Internal Variability Storylines for near-term Summer Rainfall in the Sahel

Internal climate variability is a major source of uncertainty in decadal predictions of summer rainfall in the Sahel, where it can amplify or attenuate the impact of anthropogenically forced changes. Yet, its complexity and inherent randomness make it difficult to interpret, communicate, and integrate into decision-relevant information. 

Here we introduce a novel framework to construct physically plausible storylines of internal variability (IVSs), based on a large set of multi-model large ensembles, which provides a robust sampling of internal variability and model uncertainty. Selecting Pacific Decadal Variability (PDV) and Atlantic Multidecadal Variability (AMV) as remote drivers of Sahelian climate, we identify coherent trajectories of internal variability that are physically interpretable.

We find that the proposed IVSs capture - and separate -  a range of plausible near-term, decadal futures shaped by Atlantic and Pacific variability patterns, shedding light on the dynamics that modulate decadal rainfall regimes. In particular we identify a high-impact storyline of a positive AMV in-phase with a negative PDV leads to wet anomalies which roughly double the forced response over the Western Sahel, while the contrasting storyline largely offsets the effect of climate change in this region. 

By combining physical understanding with robust statistical methods and multi-model large ensembles, the proposed method seeks to bridge the gap between climate science and action, providing more interpretable and decision-relevant regional climate information.