Possible effects of anthropogenic and volcanic aerosols on the ITCZ and rainfall over South America

Tropical rainfall variability strongly affects ecosystems, agriculture and food security across South America (e.g. the Amazon, Brazilian breadbasket regions, etc.).It is influenced by mechanisms of ocean-atmospheric modes of variability, but themodes can be modulated by external forcing factors, and consequently, it is relevant to study and compare forced and unforced variability. Here, we compare theeffects of anthropogenic and volcanic aerosols on tropical South American rainfall. We use three reanalyses of the 20th and 21st centuries and contrast themwith rainfall and streamflow observations reaching back to the 1920s. We also usea paleo-reanalysis dataset family to analyse five large volcanic eruptions in thelate 18th and early 19th century. Our results show concurrent changes in nearsurface temperature, altered atmospheric mechanisms and precipitation, associated with negative radiative forcing from meridionally asymmetric and increasingtropospheric concentration. Abrupt stratospheric injections modified the land-ocean temperature contrast, weakened moisture transport and caused droughts.The analysis of meridional energy transport links aerosol forcing to changes inthe interhemispheric temperature gradient (IHTG), atmospheric energy redistribution and changes in features of global atmospheric circulation. Tropospheric aerosols progressively modified the IHTG, regional circulation and the latitudinal position of rainfall, consistent with global changes in atmospheric circulation,specifically in the location of the strong ascent of moist air (ascending branch ofthe Hadley cell). Conversely, volcanic aerosols were related to overall cooling andreductions in rainfall. We discuss these dynamics against recent studies based onatmospheric simulations, while our study uses reanalysis.