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International Journal of Geophysics
Volume 2012 (2012), Article ID 354571, 3 pages
Advances in Climate Processes, Feedbacks, Variability and Change for the West African Climate System
1Department of Physics and Astronomy, Howard University, Washington, DC, USA
2International Research Institute for Climate and Society, Palis ades, NY, USA
3Laboratory for Atmospheric Physics-Simeon Fongang, Cheikh Anta Diop University, Dakar, Senegal
4Caribbean Institute for Meteorology and Hydrology, Husband Heights, St. James, Barbados
Received 13 June 2012; Accepted 13 June 2012
Copyright © 2012 Gregory S. Jenkins et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The Sahel, generally identified as a semiarid agroecological zone, runs from Senegal eastward to Ethiopia it has a latitude range of 10–18°N and has been subject to intensive research in recent years. It is a region well known for multidecadal precipitation anomalies that have been observed throughout the 20th century . As a consequence of these large fluctuations in climate, economic development has been hampered and there have been increased risks to segments of the population, especially as it relates to climate impacts on agriculture and water resources. At present, many of the countries in West Africa are near the bottom of the United Nations Human Development Index. Rainfall variability serves as a contributing factor to its stifled development, in addition to other factors such as governance. Anthropogenic climate change poses a new and serious challenge with Africa being identified as the most vulnerable and the least able to adapt .
In this special issue we have collected papers that detail the most recent efforts to understand the dynamics of the West African monsoon at a scale relevant to inform decisions and examine processes such as regional modeling, atmospheric chemistry, mesoscale meteorology, and the role of land-cover.
The global oceans appear to be important in both climate change and climate variability throughout the continent but especially across the Sahel region. Through climate model simulations, the downward trends in rainfall during the years between 1970 and 1990 have been linked to the various ocean basins [3–5]; particularly a warming trend in the Indian Ocean that may have been caused by anthropogenic greenhouse gas forcing, and cooling of the North Atlantic that may have been caused by sulfate aerosols have both been implicated in the persistence of Sahel drought. However, the poor observing system in West Africa, multi-scale interactions (local-mesoscale-synoptic scale) and coupling between the land-surface and atmosphere  add complexity to a deeper understanding of the late 20th century climate trends.
For example, a key deficiency in the IPCC reports for Africa is the sign and magnitude of future precipitation changes over West Africa . Current projections are characterized by a large spread in the sign of precipitation change, with some of the coupled global climate models suggesting drying while others suggest wetter conditions . While it is possible that this spread is due to differences in the different models’ projections of sea surface temperature change, it is also well known that the scale of precipitation is problematic in the GCMs. Much of the observed precipitation occurs on meso-scale in the form of mesoscale convective systems (MCSs—squall lines, convective clusters and mesoscale convective complexes), which are organized and modulated by topography and larger scaled features such as the African Easterly Jet (AEJ) and African Easterly Waves (AEWs) [8–10]. The MCSs of West Africa show diurnal behavior and show convective strength in association with high ice concentrations and lightning [11, 12].
Another important West African feature is dust and the warm, dry airmasses that are lifted and carried from the Sahara , which may suppress precipitation [14–16], but the exact process remains unknown because of its complex but poorly observed interactions with cloud microphysics. Surface-atmosphere interactions and feedbacks through soil moisture and vegetation have been shown to be very important in West Africa. These factors may have a critical role to play with respect to the 21st century climate change, along with population increases, urbanization, and land-management practices. Studies examining the production of tropospheric ozone, another greenhouse gas, remains uncertain in West Africa as multiple sinks and sources (deposition, photolysis, biogenic source of nitrogen, lightning, biomass burning, anthropogenic sources from urban centers, heterogeneous chemical processes) have been modeled and sampled through limited field campaigns [17–19]. There are still many uncertainties and processes related to ozone chemistry within the annual cycle and over interannual timescales in West Africa that need to be quantified further.
Regional climate models afford the opportunity to better represent orography, sharp vegetation gradients, meso-scale circulation, and precipitation processes over West Africa and some improvements in simulated climate have been reported. However, the ability to simulate present and future changes in global ocean and tropical atmosphere in GCMs can strongly influence the lateral boundary conditions and may lead to regional climate biases within any single regional climate model simulation. The Coordinated Regional Climate Downscaling Experiment (CORDEX) has been initiated to evaluate regional model performance and produce climate projections from present boundary conditions using reanalysis forcing and future boundary conditions from GCMs. For regions such as the Sahel, it is anticipated that uncertainty in future precipitation changes associated with higher greenhouse gas concentration will result in reduced CORDEX activities. Early results from CORDEX over West Africa show improvement using 1989–2007 ERA-interim boundary conditions  but biases still exist.
One critical concern for understanding weather or longer time-scale climate variability or anthropogenic change is the lack of coastal ocean measurements, continental and upper air measurements in West Africa and the continent of Africa except in a few regions . These ground-based measurements provide evaluation of GCMs and help to identify biases in the model. Satellite products such as the Tropical Rainfall Measurement Mission (TRMM) daily and 3 hr rain estimates are currently being used for model evaluation however, they provide only an incomplete view of the meteorological aspects, especially as they relate to meso-scale convective systems. Coastal measurements around West Africa are extremely sparse around the West African coastline and greater coverage will be needed to monitor sea level changes and other ocean measurements (salinity, temperature, wave heights, and directions) during the 21st century.
In this special journal issue we attempt to examine some of these issues with contributions that include, land-use change, atmospheric chemistry, AEWs and tropical cyclones, seasonal climate forecasts and their applications, regional climate modeling, and observations from the recent African Monsoon Multidisciplinary Analysis (AMMA) campaign [22–24] over West Africa and the downstream Atlantic. We believe that these contributions provide the basis for connecting the processes that may be indirectly or directly responsible for climate variability and potential climate change in West Africa. The quest for deeper understanding may ultimately serve as a means for reducing the vulnerability for present and future generations of West Africans.
We thank many anonymous reviewers for their careful reviews and constructive suggestions in improving each paper.
Gregory S. Jenkins
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