Flaounas E. auteur aut IRD Janicot Serge auteur aut IRD Bastin S. auteur aut IRD Roca R. auteur aut IRD text journalArticle eng text/pdf born digital access 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. specialized West African monsoon onset WRF model Dry-air intrusions 032 021 38 3-4 685-708 2012 0930-7575 https://www.documentation.ird.fr/hor/fdi:010054392 10.1007/s00382-011-1255-2 0930-7575 [F B010054392] https://www.documentation.ird.fr/hor/fdi:010054392 https://www.documentation.ird.fr/intranet/publi/2012/03/010054392.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2012-04-04 2017-08-23 fdi:010054392 fre