Background
The authors write that "despite recent progress in the monitoring and understanding of the WAM [West African Monsoon] within the framework of the African Monsoon Multidisciplinary Analysis (AMMA), there are still large uncertainties in projections of future climate in this region, such that even the sign of future precipitation change is uncertain [italics added]," citing Solomon et al. (2007).

What was done
Zheng and Braconnot, as they describe it, "revisit the results of PMIP2 simulations over Africa using two approaches." The first "considers the ensemble of simulations in order to determine how well the PMIP2 models [of today] reproduce some of the basic features of the summer monsoon precipitation," while the objective of the second is "to understand model differences by considering model characteristics for present-day climate and their sensitivities to insolation change."

What was learned
First of all, the two scientists report that the "meridional temperature gradient is underestimated between 0° and 20°N by the PMIP2 model median, resulting in a smaller gradient of sea level pressure between the Gulf of Guinea and [the] Sahel," which helps to explain "a lower than observed low-level moisture flux and an underestimate of rainfall intensity when compared with observations." Second, they say that "the northward extent of the rain belt and the intensity of precipitation change are underestimated." Third, they indicate that "the models overestimate the solar radiation." Fourth, they acknowledge that the models "underestimate the cloud radiative forcing in deep and moderate convective regimes." And fifth, they state that "some of the models have too strong a coupling between the latent heat and convection in deep convective regimes."

What it means
Quite clearly, there is still a lot of work to be done by the climate modeling community before their models are, as the saying goes, "ready for prime time," especially in the case of the West African Monsoon.

Reference
Solomon, S., Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M.B., Miller Jr., H.L. and Chen, Z. (Eds.). 2007. Climate Change 2007: The Physical Science Basis. Cambridge University Press, Cambridge, United Kingdom.