Modelling the Impact of Coastal Urbanization on the West African Summer Monsoon Climate

Abstract

A lot of research has been done to understand and improve the relationship between land use and land cover modification vis-a-vis their interaction with the West African climate variability and climate change. However, the urbanization influence on the West African climate is not yet an elaborately researched subject of studies. In this context, this thesis examines the impact of coastal urbanization on the West African summer climate, by using the Regional Climate Model version 4 (RegCM4) coupled with the Community Land Surface Model version 4.5 (CLM4.5). A series of experiments were performed, in the present-day climate (1984-2005) and the RCP8.5 far future (2079- 2100), at 25 Km of horizontal resolution over the West African domain. Two types of simulations were performed with and without modification of the natural vegetation land cover with the urban parameterization (CLMU). Results from the model evaluation show the good performance of RegCM4 to simulate the main climatic variables and atmospheric circulation over West Africa during the June-September (JJAS) summer months. Arguably, RegCM4 reproduces well the spatio-temporal pattern of rainfall and temperature over West Africa in comparison with observations datasets. Again, the model’s capability to reproduce the West African atmospheric circulation drives and atmospheric variables compared to reanalyses was examined. RegCM4 gives a good representation of atmospheric circulation from the lower to the upper troposphere. The projected changes in West African climate under RCP8.5 and both RCP8.5 and urbanization were evaluated. For all the simulations, a significant warming is expected over the whole West Africa and will be more pronounced in the Sahel-Sahara at the end of the 21st century. A significant increase in temperature is also expected along the West African coastal region which corresponded to the ‘perturbed’ region. In the precipitation simulations, all the different simulations projected drier conditions in visually the entire West African region. However, the expected change is less significant in the combined urban expansion and RCP8.5 simulations. Evaluation of contribution due to urbanization alone shows that the change in the land cover gave a response of an average increase in temperature of approximately 3ºC over the urbanized region. The results imply that urban effects can reach the same magnitude as global warming. This warming could be a result of the urban heat island processes. Precipitation in the urbanized region and their sub-urban regions shows an increase of at least ~10%. This means that urbanization has both local and regional effects on the precipitation in West Africa. Furthermore, the results show that the characteristics of some atmospheric circulation such as AEJ and AEWs would change in the future climate. For example, a southward displacement of the AEJ position was observed which can explain the projected drier conditions in the region, especially in the Sahel part. Moreover, it should be noted that urbanization influences the atmospheric circulation drivers through the AEJ and AEWs. More convective activities are found under urban conditions and the sensitivity of the AWEs to the land surface conditions is noticed.