Changes in large-scale dynamics over West Africa—the strength and position of zonal jets—are a key interim step by which local and remote forcing is communicated into changes in rainfall. This study identifies a key mode of jet variability and demonstrates how it is strongly coupled with rainfall. The approach provides a quantitative framework to assess jet–rainfall coupling and a useful tool to investigate the concerning spread in CMIP5 rainfall projections over the West African Sahel. It is shown that many CMIP5 simulations fail to capture this coupling, indicating a fundamental limitation in their ability to predict future rainfall conditions. The results demonstrate that West African rainfall in the coming CMIP6 ensemble should be interpreted with caution; key atmospheric processes that deliver rainfall must be validated before conducting detailed analysis on rainfall.