QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 135: 894–913 (2009) Published online 28 April 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.414 The impact of moist processes on the African Easterly Jet–African Easterly Wave system R. J. Cornforth a *, B. J. Hoskins b and C. D. Thorncroft c a Department of Meteorology, University of Reading, UK b Grantham Institute for Climate Change, Imperial College, London, UK c Department of Earth and Atmospheric Sciences, University at Albany, SUNY, USA ABSTRACT: The African Easterly Jet–Easterly Wave (AEJ–AEW) system was explored in an idealised model. Prescribed zonally symmetric surface temperature and moisture profiles determine the AEJ which becomes established through meridional contrasts in dry and moist convection. As in previous studies, a realistic AEJ developed with only dry convection. Including moist processes, increased its development rate, but reduced its speed and meridional extent. AEWs grew through barotropic–baroclinic conversions. Negative meridional potential vorticity (PV) gradients arose in the zonally symmetric state through the intrusion of the low-PV Saharan boundary layer. Since moist processes strengthened this significantly through diabatically generated PV in the Intertropical Convergence Zone, moist AEWs were three times stronger. Larger barotropic conversions and faster AEJ development increased the moist wave growth-rate. Jet-level and northerly low-level amplitudes grew, but in the moist case the low-level amplitudes weakened as the AEW interacted with convection, consistent with their absence from observations during the peak monsoon. Striking dependencies between the AEJ, AEW and rainfall existed. Two time-scales governed their evolution, depending on the transfer coefficients: (1) the AEJ’s replenishment rate influenced by heat fluxes, and (2) the wave growth-rate, by damping, and the slower jet development rate. Moist AEWs were characterized by intermittent growth/decay, with growth preceded by increased mean rainfall and later, weakening AEJs. These dependencies established an internal 8–10-day variability, consistent with intra-seasonal observations of 9-day rainy sequences. This internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist AEW life cycle. Copyright c  2009 Royal Meteorological Society KEY WORDS moist convection; dynamics; intraseasonal variability; general circulation model Received 19 June 2008; Revised 27 February 2009; Accepted 2 March 2009 1. Introduction A crucially important component of the West African monsoon (WAM) and its associated rainfall variability, is the African Easterly Jet–African Easterly Wave (AEJ– AEW) system. Despite this, the system continues to pose a tough challenge for general circulation models (GCMs) to simulate realistically, given the many physical processes involved and given the multitude of spatio- temporal scales they act over (Parker et al., 2005; Peyrill ´ e et al., 2007). For the West African population, any intra-seasonal rainfall variability impacts severely on the local agri- cultural and water resources, with equally severe con- sequences on health. This is especially true for the pop- ulation living in the semi-arid Sahel region. This region endures the full impact of the AEJ–AEW system and yet is the most ill-placed to suffer the space, time, and intensity variability in rainfall that this system brings. Improved skilful prediction of the monsoon rains over ∗ Correspondence to: R J Cornforth, Department of Meteorology, Uni- versity of Reading, 2 Earley Gate, Whiteknights, Reading, Berkshire RG6 2AU, UK. E-mail: r.j.cornforth@reading.ac.uk longer time-scales would be of enormous benefit, aid- ing in the mitigation of high-impact events such as the flooding during the summer of 2007, and the drought conditions of the last few decades. Improved simulation of the WAM in GCMs would also benefit downstream simulations where the AEJ– AEW system impacts tropical cyclogenesis (Landsea and Gray, 1992). This is thought to occur by direct transformation of a cold-core AEW into a warm-core system as shown by Pytharoulis (1999), or, as Berry and Thorncroft (2005) have suggested, by the amplifi- cation of an AEW, through merger of embedded vor- tices with one another and with diabatically generated potential vorticity (PV) anomalies associated with topog- raphy near the west coast, to produce a significant PV feature on leaving the West African coast that can rapidly undergo tropical cyclogenesis over the Atlantic. But in order to improve the simulation of the WAM, and thus improve our understanding of the intrasea- sonal variability of the rainfall, we still need to under- stand some fundamental truths. These include the evo- lutions and structures of the AEJ and the AEWs, their relationship to one another, and most importantly, how these change when moist processes are present. These Copyright c  2009 Royal Meteorological Society