ALBERT

Description: The present study utilizes three high-resolution simulations from the Regional Climate Model version 4 (RegCM4) to examine the late 21st century changes (2080–2099) in the West African Monsoon (WAM) features. A set of three Earth System Models are utilized to provide initial and lateral boundary conditions to the RegCM4 experiments. Our analysis focuses on seasonal mean changes in WAM large-scale dynamical features, along with their connections with the summer monsoon precipitation. In the historical period, the simulation ensemble means mimic reasonably well the intensity and spatial distribution of the WAM rainfall as well as the WAM circulation patterns at different scales. The future projection of the WAM climate exhibits warming over the whole West Africa leading to precipitation reduction over the Sahel region, and a slight increase over some areas of the Guinea Coast. The position of the African Easterly Jet (AEJ) is shifted southward and the African Easterly Waves (AEWs) activities are reduced, which affect in turn the WAM rainbelt characteristics in terms of position and strength. Overall the changes in simulated AEJ and AEWs contribute substantially to reduce the seasonal summer mean precipitation in West Africa by the late 21st century, with prevailing negative changes in the Savanna-Sahel region. To further explore the robustness of the relationships revealed in this paper, future studies using different high-resolution regional climate models with large ensemble are recommended.

Description: In West Africa (WA), interest in solar energy development has risen in recent years with many planned and ongoing projects currently in the region. However, a major drawback to this development in the region is the intense cloud cover that reduces the incoming solar radiation when present and causes fluctuations in solar power production. Therefore, understanding the occurrence of clouds and their link to the surface solar radiation in the region is important for making plans to manage future solar energy production. In this study, we use the state-of-the-art European Centre for Medium-range Weather Forecasts ReAnalysis (ERA5) dataset to examine the occurrence and persistence of cloudy and clear-sky conditions in the region. Then, we investigate the effects of cloud cover on the quantity and variability of the incoming solar radiation. The cloud shortwave radiation attenuation (CRASW↓) is used to quantify the amount of incoming solar radiation that is lost due to clouds. The results showed that the attenuation of incoming solar radiation is stronger in all months over the southern part of WA near the Guinea Coast. Across the whole region, the maximum attenuation occurs in August, with a mean CRASW↓ of about 55% over southern WA and between 20% and 35% in the Sahelian region. Southern WA is characterized by a higher occurrence of persistent cloudy conditions, while the Sahel region and northern WA are associated with frequent clear-sky conditions. Nonetheless, continuous periods with extremely low surface solar radiation were found to be few over the whole region. The analysis also showed that the surface solar radiation received from November to April only varies marginally from one year to the other. However, there is a higher uncertainty during the core of the monsoon season (June to October) with regard to the quantity of incoming solar radiation. The results obtained show the need for robust management plans to ensure the long-term success of solar energy projects in the region.

Description: This study investigates the changes in West African monsoon features during warm years using the Regional Climate Model version 4.5 (RegCM4.5). The analysis uses 30 years of datasets of rainfall, surface temperature and wind parameters (from 1980 to 2009). We performed a simulation at a spatial resolution of 50 km with the RegCM4.5 model driven by ERA-Interim reanalysis. The rainfall amount is weaker over the Sahel (western and central) and the Guinea region for the warmest years compared to the coldest ones. The analysis of heat fluxes show that the sensible (latent) heat flux is stronger (weaker) during the warmest (coldest) years. When considering the rainfall events, there is a decrease of the number of rainy days over the Guinea Coast (in the South of Cote d’Ivoire, of Ghana and of Benin) and the western and eastern Sahel during warm years. The maximum length of consecutive wet days decreases over the western and eastern Sahel, while the consecutive dry days increase mainly over the Sahel band during the warm years. The percentage of very warm days and warm nights increase mainly over the Sahel domain and the Guinea region. The model also simulates an increase of the warm spell duration index in the whole Sahel domain and over the Guinea Coast in warm years. The analysis of the wind dynamic exhibits during warm years a weakening of the monsoon flow in the lower levels, a strengthening in the magnitude of the African Easterly Jet (AEJ) in the mid-troposphere and a slight increase of the Tropical Easterly Jet (TEJ) in the upper levels of the atmosphere during warm years.

