Radiation flux variability over West Africa: the role of clouds, aerosols and water vapour in observations and CMIP5 models

We explore the ability of CMIP5 models to recreate the seasonal variability in top-of-the-atmosphere (TOA) and surface radiation fluxes over West Africa. This provides an important test of the CMIP5 models’ ability to describe the radiative energy partitioning, which is of key relevance for the current climate and its future changes, including to the West African monsoon. As a primary reference, we use 15 years of the monthly CERES EBAF/SYN1deg products, which we validate using other satellite products, reanalysis and surface station data. We interpret this with respect to cloud processes, column water vapour and aerosol loading across a number of distinct regions within the wider area of West Africa, including the semi-arid Sahel and tropical coastal regions.

We find that the CMIP5 multi-model mean is generally within the range given by the reference products, though the range of model seasonal cycles is large. We use seasonal and regional contrasts in all-sky fluxes to identify aspects which may be affected by the timing and progression of the west African monsoon. We compare coupled model output with atmosphere-only models with prescribed sea surface temperatures (SSTs). We find that although there are some improvements, for example the capture of the `little dry season' in the coastal region, there are still differences in the timing and extent of the monsoon. Furthermore, we examine and contrast the co-variability of clear-sky downwelling surface fluxes with water vapour and aerosol optical depth, both in EBAF/SYN1deg and the CMIP5 models.