High-resolution regional climate modeling for the Volta region of West Africa

[1] The Volta region is a climate-sensitive semiarid to subhumid region in West Africa. To investigate the impact of expected global climate change on regional water availability, regional climate modeling was performed. Two time slices (1991-2000 and 2030-2039) of the ECHAM4 scenario IS92a were dynamically downscaled with MM5 to a spatial resolution of 9 km. The quality of MM5 simulations in reproducing regional climate was assessed using reanalysis data for initial and boundary conditions. Although an underestimation of coastal rainfall was detected, sufficient accuracy in the Volta Basin could be achieved. The regional climate simulations show an annual mean temperature increase of 1.2-1.3 degrees C in the Volta region. This temperature change significantly exceeds interannual variability. A mean annual change in precipitation from -20% to +50% (similar to -150 to +200 mm) is simulated, with a spatial mean increase of 5% (similar to 45 mm). In the rainy season, rainfall predominantly increases, whereas a strong decrease is found for April, which is connected to a delay in the onset of the rainy season. In addition, interannual variability in the Volta region increases in the early stage of the rainy season. The climate change signals in infiltration excess and evapotranspiration show a nonlinear response to precipitation change. Aridity, expressed by the de Martonne aridity index, does not change significantly. The change signal in precipitation predominantly lies within the range of interannual variability. In contrast, the decrease in April exceeds interannual variability in the Sahel region.