Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: 1417 – 1430, July 2011 B The impact of deep convection on the West African summer monsoon climate: a regional climate model sensitivity study M. B. Sylla, a * F. Giorgi, a P. M. Ruti, b S. Calmanti b and A. Dell’Aquila b a Earth System Physics (ESP) Section, International Centre for Theoretical Physics (ICTP), Trieste, Italy b UTMEA-CLIM, Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Casaccia Center, Rome, Italy *Correspondence to: M. B. Sylla, International Centre for Theoretical Physics (ICTP), Earth System physics Section (ESP), Strada Costiera 11, PO Box 586, I-34151 Trieste, Italy. E-mail: msylla@ictp.it The role of the representation of deep convection on key elements of the West African summer monsoon climate is addressed using the Regional Climate Model RegCM3. Two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared. Results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower-level baroclinicity favoring intensification of the jet core and leading to a more realistic African easterly jet. In addition, although the isentropic potential vorticity (IPV) is lower when the convection scheme is switched off, African easterly waves (AEWs) are still generated and propagate westwards but they dissipate around the west coast. Substantial differences between the two simulations occur mainly at the 6- to 9-day time-scale over land, when much weaker activity is simulated in the absence of convection. This indicates that orographic friction and low-level large-scale moisture convergence, generating high values of latent heat and IPV, may play the dominant role in the genesis and growth of AEWs and that deep convection acts to strengthen the overall wave activity and to favor their west coast development. Copyright c  2011 Royal Meteorological Society Key Words: regional climate model; deep convection scheme; West African monsoon circulation; African easterly jet; African easterly waves Received 7 September 2010; Revised 19 April 2011; Accepted 27 April 2011; Published online in Wiley Online Library 17 June 2011 Citation: Sylla MB, Giorgi F, Ruti PM, Calmanti S, Dell’Aquila A. 2011. The impact of deep convection on the West African summer monsoon climate: a regional climate model sensitivity study. Q. J. R. Meteorol. Soc. 137: 1417 – 1430. DOI:10.1002/qj.853 1. Introduction The main structure of the West African monsoon (WAM) circulation is critically affected by atmospheric deep convection, from the scale of a single convective cell up to the scale of synoptic systems (Redelsperger et al., 2002). One of the most prominent features during the boreal summer season is the mid-tropospheric (600–700 mb) African easterly jet (AEJ), a region of strong zonal winds (up to ∼10 m s −1 ) extending from East to West Africa and crossing the Atlantic Ocean. The disturbances around this zonal circulation pattern, so-called African easterly waves (AEWs), have been identified as the main mechanism in or- ganizing rainfall patterns (Diedhiou et al., 1999) and there- fore as key drivers of climate variability in this region. It is difficult to isolate the role of deep convection in maintaining the jet from its role in the propagation of AEWs (Leroux and Hall, 2009). However, to gain insight into the uncertainties inherent in the modeling of climate variability over this region (Ruti and Dell’Aquila, 2010), we test different settings of the deep convection scheme embedded in a Copyright c  2011 Royal Meteorological Society