Abstract
West Africa exhibits decadal patterns in the behaviour of droughts and floods, creating challenges for effective water resources management. Proposed drivers of prolonged shifts in hydrological extremes include the impacts of land-cover change and climate variability in the region. However, while future land-degradation or land-use are highly unpredictable, recent studies suggest that prolonged periods of high-flows or increasing flood occurrences could be predicted by monitoring sea-surface temperature (SST) anomalies in the different ocean basins. In this study, we thus examine: i) what ocean basins would be the most suitable for future seamless flood-prediction systems; ii) how these ocean basins affect high-flow extremes (hereafter referred as extreme streamflow); and iii) how to integrate such nonstationary information in flood risk modelling. We first use relative importance analysis to identify the main SST drivers modulating hydrological conditions at both interannual and decadal timescales. At interannual timescales, Pacific Niño (ENSO), tropical Indian Ocean (TIO) and eastern Mediterranean (EMED) constitute the main climatic controls of extreme streamflow over West Africa, while the SST variability in the North and tropical Atlantic, as well as decadal variations of TIO and EMED are the main climatic controls at decadal timescales. Using regression analysis, we then suggest that these SST drivers impact hydrological extremes through shifts in the latitudinal location and the strength of the Intertropical Convergence Zone (ITCZ) and the Walker circulation, impacting the West African Monsoon, especially the zonal and meridional atmospheric water budget. Finally, a nonstationary extreme model, with climate information capturing regional circulation patterns, reveals that EMED SST is the best predictor for nonstationary streamflow extremes, particularly across the Sahel. Predictability skill is, however, much higher at the decadal timescale, and over the Senegal than the Niger catchment. This might be due to stronger impacts of land-use (-cover) and/or catchment properties (e.g. the Inner Delta) on the Niger River flow. Overall, a nonstationary framework for floods can also be applied to drought risk assessment, contributing to water regulation plans and hazard prevention, over West Africa and potentially other parts of the world.
Highlights
Droughts and floods are responsible for approximately 80% of fa talities, and around 70% of economic losses that are related to natural hazards in sub-Saharan Africa (Vicente-Serrano et al, 2012)
While future land-degradation or land-use are highly unpredictable, recent studies suggest that prolonged periods of high-flows or increasing flood frequency could be predicted by monitoring SSTa in the different ocean basins that influence the region’s climate (Sidibe et al, 2019)
This study first examines the contributions of the different ocean basins at both interannual and decadal timescales and their larger-scale mechanisms
Summary
Droughts and floods are responsible for approximately 80% of fa talities, and around 70% of economic losses that are related to natural hazards in sub-Saharan Africa (Vicente-Serrano et al, 2012). Despite persistent widespread droughts, damaging floods have occurred in the Sahelian region of West Africa, e.g. the 1996, 1998 and 2010, 2012 and 2013 floods in Niamey (Tarhule, 2005; Descroix et al, 2013; Sighomnou et al, 2013) This persistence in flood occurrence could be linked to the post-1990s prolonged increase in base flows in many regions of West Africa, especially in the central Sahel regions (Roudier et al, 2014; Sidibe et al, 2018). Little has been done to under stand the shifting patterns of extreme streamflow (i.e., high-flow or floods) over West Africa, as well as the underlying climate mechanisms
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