Model, proxy and isotopic perspectives on the East African Humid Period

Both North and East Africa experienced more humid conditions during the early and mid-Holocene epoch (11,000–5000 yr BP; 11–5 ka) relative to today. The North African Humid Period has been a major focus of paleoclimatic study, and represents a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean, atmosphere and land surface feedbacks. Meanwhile, the mechanisms that caused the coeval East African Humid Period are poorly understood. Here, we use results from isotope-enabled coupled climate modeling experiments to investigate the cause of the East African Humid Period. The modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall. Our simulations show that the orbitally-induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the East African Humid Period, and this scenario agrees well with regional lake level and pollen paleoclimate data. Changes in zonal moisture flux from both the Atlantic and Indian Ocean account for the simulated increase in precipitation from June through November. Isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western East African Rift Valley in particular experienced more humid conditions due to the influx of Atlantic moisture and enhanced convergence along the Congo Air Boundary. Our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the East African region.

Research Highlights
► In this study we investigate the cause of the East African Humid Period (EAHP) using both model simulations and paleoclimate proxy data
► Our study suggests that the EAHP was caused by an increase in dry/transitional season precipitation and the subsequent reduction of precipitation seasonality
► We discuss the two primary mechanisms that likely caused increased dry season precipitation: 1) an increase in moisture flux from the Atlantic Ocean to the Congo Air Boundary and 2) increased convergence and precipitation in the western Indian Ocean and easternmost Africa
► The isotopic changes simulated by the model are in good agreement with proxy leaf wax isotopic data if we assume a seasonal bias in the proxy data.
► The large change in the isotopic composition of precipitation at Lake Tanganyika inferred from proxy data reflects a change in moisture source; vapor source distribution simulations confirm that this is likely the case.