Key Mechanisms Leading to the East-West Asymmetric Impacts of Convectively Coupled Kelvin Waves on the West African Monsoon

Convectively coupled Kelvin waves (CCKWs) are among the most influential sources of synoptic-to-intraseasonal variability affecting the West African monsoon. Yet, how their impacts differ across the West Africa and why rainfall transitions occur earlier in some regions than others remain poorly documented. Using daily observations and reanalysis fields for 1981-2024, this study investigates Kelvin-wave modulation of West African rainfall during July-September, focusing on the contrasting responses of eastern and western West Africa. Composite analyses reveal a coherent wet (phase 2) and dry (phase 6) structure, but with markedly sharper and earlier transitions in the east. The Intertropical Discontinuity and vertical wind shear show little longitudinal dependence, indicating that surface and mesoscale factors do not drive the asymmetry. Instead, the key mechanism originates from the vertical generation of humidity anomalies by Kelvin-wave-induced subsidence and ascent. Eastern West Africa exhibits deeper mid-level drying during the wet-to-dry transition and earlier moistening during the dry-to-wet transition, producing stronger humidity anomalies $q^{\prime }$ . These anomalies are subsequently redistributed horizontally by the mean monsoon flow, yielding earlier sign reversals in the flux term $\bar{u}{q}^{\prime }$ . Western regions, with a weaker vertical response, transition more gradually. This integrated vertical-horizontal mechanism explains the spatial heterogeneity of Kelvin-wave impacts and highlights the need for forecasting systems to resolve regional differences in vertical humidity sensitivity.