Tropical Indian Ocean warming intensifies drought over South America during the dry season

Abstract In recent decades, dry-season droughts over South America have intensified under the background of global warming. While previous work has emphasized the roles of tropical Pacific and Atlantic variability in shaping South American hydroclimate, the role of tropical Indian Ocean warming remains less well constrained. In this study, we combine observational analysis together with atmospheric model experiments to show that Indian Ocean Basin Mode (IOBM) warming excites a stationary Rossby wave train that propagates into the South Pacific–South American sector, producing an anomalous upper-tropospheric high-pressure over South America and an enhanced South Atlantic anticyclone. These circulation anomalies strengthen subsidence and reduce moisture supply over the southern Amazon and central Brazil, thereby suppressing precipitation and intensifying drought. In addition, reduced cloudiness further increases surface downward shortwave radiation and near-surface warming, providing a local cloud–radiation amplification. Together, our results identify tropical Indian Ocean warming as an important and underappreciated source of dry-season predictability for South American hydroclimate and associated drought risk.