Impact of Climate Change on West African Monsoon Features and their Relationship With Convection and Precipitation

Abstract

West African region, particularly over Sahel region, has experienced a long-term climate variability and climate change during the last decades. This episode has severe consequences for local population whose economy depends mainly on rain-fed agriculture. Therefore, improved skilled prediction of the monsoon rains over longer time-scales would be of enormous benefit, aiding in the mitigation of high-impact events. As part of efforts to improve on Monsoon system, this study investigates how global warming affect West African Monsoon features characteristics such as AEJ and AEWs and their interaction between convection and precipitation patterns. The latest version of the International Centre for Theoretical Physics Regional Climate Model (RegCM4) driven by three CMIP5 Global Climate Models (GCMs) is used at 25 km of grid spacing over West Africa. A set of experiments was performed to investigate the impact of lateral boundary forcings on the simulation of monsoon precipitation and the relationship with regional circulation features. In particular, we examine and inter-compare the models performance with theirs ensemble-mean in simulating the mean climatology, convection patterns and West African Monsoon features for both historical (1985-2004) and future (2080-2099) periods during the boreal summer season (JJAS). Secondly, we examine how convection, AEJ, AEWs, precipitation and their interaction are changing in the future (2099-2080, under two GHG scenarios RCP4.5 and RCP8.5 and the referenced period 1985-2004). We firstly represent the meridional temperature gradient and potential temperature (related to baroclinicity) averaged between 10oE and 20oW to examine the strength and weakness of the mid-tropospheric AEJ. To evaluate the most favorable conditions supported the disturbance around the AEJ core, the analysis of isentropic potential vorticity (IPV) budget is presented. Firstly, the results show that the RegCM4 experiments along with their multimodel ensemble generally reproduce the location of the main precipitation characteristics and improve upon the corresponding driving GCMs. However, the provision of different forcing boundary conditions leads to substantially different precipitation magnitudes and spatial patterns. For instance, while RegCM4 nested within GFDL-ESM-2M and HadGEM2-ES exhibits some underestimations of precipitation and an excessively narrow Intertropical Convergence Zone, the MPI-ESM-MR driven run produces precipitation spatial distribution and magnitudes more similar to observations. Such a superior performance originates on one hand from a much better simulation of the interactions between baroclinicity, temperature gradient and African Easterly Jet and on the other hand from an improved connection between the Isentropic Potential Vorticity, its gradient and the African Easterly Waves dynamics. The results show that among the model simulations, some project wetter climate along the Gulf of Guinea and drier conditions along the entire Sahel while other project a drier condition along the both region. Mostly, the RegCM4 simulations simulate contrasting changes in vertical motion over the complex terrains (negative) during the present day while the higher values are found over the mountaintops and the positive value prevail during the future period. These drier conditions prevail along the low and high complex terrains and they probably result from a weakening of moisture convergence from Atlantic Ocean into land as a leading to a reduction of precipitation and weakening of West African Monsoon. Given observed change in the far future climate, the result show that the characteristics of AEJ and AEWs are changing in the future. Such changes in the location and intensity of AEJ and AEWs and the magnitude of both AEJ and AEW winds carry implications in the relationship between AEJ and precipitation, AEWs and precipitation over the West African region, particularly in the Sahel region. We conclude that a good performing GCM in terms of monsoon dynamical features (in this case MPI-ESM-MR) is needed to drive RCMs in order to achieve a better representation of the West Africa summer monsoon precipitation.