Quantitative reconstructions of annual rainfall in Africa 6000 years ago:: Model-data comparison

This paper provides the first set of quantitative reconstructions of annual precipitation for mid-Holocene Africa, based on pollen data. The estimates of precipitation are based on 85 pollen sites C-14 dated at 6000 +/- 500 years B.P and distributed over the whole of Africa. To improve the reliability of the pollen-based climate reconstruction, two methods are used: the ''modern analogues technique'' (MAT) and the ''plant functional types'' (PFT) methods. We then conduct a model-data comparison for five distinct regions, allowing an evaluation of model outputs ( the Sahara-Sahel, the eastern Sahara, western equatorial Africa, East Africa, and Madagascar). The pollen-inferred reconstructions are compared with 21 mid-Holocene simulations yielded by Atmospheric General Circulation Models (AGCMs), and coupled ocean-atmosphere-vegetation models (OAVGCMs). The large-scale feature of the hydrological changes is shown to be well captured by most of the models. Data show that during the mid-Holocene, the Sahara was considerably wetter than today (+200 to +700 mm/yr). The results reinforce the conclusion that the AGCMs significantly underestimate this precipitation increase in the Sahara whereas the OAVGCM simulations are in accordance with the data. Our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in East Africa and Madagascar. The model-data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen-based reconstruction shows no change or only slight drying there.