Impacts of Lateral Boundary Conditions on Regional Climate Projections over West Africa

Climate simulation and projections using regional climate models (RCMs) are sensitive to the choice of lateral boundary conditions (LBCs) which often are derived from global climate models (GCM). These LBCs used to drive RCMs describe the atmospheric conditions at the boundaries regarding atmospheric dynamics as well as atmospheric physics, including wind, atmospheric pressure, temperature and humidity. Understanding the impact of each component of LBCs is important for understanding and reducing the uncertainty in and improving the accuracy of RCM climate simulations and projections. In this study, the ICTP Regional Climate Model Version 4 (RegCM4) is used to investigate the impact of LBC on projected future changes of regional climate in West Africa. To examine this, present, future and several modified experiments are conducted with various combinations of LBCs and other climate change factors (including CO2 concentration and sea surface temperature (SST)) using RegCM4, and differences among the experiments are compared to identify the most important drivers for RCMs. The LBCs are derived from the global climate model ECHAM5. When driven by changes in all factors, the RegCM4-produced future climate changes include drier conditions in Sahel and wetter conditions along the Guinean coast. The impact of CO2 concentration alone (in the RCM context) or atmospheric dynamics alone is not significant. Changes in the atmospheric humidity alone at the domain boundary lead to a wetter Sahel due to the northward migration of rain belts during summer. This impact, although significant, is offset and dominated by other factors that have a stronger impact. Changes of atmospheric temperature at the domain boundaries alone lead to a drier future over most of the model domain. This, when combined with SST changes over Ocean, produces a future prediction that closely resemble the changes caused by all factors combined. Further analysis demonstrates that the changes of moisture flux convergence dominate the projected precipitation changes. Moreover, it is founded that the response of the RCM climate to different climate change factors is primarily linear in that the projected changes driven by all factors combined are close to the sum of projected changes due to each individual factor alone. Findings from this study may be region-dependent, which will be examined in follow-up studies