AMMA-CATCH studies in the Sahelian region of West-Africa: An overview

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The African Monsoon Multidiciplinary Analysis (AMMA) is an international and interdisciplinary experiment designed to investigate the interactions between atmospheric, oceanic and terrestrial systems and their joint controls on tropical monsoon dynamics in West Africa. This special issue reports results from a group of AMMA studies regrouped in the component “Couplage de l'Atmosphere Tropicale et du Cycle Hydrologique” (CATCH). AMMA-CATCH studies focus on measuring and understanding land surface properties and processes in West Africa, the role of terrestrial systems in altering boundary layer dynamics, and thus the potential that surface hydrology and biology, and human land use practices, may directly or indirectly affect monsoon dynamics and rainfall in the region. AMMA-CATCH studies focus on three intensively instrumented mesoscale sites in Mali, Niger and Benin that sample across the 100-1300 mm/annum rainfall gradient of the Sahel, Sudan and North-Guinean bioclimatic zones. Studies report on i) surface-boundary layer interactions that may influence atmospheric convergence and convective processes and thus rainfall type, timing and amount; ii) vegetation dynamics at seasonal to decadal time-scales that may respond to, and alter, atmospheric processes; iii) surface-atmosphere fluxes of heat, water and carbon dioxide that directly influence the atmosphere; iv) soil moisture variability in space and time that provide the proximate control on vegetation activity, evapotranspiration and energy balance; and v) local and mesoscale modeling of hydrology and land surface-atmosphere exchanges to assess their role in the hydrological, atmospheric and rainfall dynamics of West Africa. The AMMA-CATCH research reported in this issue will be extended in future years as measurements and analysis continue and are concluded within the context of both CATCH and the wider AMMA study. This body of research will contribute to an improved understanding of the functioning of the coupled West African system, and enhance our ability to model and predict rainfall, vegetation and biogeochemical dynamics across time-scales (day, year, decade, century), and in response to changing climate and land use. Such information is vital for policy makers and managers in planning for future economic development, sustainability and livelihoods of the growing populations of the region