Explaining the Modulation of Forced Precipitation Changes with Internal Variability Storylines: The case of decadal Sahel Precipitation Changes

Abstract. Decadal Sahel rainfall is strongly influenced by internal climate variability, which can substantially modulate anthropogenically forced changes, particularly in the near- and mid-term. Yet, the complexity and inherent randomness of internal variability make it difficult to interpret, communicate and integrate into decision-relevant climate information. Here, we develop internal variability storylines of Sahel summertime precipitation using a large model ensemble. We identify the Atlantic Multidecadal Variability (AMV) and Pacific Decadal Variability (PDV) as two key oceanic drivers of decadal Sahel rainfall variability that act through distinct atmospheric mechanisms: AMV-related sea surface temperature changes are consistent with changes in the Atlantic inter-hemispheric temperature gradient, shifting the meridional position of the Intertropical Convergence Zone (ITCZ) and associated monsoon circulation, whereas positive PDV increases tropospheric stability and subsidence over the Sahel, suppressing convection. By conditioning on the combined phases of AMV and PDV, we construct physically coherent climate storylines that illuminate how decadal internal variability can organise tropical circulation and Sahel precipitation anomalies. Their effects reinforce most strongly in out-of-phase configurations, with AMV+/PDV− producing a wet Sahel, and AMV−/PDV+ a dry Sahel, while in-phase states show weak responses. These combined influences are captured by the North Atlantic Relative Index (NARI), which combines thermodynamic and dynamic impacts of oceanic forcings on the Sahel. A positive NARI corresponds to AMV+/PDV−, whereas a negative NARI is associated with AMV−/PDV+, with NARI explaining 37% of Total Sahel precipitation variability. Understanding the pathways of oceanic drivers reveals how they modulate the forced response: around mid-century, opposing storylines can more than double or cancel forced regional precipitation changes. Overall, our results demonstrate how internal variability storylines can extract physically grounded decadal climate information from large ensembles, providing a framework complementary to initalised predictions for regional climate information.