Abrupt Changes in the Water Balance of Tropical West Africa during the Late Quaternary

Shanahan, T.M., J.T. Overpeck, C.A. Scholz, J.W. Beck, J. Peck, and J.W. King

2008 Journal of Geophysical Research 113: 10.1029/2007JD009320



ABSTRACT

Understanding the causes of tropical climate variability requires an understanding of not only the temporal patterns of these changes, but their spatial extent and magnitude. In this study, we employ a water balance modeling approach to simulate the magnitude of hydrologic changes associated with the geologic record of past lake stands at Lake Bosumtwi, West Africa. Our results indicate that lake level lowering during the late glacial (16 ka) was likely the result of a ~20% decrease in precipitation, consistent with general circulation modeling (GCM) results and existing paleohydrological estimates in Africa. In contrast, our observation-based estimate of hydrologic change during the early Holocene (~5–10 ka) is at odds with the magnitude and direction of hydrologic changes predicted by GCM results, suggesting that the models may not adequately reproduce the processes associated with large-scale changes in the West African monsoon. Our results for drought events at 12 and 8.7 ka are also inconsistent with GCM predictions of a precipitation dipole over West Africa, but depend strongly on assumptions about other hydrologic parameters such as relative humidity and temperature. Our water balance model also indicates that significant hydrologic changes occurred in response to changes in orbital forcing at ~70–80 ka, possibly associated with an eccentricity-driven shift in precession. Drought conditions also occurred during the same time as the European ‘‘Little Ice Age,’’ indicating a connection between relatively minor Northern Hemisphere cooling and drought conditions in Africa. These results suggest that the West African monsoon has the potential to respond in a highly nonlinear way to forcing involving changes in the solar radiation over Africa.