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The West African monsoon onset in 2006: sensitivity to surface albedo, orography, SST and synoptic scale dry-air intrusions using WRF

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Abstract

In order to test the sensitivity of the transitional phase of the 2006 West African monsoon (WAM) onset to different mechanisms, weather research and forecasting (WRF) model simulations have been carried out addressing the role of the Saharan heat low (SHL) and its sensitivity to the albedo field and to the northern Africa orography, and the role of the sea surface temperature (SST) in the eastern tropical Atlantic and Mediterranean. Lowering albedo over the desert region induces a northward location of the inter-tropical convergence zone (ITCZ), while removing mountains in North Africa reduces rainfall over West Africa. Shifting SST forward by 15 days leads to a northward location of the ITCZ before the WAM onset. However none of these factors modifies the timing of the WAM onset in 2006. The transitional phase of the 2006 WAM onset has been examined in more detail. The enhancement of SHL intensity, combined with the development of the oceanic cold tongue in the Guinea gulf, leads to low-level moisture flux divergence in the ITCZ reducing rainfall and increasing low-level humidity over the Sahel. However, weakening of convection can be clearly attributed to dry-air intrusions in mid-levels, originating from the subtropical westerly jet and associated with Rossby wave pattern over North Africa. Sensitivity tests on the synoptic scale forcing outside of the WRF model domain confirm the dominating role of large-scale dynamics to control the transitional phase of the WAM onset and its timing. However it is shown that the regional factors can modulate this larger scale forcing.

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Acknowledgments

The authors are grateful for the help of Philippe Weill to install the model on the IPSL servers. We are also thankful to the Climate Diagnostics Center (NOAA, Boulder, CO) and to the NCEP for the OLR and Final Analyses (FNL) data. Finally we are grateful to all three reviewers for their constructive comments that contributed greatly to the improvement of this article. Based on French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially from France, UK, US and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available on the AMMA International website http://www.amma-international.org.

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Authors and Affiliations

  1. LATMOS/IPSL, Université Pierre et Marie Curie (UPMC), Boite 102, 4 Place Jussieu, 75252, Paris cedex 05, France

    Emmanouil Flaounas

  2. LOCEAN/IPSL, IRD, UPMC, Paris, France

    Serge Janicot

  3. LATMOS/IPSL, CNRS, UPMC, Paris, France

    Sophie Bastin

  4. LMD/IPSL, CNRS, UPMC, Paris, France

    Rémy Roca

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  1. Emmanouil Flaounas
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  2. Serge Janicot
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  4. Rémy Roca
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Correspondence to Emmanouil Flaounas.

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Flaounas, E., Janicot, S., Bastin, S. et al. The West African monsoon onset in 2006: sensitivity to surface albedo, orography, SST and synoptic scale dry-air intrusions using WRF. Clim Dyn 38, 685–708 (2012). https://doi.org/10.1007/s00382-011-1255-2

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