Mid-holocene and Last Glacial Maximum African monsoon changes as simulated within the Paleoclimate Modelling Intercomparison Project

As part of the Paleoclimate Modelling Intercomparison Project (PMIP), several atmospheric general circulation models have performed climate simulations of the mid-Holocene (6 ka BP) and of the Last Glacial Maximum (LGM) climates (21 ka BP) using the same experimental design for boundary conditions, insolation and CO2 forcing. PMIP results are used in this study to investigate how the position of the intertropical convergence zone (ITCZ) changes over West Africa throughout the seasonal cycle in these two climates. The Mid-Holocene corresponds to a period of enhanced seasonal cycle with colder winters and warmer summers, leading to an increase of the African monsoon. During LGM, the presence of large ice sheets and lower CO2 give rise to a colder climate and a reduced summer monsoon. For the two time periods and all the models, it is found that over West Africa, both the atmospheric low level temperature gradient and the position of the maximum of surface temperature control the position of the ITCZ. The energetic changes associated with the change in the hydrological cycle are driven by latent heat release in the atmosphere within the rainbelt and by radiative fluxes north of the ITCZ. The sensible heat flux plays also a non-negligible role over the desert during mid-Holocene. Differences between the PMIP simulations reflect differences in model parameterization, including surface processes and clouds. However, for a given model, the partitioning between the changes in the different heat fluxes is quite similar for the mid-Holocene and the LGM climate change. (C) 2000 Elsevier Science B.V. All rights reserved.