6 month report

Sylwia Trzaska

Diagnostic and predictability studies using observed rainfall and NCEP reanalysis data conducted during the first year of the programme have highlighted the importance of local surface conditions in the seasonal evolution of the West African Monsoon via low-level Moist Static Energy (MSE) gradients. Spring (March-April) values of such gradients appear to be usefull predictors of the seasonal rainfall amounts in the Sahel. In particular July-September wet seasons are usually preceeded by stronger gradients in Spring and a later northward shift of the rainbelt. - The relative role of the continental vs sea surface conditions in the formation and relaxation of these gradients and the seasonal evolution of the mosoon are studied through numerical experiments. The Meteo-France's AGCM ARPEGE-Climat is forced by perturbed SST associated with dry/wet sahelian season and by perturbed seasonal cycle of vegetation supposed to influence local moisture recycling. These ensembles of 6 annual cycle simulations are currently being done. - Preliminary studies aimed at validating the model in the West African region have been done on an ensemble of 8 50-yr simulations forced by observed SST (1948-98) and made available by CERFACS (France). Diagnostics revealed a quite high skill of the model in simulating the interannual variability of the sahelian rainfall but also the shift into a persistently dry period at the end of 1960's. However, during this multi-decadal dry period, the wettest years are better reproduced than the dryiest ones. Of particular interest is the good simulation of the time evolution of the monsoon during wet years with the aforementionned delayed northward shift of the rainbelt. This result highlights the importance of simulationg the whole cycle of African Monsoon, especially the spring conditions. Further investigations focusing on the role of soil moisture vs SST in this evolution are needed.