Simulated Responses of the West African Monsoon and Zonal-Mean Tropical Precipitation to Early Holocene Orbital Forcing

This study seeks to improve our mechanistic understanding of how the insolation changes associated with orbital forcing impact the West African monsoon and zonal-mean tropical precipitation. We impose early Holocene orbital parameters in simulations with the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model, either with fixed sea surface temperatures, a 50-m thermodynamic slab ocean, or coupled to a dynamic ocean (CM2.1). In all cases, West African Monsoon rainfall expands northward, but the summer zonal-mean Intertropical Convergence Zone does not-there is drying near 10 degrees N, and in the slab ocean experiment a southward shift of rainfall. This contradicts expectations from the conventional energetic framework for the Intertropical Convergence Zone location, given anomalous southward energy fluxes in the deep tropics. These anomalous energy fluxes are not accomplished by a stronger Hadley circulation; instead, they arise from an increase in total gross moist stability in the northern tropics. Plain Language Summary Fossils, sediment records, and other evidence show that 10,000 years ago (10 ka), much of Northern Africa was substantially wetter. At the time, due to natural orbital variations, the Earth was closest to the sun during Northern Hemisphere (NH) summer, as opposed to NH winter today. This enhanced the equator-to-pole difference in incoming sunlight during NH summer. To better understand how this affected tropical rainfall, including the West African monsoon, we analyze atmospheric general circulation model simulations with either the modern or 10 ka orbital configuration. To isolate the role of sea surface temperature changes, one set of simulations fixes them at modern values, another represents the ocean as a static 50-m slab of water, and a third allows the ocean circulation to respond to the sunlight changes to respond to the sunlight changes. In all cases, the West African monsoon expands northward with the 10 ka orbit, consistent with fossil evidence. However, the NH summer tropical rainfall maximum moves southward, counter to the conventional understanding of how sunlight changes affect rainfall. Adjustments of the atmospheric circulation and its energy transport explain this result. This work improves our physical understanding of how the West African monsoon and the broader tropical precipitation respond to sunlight changes.