El Niño, the positive phase of ENSO, and a positive IOD occur simultaneously, the IOD intensifies and modulates the characteristic El Niño signal over the region. Specifically, destructive interference between effects of El Niño and the IOD at around 20ºS in central-eastern SA, confines positive precipitation anomalies to SESA, reinforces negative anomalies over northeastern Brazil, and strengthens positive temperature anomalies. These modulations scale with event intensity, where the stronger large-scale climate driver dominates the signal. In contrast, the influence of their negative phases yield inconsistent results across methods, combined with nonlinear behavior and the limited number of observed events, constrain the ability to draw robust conclusions. Accounting for the IOD is essential for accurately assessing ENSO effects over SA and highlights the value of multi-method approaches for robust climate characterization.