West African Monsoon Decadal Variability and Surface-Related Forcings: Second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)

The Sub-Sahara Africa is a diverse climatic and economically fragile region and dramatic change over the Sahelian Africa from wet conditions in the 1950s to much drier conditions in the 1970s-1980s and then to partial recovery from the 1990s represents one of the strongest interdecadal climate variability and the longest drought on the planet in the twentieth century. A major climate feature in the Sahelian Africa is the West African monsoon (WAM), which variability dominants the climate variability there. However, the CMIP5 coupled models consistently underestimate the WAM decadal variability and the drought.

In past several decades, the West African monsoon community has recognized the importance of external forcings: oceans, land processes including land cover and land use change (LULCC), aerosols, and greenhouse gases, on WAM variability, especially their roles in the Sahel drought. However, most of these studies only focused on one external forcing with one single model.

The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings in the Sahel climate system at seasonal to decadal scales. The project’s strategy is to apply prescribed observationally based anomaly forcing, i.e., “idealized but realistic” forcing, in simulations by climate models. The goal is to assess these forcings’ effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions.

The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the 20th century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60% of the precipitation difference during the period. The present paper also addresses the role of SSTs in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone (ITCZ) before the WAM onset, and the latter mainly contributes to the summer WAM drought.

The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40% of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified.

In the dust experiment, the direct impact of dust on the radiation budget and its influence to the Sahel rainfall are evaluated using the GOCART dust data and its effect also contributes to the drought (less than 20% of the drought). In addition, some preliminary results for impact of the greenhouse and global warming on the Sub-Sahara climate decadal variability is discussed.

WAMME is the first attempt to use multi-GCMs and RCMs to collectively explore the roles of multiple external forcing in WAM variability. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land and aerosols conditions combine synergistically to favor drought. WAMME2’s achievement provide better understanding of relative importance of various forcing and possible feedback mechanisms, complementary to experiments under CMIP, which are focused more on impacts of emission control scenarios. In the recent CLIVAR land-monsoon initiative, land/atmosphere feedbacks (vegetation, dust) were identified for further investigation. The WAMME project provides a proto-type approach for multi-model experiments to have comprehensive understanding of monsoon/land-surface interactions.