Seasonal Variability, Long-Term Trends, and Extreme-Value Statistics of ERA5 Wave Climate Along the West African Coast, 1940–2024

West African coastal zones are increasingly exposed to climatic and anthropogenic pressures that may enhance erosion, marine flooding, and changes in offshore wave conditions. This study investigates the spatial variability, seasonality, long-term trends, and extreme statistics of wave climate along the West African coast over 1940–2024 using six-hourly ERA5 data at twenty offshore stations from Mauritania to Sao Tome and Principe. Significant wave height (Hs), mean wave period (Tm), and mean wave direction (Dir) were analyzed. The results identify three broad wave-climate domains: a northwestern African domain dominated by energetic North Atlantic swells, a central transition domain with lower wave energy and higher directional variability, and a Gulf of Guinea domain influenced by persistent southerly to southwesterly wave components. The highest recorded Hs reaches about 4.25 m at Saint-Louis, while values below 1 m occur near Guinea. Seasonal contrasts show more energetic and directionally concentrated conditions during DJFM in the northwestern domain, whereas JASO is associated with lower wave heights and broader directional contributions. Long-term trends were assessed using the Hamed and Rao modified Mann–Kendall test, with Sen’s slope used to quantify trend magnitudes. Significant positive trends were found for monthly mean Hs and monthly Hs,P95, with Sen’s slopes ranging from 0.0106 to 0.0271 m decade−1 and from 0.0120 to 0.0369 m decade−1, respectively. These results indicate a stronger increase in high-percentile wave conditions than in mean conditions. Extreme wave statistics based on Generalized Extreme Value distributions show 100-year return levels of 4.32 m at Saint-Louis, 2.65 m at Conakry, and 3.24 m at Sao Tome West. Overall, this study provides a consistent offshore wave-climate baseline for West Africa, while emphasizing that coastal-impact applications require consideration of nearshore transformation, water levels, bathymetry, and uncertainties in the early ERA5 record.