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WRF/ARPEGE-CLIMAT simulated climate trends over West Africa

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Abstract

The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981–1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally.

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Acknowledgments

The authors are most thankful to the anonymous reviewers for their constructive suggestions and substantial contribution to the paper. N. Vigaud would like to acknowledge the financial support of the RESSAC (Vulnérabilité des Ressources en Eau Superficielle au Sahel aux Evolutions Anthropiques et Climatiques à moyen terme) program from the Agence Nationale pour la Recherche (ANR-06-VULN-017-02). This study was supported by the European Commission 6th Framework Program (ENSEMBLES, contract GOCE-CT-2003-505539). Regional experiments were performed using HPC ressources from GENCI-[CCRT/CINES/IDRIS] (Grant 2009-91320). ECMWF ERA-40 re-analyses used in this study were obtained from the ECMWF data server, IRD in situ daily precipitation data, CMAP and GPCP rainfall from the referring institutions.

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Authors and Affiliations

  1. Centre de Recherches de Climatologie, UMR 5210 CNRS, Université de Bourgogne, Dijon, France

    N. Vigaud, P. Roucou & B. Fontaine

  2. Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India

    S. Sijikumar

  3. CNRM/GAME, URA 1357 CNRS/Météo-France, Toulouse, France

    S. Tyteca

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  1. N. Vigaud
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  2. P. Roucou
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  3. B. Fontaine
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  4. S. Sijikumar
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Correspondence to N. Vigaud.

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Vigaud, N., Roucou, P., Fontaine, B. et al. WRF/ARPEGE-CLIMAT simulated climate trends over West Africa. Clim Dyn 36, 925–944 (2011). https://doi.org/10.1007/s00382-009-0707-4

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