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Dynamics of the boreal summer African monsoon in the NSIPP1 atmospheric model

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Abstract

A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic, SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought. This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed, by anthropogenic climate change.

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Acknowledgments

The authors wish to thank Max Suarez and Phil Pegion (NASA/GSFC) for their selfless scientific and technical support, and the two reviewers, Michela Biasutti and Guojun Gu, for their helpful, constructive comments. This study was supported by NASA’s Seasonal to Interannual Prediction Project, through Interagency Agreement W-19,750 and by NOAA, through Grant NA16GP1575. PC was also supported by NOAA and NSF through research grants NA16GP1572, NA16GP2020, and ATM-0337846, and by the National Natural Science Foundation of China (NSFC) through Grant 40128003. The International Research Institute for climate prediction, a unit of the Earth Institute at Columbia University, is funded by a cooperative agreement between NOAA and Columbia University. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.

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  1. International Research Institute for climate prediction, The Earth Institute at Columbia University, P. O. Box 1000, Palisades, NY, 10964-8000, USA

    Alessandra Giannini

  2. National Center for Atmospheric Research, Boulder, CO, USA

    R. Saravanan

  3. Department of Oceanography, Texas A&M University, College Station, TX, USA

    Ping Chang

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  1. Alessandra Giannini
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  2. R. Saravanan
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  3. Ping Chang
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Correspondence to Alessandra Giannini.

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Giannini, A., Saravanan, R. & Chang, P. Dynamics of the boreal summer African monsoon in the NSIPP1 atmospheric model. Climate Dynamics 25, 517–535 (2005). https://doi.org/10.1007/s00382-005-0056-x

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