Dynamics of Intense Multi-Day Wet Spells over West Africa

Abstract

Intense multi-day wet spells, characterized by prolonged heavy rainfall, significantly influence West Africa’s hydroclimate, driving flood risks and shaping socio-economic vulnerabilities. However, their dynamics remain poorly understood due to observational data limitations, methodological inconsistencies, and the complex interplay of atmospheric processes. The study defines intense 3-day and 5-day wet spells as rainfall accumulations exceeding the 90th percentile, with specific wet-day criteria and addresses critical gaps in understanding their characteristics by investigating the drivers, and future changes of intense 3-day and 5-day wet spells over West Africa from 1982–2020 to enhance climate resilience and disaster preparedness. Utilizing a novel high-resolution gridded rainfall dataset (KASS-D), developed from 239 quality-controlled rain gauge stations (1982–2020) via SPHEREMAP interpolation, the research evaluates rainfall variability across the Guinea Coast, Savannah, and Sahel. KASS-D outperforms global datasets (GPCC, CHIRPS, IMERG) in capturing daily rainfall, onset/cessation periods, and extreme events, with lower root-mean-square error and higher probability of detection, making it a robust tool for hydrometeorological analyses in data-sparse regions and was selected as the standard for characterizing wet spells. Spatial analyses reveal a latitudinal gradient, with the Guinea Coast and Cameroon Highlands exhibiting the highest frequencies and intensities. Temporal trends indicate a post-2000 increase in large-scale, longer-duration wet spells, contributing significantly to annual rainfall (20% in the Sahel, 16% for 3-day and 14% for 5-day spells in the Savannah, 9% in the Guinea Coast). A composite analysis of wet and dry spells reveals a dipole-like atmospheric structure influencing multi-day rainfall events. Wet spells are preceded by easterly anomalies, with a weakened AEJ and strengthened TEJ that enhance deep convection, while dry spells involve offshore moisture export and suppressed vertical development due to deeper moisture depth. Intense 3- and 5-day wet spells is associated with high total column water vapor and strong low-level moisture flux convergence fueled by Atlantic and Gulf of Guinea moisture advection, dual cyclonic systems over the Atlantic and Sahel maintained by Southern Hemisphere anticyclonic flow. Their lifecycle involves vertically coupled dynamics with strong low-level southwesterlies and upper-level easterlies during initiation and peak phases, while the decay phase involves weakened inflows and reduced humidity. Multi-day wet spells significantly increase flood risks in West Africa, especially in the Sahel and Guinea Coast. Their predictability via atmospheric signals like SHL dynamics and AEWs can enhance early warnings. Adjusted observation datasets using climate change signal obtained from the ensemble mean of 23 NEX-GDDP-CMIP6 models under SSP2-4.5 and SSP5-8.5 scenarios, suggest intensified wet spell frequency and magnitude, particularly under SSP5-8.5, with increases exceeding 15% in coastal zones and the Niger Delta. Frequency trends show annual rises of 0.02–0.29 events/year in the Savannah and Sahel, signaling a shift toward a wetter, more extreme hydroclimate. These changes heighten risks to agriculture, urban infrastructure, and water management, necessitating adaptive strategies. The findings underscore the critical role of wet spells in flood risks, particularly in the Sahel and Guinea Coast, where prolonged rainfall amplifies socio-economic impacts. The predictability of atmospheric precursors offers opportunities to enhance early warning systems, with lead times of 3–5 days. Recommendations include expanding gauge networks, integrating KASS-D into forecasting platforms, and fostering regional cooperation for transboundary flood management. By informing flood risk models, community preparedness, and climate services, this research provides actionable insights for policymakers to develop anticipatory disaster risk reduction mechanisms, strengthening resilience in West Africa’s climate-vulnerable communities.