Advancing the Theory of Coupled Land-Atmosphere-Ocean Feedbacks Governing the Bimodal Rainfall Pattern Over Coastal Southwest Nigeria

The bimodal rainfall pattern of coastal Southwest Nigeria, characterized by two rainy season peaks in April-June and September-October separated by a July-August minimum, is a climatological signature of fundamental importance whose governing mechanisms remain incompletely theorized in the scientific literature. This study advances the theoretical understanding of the coupled land-atmosphere-ocean feedbacks responsible for maintaining the bimodal rainfall structure over coastal Southwest Nigeria. An observational climatology of the bimodal pattern is established using 30 years of daily NiMet records from 15 stations, augmented by ERA5 reanalysis atmospheric variables and NOAA ERSST SST data. Theoretical analysis of the feedback mechanisms is structured around three competing hypotheses: Gulf of Guinea upwelling-induced SST cooling, continental surface energy balance modification, and large-scale ITD double passage dynamics. A series of WRF regional model sensitivity experiments with prescribed SST perturbations, modified land surface parameters, and altered lateral boundary conditions was designed to isolate the contribution of each hypothesized mechanism. Results indicate that the August Gulf of Guinea cold tongue SST pattern is the necessary but not sufficient condition for maintaining the rainfall minimum, requiring co-occurrence of continental surface heating anomalies to produce the observed magnitude of July-August suppression. Original theoretical contributions include derivation of a coupled SST-continental heating criterion for bimodal pattern maintenance and identification of a previously undescribed land surface feedback that amplifies the rainfall minimum through reduced moisture recycling. Keywords: bimodal rainfall, Southwest Nigeria, Gulf of Guinea, land-atmosphere coupling, SST feedback.