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Modes of climate variability are known to influence rainy season onset, but there is less understanding of how they impact flood timing. We use streamflow reanalysis and gauged observation data sets to examine the influence of the Indian Ocean Dipole and El Nino-Southern Oscillation across sub-Saharan Africa. We find significant changes in flood timing between positive and negative phases of both Indian Ocean Dipole and El Nino-Southern Oscillation; in some cases the difference in the timing of annual flood events is more than three months. Sensitivity to one or other mode of variability differs regionally. Changes in flood timing are larger than variability in rainy season onset reported in the literature, highlighting the need to understand how the hydrological system alters climate variability signals seen in rainy season onset, length, and rainfall totals. Our insights into flood timing could support communities who rely on flood-based farming systems to adapt to climate variability.

Plain Language Summary Patterns of climate variability such as the El Nino-Southern Oscillation or the Indian Ocean Dipole affect the timing of the start of the rainy season, but little is known about how this translates into changes in the timing of the annual flood in rivers. We use computer model reconstructions and observed records of river flows in Africa to understand how these patterns of climate variability are changing the timing of river floods. In eastern and southern Africa, in particular, the differences in flood timing can be more than three months. This information could help farmers in African floodplains to adapt their water management, planting, and cropping practices to these patterns of climate variability.