Fingerprints of External Forcings on Sahel Rainfall
Precipitation in the Sahel, the semi-arid region just south of the Sahara desert, affects a large and rapidly growing population. Regional rainfall has been shaped by two main external factors: aerosols and greenhouse gases. Decreasing aerosol burden over the North Atlantic is expected to increase regional rainfall, as is increasing greenhouse gas emissions. Can the responses to these forcings be distinguished in observations?
A better understanding of the historical changes in precipitation in the Sahel can help to better understand the future changes in rainfall in this region.
In this study, we show that CMIP5 models project the emergence of a detectable signal of aerosol forcing in the middle of the twentieth century and a detectable signal of greenhouse gas forcing at the beginning of the twenty-first. However, the signals of both aerosol and greenhouse gas forcing in observations emerge earlier and are stronger than in the models, far stronger in the case of aerosols. The similarity between the response to aerosol forcing and the leading mode of internal variability makes it difficult to attribute this model-observation discrepancy to errors in the forcing, errors in the forced response, model inability to capture the amplitude of internal variability or some combination of these. For greenhouse gases, however, the forced response is distinct from internal variability as estimated by models, and the observations are largely commensurate with the model projections.