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A further assessment of vegetation feedback on decadal Sahel rainfall variability

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Abstract

The effect of vegetation feedback on decadal-scale Sahel rainfall variability is analyzed using an ensemble of climate model simulations in which the atmospheric general circulation model ICTPAGCM (“SPEEDY”) is coupled to the dynamic vegetation model VEGAS to represent feedbacks from surface albedo change and evapotranspiration, forced externally by observed sea surface temperature (SST) changes. In the control experiment, where the full vegetation feedback is included, the ensemble is consistent with the observed decadal rainfall variability, with a forced component 60 % of the observed variability. In a sensitivity experiment where climatological vegetation cover and albedo are prescribed from the control experiment, the ensemble of simulations is not consistent with the observations because of strongly reduced amplitude of decadal rainfall variability, and the forced component drops to 35 % of the observed variability. The decadal rainfall variability is driven by SST forcing, but significantly enhanced by land-surface feedbacks. Both, local evaporation and moisture flux convergence changes are important for the total rainfall response. Also the internal decadal variability across the ensemble members (not SST-forced) is much stronger in the control experiment compared with the one where vegetation cover and albedo are prescribed. It is further shown that this positive vegetation feedback is physically related to the albedo feedback, supporting the Charney hypothesis.

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Notes

  1. Note that the coefficient of variation is only shown where the mean rainfall is larger than 1 mm/day.

  2. Five members have been randomly plotted.

  3. The internal components are as described in Sect. 3 derived by subtracting the ensemble mean from every ensemble member. The results of the spectral analysis of the forced components are strongly determined by the common SST forcing and therefore not shown. The 90 % confidence limit has been derived from the ensembles.

  4. All modeled anomalies shown are at least 95 % statistically significant as derived from a t test using intra-ensemble standard deviations.

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Acknowledgments

The authors thank two anonymous reviewers for their constructive comments that helped improving the paper.

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Authors and Affiliations

  1. Abdus Salam International Centre for Theoretical Physics, Earth System Physics Section, Strada Costiera 11, 34151, Trieste, Italy

    Fred Kucharski

  2. Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD, 20742, USA

    Ning Zeng & Eugenia Kalnay

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  1. Fred Kucharski
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  2. Ning Zeng
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  3. Eugenia Kalnay
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Correspondence to Fred Kucharski.

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Kucharski, F., Zeng, N. & Kalnay, E. A further assessment of vegetation feedback on decadal Sahel rainfall variability. Clim Dyn 40, 1453–1466 (2013). https://doi.org/10.1007/s00382-012-1397-x

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