This thesis examines the structure of the Tropical Easterly Jet (TEJ) in a General Circulation Model (GCM). The TEJ is observed only during the Indian summer monsoon period and is strongest during July and August. The jet structure simulated by an atmospheric GCM (CAM-3.1) in July has been compared with reanalysis data. The simulated TEJ was displaced westward by ~ 25◦ when compared to observations. The removal of orography had no impact on the jet structure. This demonstrated that the Tibetan Plateau did not play an important role in the location and structure of the jet. The changes in cumulus scheme in the GCM had a large influence on the location of the jet maxima. To examine the factors which control the location and structure of the jet, a series of experiments were conducted using an aqua-planet version of the model. The impact of different sea surface temperature (SST) profiles was studied. The rainfall in the GCM was primarily in the regions where the SST attained a maximum. By altering the location of SST maximum (and hence the rainfall maximum), the impact of location of rainfall maximum on the location and structure of the jet was studied. When the rainfall maximum was located close to the equator, it did not generate a strong jet but had an influence on the vertical structure of the jet. A large number of simulations were conducted with multiple rainfall maxima and the need for these was demonstrated since only then was the observed jet structure well simulated. Based on the simulations, it was concluded that the simulation of the TEJ by CAM-3.1 was unrealistic because of large unrealistic rainfall over Saudi Arabia in this GCM. Equatorial heating has been shown to be important to simulate proper jet structure. The zonal structure of the jet was also influenced by rainfall in the Pacific Ocean. Although the aqua-planet configuration of the CAM-3.1 GCM provided several useful insights, the simulation was not perfect on account of errors in the simulation of the temperature profile in the lower troposphere. An ideal-physics configuration of the GCM was used. This removed the cumulus physics and instead imposed the observed heating pro-files. Both upper tropospheric friction and radiative-convective atmospheric temperatures were required to simulate the TEJ. The problems with the simulation of structure in the jet exit region was corrected by using radiative-convective atmospheric temperatures that were qualitatively similar to those observed in northern hemisphere summer time. The ideal-physics configuration reconfirmed that the Saudi Arabian rainfall was responsible for the westward shift of the TEJ in the simulations. The ideal-physics simulations showed that the simple analytical model proposed by Gillin1980 was not suitable for the simulation of TEJ. The above the simulations indicate that a shift in the location of the jet is related to a shift in the rainfall pattern. Based on this insight one would expect that the jet location will be different in good and bad monsoon periods. This is indeed the case. In July 2002 the Indian monsoon failed after beginning well in June. In June the TEJ is consequently located west ward compared to July. The same situation prevails even in good and poor monsoon years. In a good monsoon year (July 1988) the jet maximum is located westward when compared to a bad monsoon year (July 2002). In this thesis we have clearly demonstrated the role of anomalous rainfall on the location of the TEJ. This thesis has shown that an accurate simulation of the TEJ depends upon the accurate simulation of various rainfall centers that act as multiple heat sources in the atmosphere. The rainfall in the equatorial region does not influence the strength of the TEJ but alters the vertical structure of the jet. The strength the jet is dependent on the intensity of rainfall and the latitudinal distance from the equator. The complex vertical structure of the jet is not simulated by simple analytical models of the jet.

La Mousson Ouest Africaine est un système complexe, qui présente des variations interannuelles très importantes. Ces variations se traduisent par les fluctuations considérables des précipitations sur le continent. Avoir une meilleure compréhension du système complexe qu'est la mousson africaine peut permettre de mieux cerner les mécanismes qui influencent la production des précipitations sur le Sahel. La première partie de ce travail de thèse consiste en une comparaison entre deux saisons de mousson aux régimes de précipitations différents afin d'extraire les caractéristiques communes et différentes, pour les appliquer à l'étude de la saison de mousson durant laquelle s'est déroulée la campagne AMMA. Cette étude est complétée par une compréhension des processus dynamiques et thermodynamiques qui sont à l'origine du déclenchement de l'Onset de la mousson, et qui permettent l'installation des conditions favorables au développement des précipitations sur le Sahel. Le second volet est basé sur les mécanismes physiques qui entrent en compte dans le cycle de vie des lignes de grains sur le Sahel. Enfin, la dernière partie traite de l'impact des systèmes de méso - échelle sur leur environnement en terme de bilan de chaleur et d'humidité dans le quadrilatère sud de l'expérience AMMA.