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Abstract

The Congo River Basin is one of the world’s major tropical rainfall centres, sustaining dense rainforests, river discharge, regional climate regulation and millions of people. Here, we show that a spatially coherent aridification hotspot has emerged within the basin during 1981-2023, driven by the combined decline of precipitation and rise in atmospheric evaporative demand, challenging the classical expectation that humid tropical regions should become wetter under global warming. The results reveal that, whilst local recycling and regional continental contributions dominate the climatological moisture supply, the long-term precipitation decline is most strongly associated with weakening oceanic moisture contribution, which accounts for ~54% of the statistically explained trend, despite representing the lower moisture input. This decoupling between climatological dominance and trend-driving relevance demonstrates that the sources sustaining mean rainfall are not those controlling its long-term trajectory, a finding with broad implications for understanding hydroclimatic change in a tropical recyclingdominated system. The drying hotspot is also associated with weakened moisture convergence, atmospheric warming and vertically structured humidity changes, suggesting a declining efficiency of the atmosphere in converting available moisture into rainfall. These results underscore that understanding aridification in this region requires moving beyond conventional precipitation diagnostics towards integrated moisture-source and circulation-based frameworks, which represent a major scientific challenge in the current and future scenarios.