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Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model

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Abstract

In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the twentieth century is used as the primary model forcing. Using this coupled global model, we experiment on treating vegetation as a static boundary condition and as a dynamic component of the Earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics act as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model. Regardless how vegetation is treated and what model resolution is used, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as that observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the twentieth century drought in the Sahel.

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Acknowledgements

The authors would like to acknowledge the helpful comments from two anonymous reviewers. We thank Joe Helkowski at the University of Wisconsin for his help with the statistical analysis of the CRU precipitation data. We also thank Christine Delire for sharing a manuscript still in press.

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Authors and Affiliations

  1. Department of Civil and Environmental Engineering, University of Connecticut , Storrs, CT 06269, USA

    G. Wang

  2. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

    E. A. B. Eltahir

  3. Center for Sustainability and the Global Environment (SAGE) , Nelson Institute for Environmental Studies, University of Wisconsin-Madison, Madison, WI 53726, USA

    J. A. Foley

  4. EMS Environment Institute, Pennsylvania State University , College Station, PA 16802, USA

    D. Pollard

  5. Terrestrial Science Section, National Center for Atmospheric Research, Boulder, CO 80305, USA

    S. Levis

Authors
  1. G. Wang
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  2. E. A. B. Eltahir
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  3. J. A. Foley
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  4. D. Pollard
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  5. S. Levis
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Correspondence to G. Wang.

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Wang, G., Eltahir, E.A.B., Foley, J.A. et al. Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model. Climate Dynamics 22, 625–637 (2004). https://doi.org/10.1007/s00382-004-0411-3

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  • DOI: https://doi.org/10.1007/s00382-004-0411-3

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