Semi-arid Sahelian Africa is one region that has experienced significant changes in climate in the past century. Annual rainfall has persistently remained below the long-time average since the late sixties. Starting from the late 1980s, however, there is evidence of some rainfall recovery relative to the very dry period. Despite recent progress in understanding the effects of boundary forcing on West African monsoon (WAM) variability, the monsoon precipitation and associated important features, as well as their strong and complex interactions with oceanic, land processes, and aerosols, are still not well understood, due in part to lack of detailed observations and to the inability of general circulation models (GCMs) to simulate these features at different temporal and spatial scales.

The West African Monsoon Modeling and Evaluation project (WAMME), a CEOP Inter-Monsoon Study (CIMS) initiative, uses GCMs and regional climate models (RCMs) to address issues regarding the role of ocean-land-atmosphere interactions, land-use and water-use changes, vegetation dynamics, as well as aerosols, particularly dust, on the WAM development as well as the long-term drought and partial recovery. WAMME also has close collaborations with the African Monsoon Interdisciplinary Analysis (AMMA) project and will use AMMA field data for validation.

In the first phase, a set of experiments consisting of four spring and summer seasons in the 21th century with different climate characteristics were selected to test the state-of-the-art models' ability to simulate the WAM's basic features on diurnal, intraseasonal, seasonal, and interannual scales. The model intercomparison emphasizes the WAM evolution, its onset and demise, precipitation intensity and frequency, and some important modes. The evaluation of the simulated rainfall at diurnal, intraseasonal, and seasonal scales and its link with the large scale dynamics (such as strength of the African Easterly Jet (AEJ) and penetration of the monsoon flow) and external forcing (such as SST, meridional gradients of surface temperature and soil moisture, surface evaporation and energy partitioning) will be the primary focus. Interannual variability will also be documented. The observational data from AMMA will be applied to evaluate the model simulation. In addition, the RCM's downscaling ability will be evaluated when reanalyses as well as GCM outputs are imposed as lateral boundary conditions. The RCM's ability to reproduce the sudden jump in rainfall at monsoon onset and in improving the representation of the AEJ and diurnal variability will be the focus.

We believe the results from this first set of experiments could be a good starting point providing benchmarks for further studies to understand the roles of external forcing, and internal dynamics in WAM variability.