Evaluating the Variability of the West African Monsoon in Gridded Paleo Reconstructions

There is mounting evidence that late 20^th century multi-decadal rainfall variations in the Sahel, at the northernmost edge of the West African Monsoon region, were significantly forced by anthropogenic emissions of aerosols and greenhouse gases. Yet, a formal attribution is hampered by severe model biases and by the fact that a lack of direct observations makes models the main tool for estimating pre-industrial internal variability of rainfall.

Another possible tool is provided by multi-century field reconstructions of drought. We compare the modes of variability produced by four such reconstructions, two global reanalyses that merge wide ranging proxy data and models (the Last Millennium Reanalysis and the Paleo Hydrology Atlas) and two drought atlases based on tree-ring chronologies from outside West Africa (the West Sub-Saharan Africa Drought Atlas and the Sub-Saharan Africa Drought Atlas).

We evaluate the dominant patterns of variability and their degree of stationarity, as well as the coherence among gridded reconstructions and between the reconstructions and independent records of regional rainfall, such as flow in the river Nile and documentary evidence of drought.

We show that progress towards a reliable estimate of pre-industrial drought in West Africa requires the acquisition of in-situ paleo chronologies.