Rainfall Onset Timing and Hydroclimatic Variability in a Tropical Catchment: Insights From Observations and Climate Projections

ABSTRACT

Variability in the seasonal onset of rainfall remains a challenge in tropical regions, yet its spatial and temporal patterns remain poorly characterised at the catchment scale in West Africa. Here, we perform a station‐level analysis of rainfall onset, centre of mass dynamics and multi‐day accumulation across the Pra River Catchment. We used the Mann–Kendall trend test and linear regression to detect systematic shifts in onset over the observational record, and Kernel density estimation to characterise scale‐dependent rainfall distribution over 1‐, 5‐, 15‐ and 20‐day accumulation windows. The method is evaluated against historical simulations and future projections of the CMIP6 multi‐model ensemble under SSP2‐4.5 and SSP5‐8.5 scenarios. Results show spatial heterogeneity in rainfall onset trends, with statistically significant earlier shifts at several southern, eastern and western stations, while most stations show no significant trend. Furthermore, ENSO shows only a weak probabilistic influence on onset timing, with a significant phase response detected at Mfensi. CMIP6 simulations consistently underestimate rainfall variability and upper‐tail extremes relative to observations across all accumulation windows. Future projections indicate that climate risk in the catchment will be driven primarily by the intensification of extreme events rather than shifts in mean rainfall. Our results point toward a dominance of interannual variability over systematic regional change, and a stronger proximal control on Guinea Coast onset dynamics than the remote Pacific teleconnection by the Atlantic equatorial mode. These findings provide a quantitative basis for improving seasonal forecasting, adaptive agricultural planning and long‐term water resources governance across tropical hydrologically significant catchments.