Changes in West African Monsoon Precipitation: The Competition between Large-scale Dynamics and Local Thermodynamics

The changes in West Africa Monsoon (WAM) precipitation are studied by the convective regimes and energetic analyses on the basis of three simulations of mid-Holocene (MH), sstClim4xCO2 and abrupt4xCO2. The atmospheric only simulation of sstClim4xCO2 shows similar changes in the pattern of WAM precipitation as that in the MH. The large-scale dynamics is found to be the main contribution to the changes in WAM precipitation, although the external forcing is completely different in these two simulations. The analyses of the moist and dry static energy (MSE and DSE, respectively) are also similar in these two simulations, showing a northward shift of the MSE and DSE maxima. However, such changes are not conclusive through models in abrupt4xCO2 simulation. Interestingly, in the very a few years when the 4xCO2 is imposed, the pattern of precipitation change is similar to those of MH and sstClim4xCO2, but the pattern reversed when the coupled system reaches an equilibrium state. The warming in the ocean tends to reduce the land-sea thermal contrast that leads to a negative contribution of large-scale dynamics, while the increase of CO2 and water vapor in the atmosphere tends to enhance the local thermodynamics. The large model spread thus result in the competition between large-scale dynamics and local thermodynamics in the models.