Pacific and Atlantic teleconnections reduce uncertainty in multidecadal projections of the South American Summer Monsoon

Reliable projections of the South American Summer Monsoon (SASM) are critical for managing regional hydroclimatic risks, yet remain highly uncertain due to internal climate variability. Here, we reconstruct a robust historical SASM index ensemble from 1850 CE onward by integrating high-resolution paleoclimate proxies (tree rings and ice cores), historical documents, and instrumental observations. We further analyze future changes using large ensembles from the CESM2 and CanESM5 climate models. Our results demonstrate that multidecadal variability in the SASM is primarily driven by the Interdecadal Pacific Oscillation (IPO) and the associated changes in the Pacific Walker Circulation (PWC), whereas the influence of the tropical Atlantic sea surface temperature (SST) gradient is comparatively minor. By constraining these key Pacific modes, we reduce the uncertainty in projected SASM intensity by approximately 30%, highlighting their dominant role in shaping near-term monsoon trajectories. This study underscores the importance of improved simulation and representation of Pacific variability for advancing hydroclimate projections and informing climate adaptation strategies in tropical South America.