Dynamical Downscaling of Climate Change Projection over the African Region using the Regional Climate Model (RegCM3)

Results are presented from regional climate model experiments over the African region. Simulated present day climate (1990 to 2007) is examined and compared to climate projections for the 1990 to 2007 period. The model used is the resident regional climate model at the International Centre for Theoretical Physics referred to as RegCM. Horizontal resolution was fixed at 50km as an optimal compromise between computing power and model performance. For the present day simulation, referred to here as the ‘control’ case, boundary conditions were derived from the ERA-Interim database. The projected climate simulation, referred to here as the ‘scenario’ case, is intended to respond to a forcing described by the A1B emission scenario. The boundary conditions were derived from the global climate simulation carried out with the ECHAM5 model. The control simulation (forced by ERA-Interim) performs quite well in terms of temperature and precipitation during the eighteen years of validation. The bias differences for RegCM compared with CRU dataset show the West African Monsoon penetrates too far northward during the summer, resulting a negative precipitation bias over the Guinea Coast (Sylla MB. Et al. 2009). With ECHAM forcing we found an opposite bias to that of the ERA-Interim: the monsoon is located too far southward; this illustrates the important of the boundary conditions. The scenario simulation shows a warming of 0.5 to 6 degrees for the period 2081-2100 compared to present day everywhere on the continent and for all seasons. In general, future increases in temperature correspond with areas of decrease precipitation. For example, over the Sahel region precipitation decreases by 40% while the temperatures increases by almost 5 degrees. If precipitation decreases, temperatures can increase due to both reduced cloud cover and a decrease in soil moisture and latent heat flux (i.e., a greater Bowen ratio). Compared to the ECHAM change signal we notice a slightly bigger warming over the Guinea Coast both for summer and winter season. The GCM precipitation change signal over the western African region show a drying of the Sahel and an increase of precipitation over the Guinea Coast. In the regional model the same dipole is shown but slightly shifted to the south. If we compute the two model precipitation regression on to the SST anomaly mean index over the Guinea Coast we obtain a similar precipitation anomaly pattern.