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Regional modeling of decadal rainfall variability over the Sahel

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Abstract

A regional climate model is used to investigate the mechanism of interdecadal rainfall variability, specifically the drought of the 1970s and 1980s, in the Sahel region of Africa. The model is the National Center for Environmental Prediction’s (NCEPs) Regional Spectral Model (RSM97), with a horizontal resolution of approximately equivalent to a grid spacing of 50 km, nested within the ECHAM4.5 atmospheric general circulation model (AGCM), which in turn was forced by observed sea surface temperature (SST). Simulations for the July–September season of the individual years 1955 and 1986 produced wet conditions in 1955 and dry conditions in 1986 in the Sahel, as observed. Additional July–September simulations were run forced by SSTs averaged for each month over the periods 1950–1959 and the 1978–1987. These simulations yielded wet conditions in the 1950–1959 case and dry conditions in the 1978–1987 case, confirming the role of SST forcing in decadal variability in particular. To test the hypothesis that the SST influences Sahel rainfall via stabilization of the tropospheric sounding, simulations were performed in which the temperature field from the AGCM was artificially modified before it was used to force the regional model. We modified the original 1955 ECHAM4.5 temperature profiles by adding a horizontally uniform, vertically varying temperature increase, taken from the 1986–1955 tropical mean warming in either the AGCM or the NCEP/National Center for Atmospheric Research Reanalysis. When compared to the 1955 simulations without the added tropospheric warming, these simulations show a drying in the Sahel similar to that in the 1986–1955 difference and to the decadal difference between the 1980s and 1950s. This suggests that the tropospheric warming may have been, at least in part, the agent by which the SST increases led to the Sahel drought of the 1970s and 1980s.

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Acknowledgments

The first author acknowledges the support from the US National Science Foundation, through a Fellowship in the IGERT Joint Program in Applied Mathematics and Earth and Environmental Science at Columbia University. Thanks to International Research Institute for Climate and Society (M. Benno Blumenthal) for the data provided in Data Library, and to Michela Biasutti for Fig. 7, and to Michela Biasutti, Alessandra Giannini, Isaac Held, and Vincent Moron for discussions. The second author acknowledges support from National Science Foundation Grant DMS-0139830. The third author acknowledges support from the International Research Insitute for Climate and Society (IRI) National Oceanic and Atmospheric Administration Grant NA07GP0213.

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Authors and Affiliations

  1. Institute of Marine and Coastal Sciences (IMCS), Rutgers University, 71 Dudley Road, New Brunswick, NJ, 08901, USA

    Deborah Herceg

  2. Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA

    Adam H. Sobel

  3. International Research Institute for Climate and Society (IRI), Columbia University, Palisades, NY, USA

    Adam H. Sobel & Liqiang Sun

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  1. Deborah Herceg
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  2. Adam H. Sobel
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  3. Liqiang Sun
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Correspondence to Deborah Herceg.

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Herceg, D., Sobel, A.H. & Sun, L. Regional modeling of decadal rainfall variability over the Sahel. Clim Dyn 29, 89–99 (2007). https://doi.org/10.1007/s00382-006-0218-5

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