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Abstract

We investigate the hydroclimate impacts of the major Santa Maria (1902) and Katmai (1912) eruptions, which were among the largest eruptions in the 20^th century, in the greater tropical Atlantic region (encompassing the Atlantic Ocean, Africa, and the Americas), using recently-updated early instrumental data, historical reanalysis output, and climate models. Long-term observational datasets examined include gridded and station precipitation, tropical-Atlantic-flowing river discharge (Amazon, Congo, Paraná, Niger, and Senegal), and tropical Atlantic sea surface salinity, which provides insight into the marine surface freshwater balance (evaporation minus precipitation and surface runoff). We also study the mechanisms of hydroclimate impacts using latest-generation historical atmospheric reanalysis output and a medium-sized ensemble of prescribed volcanic-forcing experiments with the atmospheric general circulation model ECHAM6. Understanding these eruption impacts allows us to contextualize the hydroclimate impacts of better-observed large eruptions since the second half of the 20th century and better anticipate the impacts of future large eruptions. We find that both these volcanic eruptions caused detectable reductions in precipitation and discharge around the tropical Atlantic Basin; however, there were significant differences due to the eruption location and seasonality. In particular, the high-latitude Katmai eruption induced northern hemisphere cooling, driving a southward displacement of the Atlantic intertropical convergence zone (ITCZ) and Hadley circulation, weakening the West African summer monsoon; in contrast, though much closer in distance to the tropical Atlantic, the low-latitude Santa Maria eruption did not induce a comparable displacement of the marine ITCZ or summer monsoon response.