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

This study assesses the individual and combined effects of the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on South America’s 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 found strong agreement across methodologies for positive phases. Pure positive IOD events are associated with positive precipitation anomalies over south-eastern South America (SESA) and negative anomalies to the north, along with strong positive temperature anomalies in both regions. WhenEl Niño, the positive phase of ENSO, and a positive IOD occur simultaneously, the IOD intensifies and modulates the characteristic El Niñosignal over the region. Specifically, destructive interference betweenthe effects of El Niñoand the IOD atprecipitation and temperature during the austral spring across central ~20°S in central-eastern South America confines positive precipitation anomalies to SESA, reinforces negative anomalies over north-eastern Brazil, and strengthens positive temperature anomalies. These modulations scale with event intensity, where the stronger large-scale climate driver dominates the signal. By contrast, the ability to draw robust conclusions is constrained as the influence of their negative phases yields inconsistent results across methods, combined with non-linear behaviour and the limited number of observed events. Accounting for the IOD is essential for accurately assessing ENSO effects over South America and highlights the value of multi-method approaches for robust climate characterisation.