Complex Investigation of Intraseasonal Oscillations in West African Rainfall and Their Predictability Using Coupled Ocean-Atmosphere Models Over Nigeria

Intraseasonal rainfall variability in West Africa, governed principally by the Madden-Julian Oscillation and African Easterly Waves, creates 10 to 90 day rainfall modulations that critically affect agricultural calendars, flood risk, and water resource management across Nigeria. This study presents a complex methodological investigation of intraseasonal oscillations in Nigerian rainfall and their predictability using state-of-the-art coupled ocean-atmosphere models. Extended-range ensemble forecasts from the ECMWF Subseasonal-to-Seasonal (S2S) prediction system are evaluated against NiMet daily station records and IMERG satellite rainfall for 100 rainy season months spanning 2016 to 2023. Spectral analysis using multi-taper methods and wavelet transform is applied to isolate intraseasonal variance in observed and model-simulated rainfall. Original contributions include the first systematic characterization of MJO phase-conditioned rainfall anomaly maps at the state level across Nigeria's six geopolitical zones, revealing that North Central and South West states exhibit the strongest and most spatially coherent MJO rainfall signal among all Nigerian regions. The ECMWF S2S model demonstrates useful intraseasonal forecast skill over Nigeria at lead times up to 25 days during active MJO events, with skill dropping sharply during suppressed MJO periods, a result with direct implications for operational sub-seasonal forecast use in NiMet's advisory services. A novel regime-dependent forecast skill communication framework is developed, enabling NiMet to issue differentiated confidence statements on sub-seasonal outlooks based on objectively assessed MJO activity. Keywords: intraseasonal oscillation, MJO, Nigeria, S2S prediction, rainfall predictability.