Mechanisms of North Atlantic Influence on the West African Monsoon: Thermodynamic or Dynamic? A Hierarchical Bayesian Approach.

The dramatic decadal swings in Sahel precipitation over the 20th century have been linked in observations and in simulations to global sea surface temperature (SST) in multiple ocean basins, especially the North Atlantic and the Global Tropics. But there is still no consensus in the scientific community on the physical pathways that relate SST to Sahel rainfall. To increase confidence in 21st century projections, it is of critical importance to understand the mechanisms that control Sahel rainfall in different models, and to evaluate the simulation of these pathways relative to observations. We design a hierarchical Bayesian model in which local column stability determines precipitation, the Global Tropics control dry static energy in the upper tropical troposphere, and the North Atlantic can influence Sahel precipitation in two ways: by directly supplying moisture, or by increasing moisture transport into the Sahel from the South. We evaluate the magnitudes of these two processes relative to each other and relative to total precipitation variability in the observations and CMIP6 simulations. The hierarchical nature of the Bayesian model allows simulations and observations to partially constrain one another while maintaining separate parameter estimates for each model. We then investigate the effect of differences in the relative importance of these processes on 20th and 21st century simulations of Sahel rainfall, and use the observed parameter values to constrain 21st century projections.