Abrupt changes in the African monsoon can have pronounced socioeconomic impacts on many West African countries. Evidence for both prolonged humid periods and monsoon failures have been identified throughout the late Pleistocene and early Holocene epochs1,2. In particular, drought conditions in West Africa have occurred during periods of reduced North Atlantic thermohaline circulation, such as the Younger Dryas cold event1. Here, we use an ocean–atmosphere general circulation model to examine the link between oceanographic changes in the North Atlantic Ocean and changes in the strength of the African monsoon. Our simulations show that when North Atlantic thermohaline circulation is substantially weakened, the flow of the subsurface North Brazil Current reverses. This leads to decreased upper tropical ocean stratification and warmer sea surface temperatures in the equatorial South Atlantic Ocean, and consequently reduces African summer monsoonal winds and rainfall over West Africa. This mechanism is in agreement with reconstructions of past climate. We therefore suggest that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the tropical Atlantic Ocean contributes to the rapidity of African monsoon transitions during abrupt climate change events.
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We wish to thank J. Yin for providing us with the extra water-hosing experiment output of the GFDL CM2.1 model. This work is supported by the National Science Foundation, Grant No. OCE-0623364 and ATM-0337846, the Climate Program Office, US National Oceanic and Atmospheric Administration, Grant No. NA050AR4311136 and the Office of Science (BER), US Department of Energy, Grant No. DE-FG02-07ER64443.
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Chang, P., Zhang, R., Hazeleger, W. et al. Oceanic link between abrupt changes in the North Atlantic Ocean and the African monsoon. Nature Geosci 1, 444–448 (2008). https://doi.org/10.1038/ngeo218
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