West African Monsoon System Research Articles

Coupled Model Simulations of the West African Monsoon System: Twentieth- and Twenty-First-Century Simulations

Abstract The ability of coupled GCMs to correctly simulate the climatology and a prominent mode of variability of the West African monsoon is evaluated, and the results are used to make informed decisions about which models may be producing more reliable projections of future climate in this region. The integrations were made available by the Program for Climate Model Diagnosis and Intercomparison for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The evaluation emphasizes the circulation characteristics that support the precipitation climatology, and the physical processes of a “rainfall dipole” variability mode that is often associated with dry conditions in the Sahel when SSTs in the Gulf of Guinea are anomalously warm. Based on the quality of their twentieth-century simulations over West Africa in summer, three GCMs are chosen for analysis of the twenty-first century integrations under various assumptions about future greenhouse gas increases. Each of these models behaves differently in the twenty-first-century simulations. One model simulates severe drying across the Sahel in the later part of the twenty-first century, while another projects quite wet conditions throughout the twenty-first century. In the third model, warming in the Gulf of Guinea leads to more modest drying in the Sahel due to a doubling of the number of anomalously dry years by the end of the century. An evaluation of the physical processes that cause these climate changes, in the context of the understanding about how the system works in the twentieth century, suggests that the third model provides the most reasonable projection of the twenty-first-century climate.

Seasonal Evolution and Variability Associated with the West African Monsoon System

Abstract In this study, the seasonal variations in surface rainfall and associated large-scale processes in the tropical eastern Atlantic and West African region are investigated. The 6-yr (1998–2003) high-quality Tropical Rainfall Measuring Mission (TRMM) rainfall, sea surface temperature (SST), water vapor, and cloud liquid water observations are applied along with the NCEP–NCAR reanalysis wind components and a 4-yr (2000–2003) Quick Scatterometer (Quik SCAT) satellite-observed surface wind product. Major mean rainfall over West Africa tends to be concentrated in two regions and is observed in two different seasons, manifesting an abrupt shift of the mean rainfall zone during June–July: (i) near the Gulf of Guinea (about 5°N), intense convection and rainfall are seen during April–June and roughly follow the seasonality of SST in the tropical eastern Atlantic, and (ii) along the latitudes of about 10°N over the interior of the West African continent, a second intense rain belt begins to develop in July a...

Impact of greenhouse warming on the West African summer monsoon

The Meteo-France climate model has been used to run a time-dependent climate change experiment in order to study the impact of increasing amounts of greenhouse gases and aerosols on the West African monsoon system. A transient climate simulation of 150 years has been performed with a coupled ocean/sea-ice/atmosphere model including stratospheric ozone with the greenhouse gas concentrations changing annually according to one of the new scenarios of the Intergovernmental Panel on Climate Change. The simulated climate change has been analyzed as the difference between two 30-year time-slices (1980–2010 and 2070–2100 respectively) with particular attention paid to the hydrological cycle. The climate of the recent period, validated in detail by comparison to reanalyses, shows that the model is able to reproduce a realistic seasonal cycle of the African monsoon. The precipitation changes simulated at the end of the twentyfirst century show an enhanced monsoon precipitation over West Africa. The analysis of the atmospheric branch of the water cycle highlights some of the parameters that control these precipitation anomalies: changes in precipitable water, water vapour recycling, moisture convergence and precipitation efficiency. These changes of the hydrological processes also cause corresponding changes in the thermal state of the atmosphere such as a smaller warming of the surface in the Sahel linked to stronger evaporation in this region. This results in changes of the mean zonal circulation through the thermal wind relationship and of position and strength of key elements of the general circulation over the West Africa (Hadley and Walker circulations, African Easterly Jet).