The African monsoon during the early Eocene from the DeepMIP simulations

Here we present a study of African climate (with a focus on precipitation) during the early Eocene (~55-50 million years ago, Ma), as simulated by an ensemble of state-of-the-art climate models under the auspices of the Deep-time Model Intercomparison Project (DeepMIP).  The early Eocene is of particular interest, because with CO₂ levels ranging between 1200-2500 ppmv (and a resulting temperature increase of ~5°C in the tropics and up to ~20°C at high latitudes) it provides a partial analogue for a possible future climate state by the end of the 21st century (and beyond) under extreme emissions scenarios.  This study is novel because it investigates the relatively little-studied subject of African hydroclimate during the early Eocene, a period from which there are very few proxy constraints, requiring more reliance on model simulations.

 

A comparison between the DeepMIP pre-industrial simulations and modern observations suggest that model biases aremodel- and geographically dependent.  However, the model ensemble mean reduces these biases and is showing the best agreement with observations.  A comparison between the DeepMIP Eocene simulations and the pre-industrial suggests that, when all individual models are considered separately, there is no obvious wetting or drying trend as the CO₂ increases.  However, concerning the ensemble mean, the results suggest that changes to the land sea mask (relative to the modern) in the models may be responsible for the simulated increases in precipitation to the north of Eocene Africa, whereas it is likely that changes in vegetation (again relative to the modern geographical locations) in the models are responsible for the simulated region of drying over equatorial Eocene Africa.  When CO₂ is increased in the simulations, at the lower levels of increased CO₂, precipitation over the equatorial Atlantic and West Africa appears to be increasing in response.  At the higher levels of CO₂, precipitation over West Africa is even more enhanced relative to the lower levels.  These precipitation increases are associated with enhanced surface air temperature, a strongly positive P-E balance and cloud cover increases.  At the lower levels of increased CO₂, anticyclonic low-level circulation increases with CO₂, drawing in more moisture from the equatorial Atlantic and causing a relative drying further north.  At higher levels of CO₂, the increased anticyclonic low-level circulation is replaced by increased south-westerly flow.

 

Lastly, a model-data (using newly-compiled Nearest Living Relative reconstructions) comparison suggests that whether the Eocene simulations (regardless of CO₂ experiment) over- or underestimate African precipitation is highly geographically dependent, with some of the CO₂ experiments at some of the locations lying within the uncertainty range of the reconstructions.  Concerning the ensemble mean, the results suggest a marginally better fit with the reconstructions at lower levels of CO₂.