Past, present, and future impacts of oceanic internal modes of variability on the rainfall over the West African Guinea Coast by Koffi Worou

The West-African monsoon system governs in the first order the annual cycle of rainfall over the Guinea Coast. During the 20th century and the last decades, much of the variability of the boreal summer Guinea Coast rainfall occurs on the interannual timescale and has been mainly driven by changes in the sea surface temperature (SST) in the eastern equatorial Atlantic, amplified by land-atmosphere processes. These SST fluctuations characterize the Atlantic equatorial mode (AEM), which is the dominant oceanic internal mode of variability in the tropical Atlantic. Positive phases of the AEM are associated with above-normal rainfall and extreme rainfall events over the Guinea Coast. In this thesis, we extend the study of teleconnection processes to the seasons beyond the monsoon season, which are not well known. The strongest SST-rainfall linkages found involve the Maritime Continent oceanic region in the Harmattan season, the tropical South Atlantic in the pre-monsoon season, and the eastern Mediterranean Sea in the post-monsoon season. Moreover, the ongoing global warming and the potential future changes according to different climatic scenarios could affect the actual known SST modes of variability and their impacts on different regions of the Earth. We make use of the climate simulation outputs from the most advanced climate general circulation models (GCMs) to study their performance in simulating the present-day AEM influence on the rainfall and extreme rainfall indices over Guinea Coast. Our results indicate that GCMs simulate realistically well the SST patterns related to the AEM phases but struggle in the representation of the rainfall responses. Using the highest emission scenario of greenhouse gases (SSP5-8.5), we obtained a future decrease in the influence of the AEM on the rainfall and extreme rainfall indices over the Guinea Coast.