West African Reservoirs Exhibit Variable Sensitivity to Large-Scale Climate Patterns

Abstract. Limited understanding of current trends in reservoir surface extent in West Africa and how large-scale climate patterns influence these dynamics challenges efforts to anticipate stress on water resources. To address this gap, we assessed reservoir sensitivity to large scale climate patterns across West Africa. We linked reservoir surface extent dynamics from 1985 to 2022 for 412 reservoirs with ten large-scale climate patterns represented by ten Sea Surface Temperature Anomaly (SSTA) indices, using supervised machine learning approaches and further analyzed the regional clustering in reservoirs responses. Most West African reservoirs, with significant trend, exhibit declining surface extent from 1985 to 2022 and respond primarily to combined Indo-Mediterranean-Atlantic climate signals rather than to single SSTA indices. Western Mediterranean Index (WMED) emerged as the dominant driver and is generally positively associated with reservoir surface extent, while the Eastern Mediterranean Index (EMED) showed contrasting effects. Secondary contributions were observed from Atlantic modes, particularly the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Mode (AMM). Regression coefficients of SSTA indices vary widely across reservoirs, with limited significant spatial clustering, indicating variable sensitivity of reservoirs to large-scale climate patterns potentially shaped by local factors. This spatial heterogeneity further stressed the need to investigate its implication for adaptation strategies, as well as the effects of local management practices and unique reservoir configuration on reservoir sensitivity to large scale climate patterns.