Examination of Boreal Summer Sahel Rainfall Variability in the Context of the Tropical Easterly Jet

The movement of precipitation around the Earth has an integral impact on fresh water availability, vegetation, and the occurrence of natural disasters, and therefore human society at large. In the African Sahel, the rainfall is the limiting factor in agriculture and the most variable characteristic of climate both spatially and temporally. In light of recent famine concerns, research on rainfall variability in the Sahel is critical as well as timely. A review of climate and environmental literature provided a perspective and a set of methodologies upon which the research could build. This research emphasizes the role of the regional and global atmospheric circulation in governing the variability of Sahel rainfall. It examines the changing spatiotemporal characteristics of the rainfall regime in the context of the Tropical Easterly Jet (TEJ), the El Nino/Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Tibetan High. Historical data was obtained from the African rain gauge network, NCEP/NCAR Reanalysis 1, and HadISST version 1.1. The results of this study show that the relationship between the TEJ and Sahel rainfall is a casual one, with a stronger TEJ enhancing upper-level divergence to promote broader, more intense vertical ascent. It also showed that diagnostic variables relevant to the general circulation, such as the maximum ascent within the rainbelt, the northward displacement of the AEJ, and the advection of precipitable water over the Sahel have a robust positive relationship with the Sahel rainfall anomaly due to the influence of the TEJ. The results also determined that a significant relationship exists in terms of the high-frequency interannual variability between negative ENSO events and the strengthening of the TEJ, which would similarly lead to anomalously wet years in the Sahel during a negative ENSO anomaly. This study posits a new framework for understanding rainfall variability in the Sahel due to its focus on the impacts of the atmospheric circulation. Furthermore, it can have important applications to understanding and forecasting droughts and floods in the Sahel, improve the modeling of Sahel climate, and provide a basis for further study of how West Africa and the TEJ fit into the global picture.