Two runs have been performed with the French ARPEGE-Climat AGCM forced by SSTs on the period 1959-1995 (version SF1) and on the period 1950-1999 (version SF2). These two simulations reproduce quite well the seasonal cycle of the West African monsoon, especially the abrupt ITCZ shift observed in averaged in the end of June, signing the onset of the summer monsoon. A detailed examination of the atmospheric dynamics associated with this shift in the model is provided, and compared to the dynamics seen in the NCEP-2 reanalyses. High consistency is found between the model and the reanalyses, and the results highlight the role of the Saharan heat low and of the westerly advection of moisture from the northeastern tropical Atlantic.