Analysis of the Inter-Annual Variability of the West African Monsoon and Its Local Expression in the South East Geopolitical Zone

The West African Monsoon (WAM) is the dominant large-scale climate system governing seasonal rainfall across Nigeria, yet its inter-annual variability expresses locally in diverse and often counterintuitive ways across different geopolitical zones. This study analyzes the inter-annual variability of the WAM and its local meteorological expression across Nigeria's South East geopolitical zone covering Abia, Anambra, Ebonyi, Enugu, and Imo states. Rainfall records from 16 NiMet stations in the zone (1990 to 2022) were analyzed in relation to established WAM variability indices including the West African Monsoon Index, ITD latitude anomalies, and monsoon onset dates derived from NiMet operational records. Multi-regression and wavelet analyses were applied to decompose variance in zone rainfall into WAM-forced and residual local components. Results reveal that WAM index variability explains 55 to 68 percent of inter-annual rainfall variance at stations in the northern sections of the zone but only 32 to 44 percent at coastal-proximate stations in Imo and Abia, where Gulf of Guinea moisture dynamics contribute an additional independent source of variability. A significant regime shift in the WAM-rainfall relationship is detected post-2010, characterized by increased intraseasonal variability and a weakening of the traditional dry-season rainfall minimum in December, potentially attributable to changing SST patterns. The study contributes to understanding of regional manifestations of WAM variability and provides a reference for bias-assessment of climate model simulations over the South East. Keywords: West African Monsoon, rainfall variability, South East Nigeria, monsoon index, inter-annual.