Description: We used the Abdu Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model version 4.5 (RegCM4.5), to investigate the potential impacts of land cover change of the Sahel–Sahara interface on the West African climate over an interannual timescale from 1990 to 2009. A simulation at 50 km grid spacing is performed with the standard version of the RegCM4.5 model (control run), followed by three vegetation change experiments at the Sahel-Sahara interface (15° N and 20° N): forest, tall grass, and short grass savanna. The impacts of land cover change are assessed by analyzing the difference between the altered runs and the control one in different sub-domains (western Sahel, central Sahel, eastern Sahel, and Guinea). Results show that the presence of forest, tall grass, and short grass savanna at the Sahel–Sahara interface tends to decrease the mean summer surface temperature in the whole domain. Nevertheless, this decrease is more pronounced over the Central Sahel when considering the forest experiment. This temperature decrease is associated with a weakening (strengthening) of the sensible (latent) heat flux in the whole domain. An analysis of the radiation field is performed to better explain the changes noted in the latent heat flux, the sensible heat flux, and the surface temperature. When considering the rainfall signal, the analysis shows that the afforestation options tend to alter the precipitation in the considered sub-domains substantially by increasing it in the whole Sahel region, with strong interannual variability. This rainfall increase is associated with an increase of the atmospheric moisture. Finally, we investigated the impacts of the afforestation options on some features of the rainfall events, and on the atmospheric dynamics during wet and dry years. All afforestation options tend to increase the frequency of the number of rainy days in regions located south of 18° N during both periods. Nevertheless, this increase is stronger over the Central and Eastern Sahel during wet years in the forest case. All afforestation experiments induce an increase (decrease) of the low-levels monsoon flux in the Eastern Sahel (western Sahel) during both periods. At the mid-levels, the three afforestation options tend to move northward and to decrease the intensity of the African Easterly Jet (AEJ) south of 13° N during wet and dry years.The intensity of the AEJ is weaker during the wet period. The vegetation change experiments also affect the Tropical Easterly Jet (TEJ), especially during wet years, by increasing its intensity over the southern Sahel. The analysis of the activity of African Easterly Waves (AEWs) patterns exhibits a decrease of the intensity of these disturbances over the Sahel during both periods. This may be due to the weakening of the meridional temperature contrast between the continent and the Gulf of Guinea due to the Sahel–Sahara surface temperature cooling induced by the afforestation. In summary, this study shows that during both periods, the increase of the atmospheric moisture due to the afforestation is associated with favorable AEJ/TEJ configurations (weaker and northward position of the AEJ; stronger TEJ) which in turn may create a stronger convection and then, an increase in the Sahel rainfall. This Sahel rainfall increase is associated with a strengthening of the intense and heavy rainfall events which may impact diversely local populations.

Description: Climate and land use/cover changes are potential drivers of change in hydrology and water use. Incidences of these factors on Bandama hydrological basin and Kossou hydropower generation (1981–2016) in West Africa are assessed in this present work. Using Landsat products of United Stated Geological Survey, results show that water bodies areas and land use have increased by 1.89%/year and 11.56%/year respectively, whereas herbaceous savanna, savanna, forest and evergreen forest coverage have been reduced by 1.39%/year, 0.02%/year, 2.39%/year and 3.33%/year respectively from 1988 to 2016. Hydroclimatic analysis reveals that streamflow presents greater change in magnitude compared to rainfall though both increasing trends are not statistically significant at annual scale. Streamflow varies at least four (two) times greatly than the rainfall (monthly and seasonally) annually except during driest months probably due to land use/cover change. In contrast, Kossou hydropower generation is significantly decreasing (p-value 0.007) at both monthly and annual scales possibly due to water abstraction at upstream. Further works are required to elucidate the combined effects of land use/cover and climate changes on hydrological system as well as water abstraction on Kossou generation.

Description: Extreme climate events, either being linked to dry spells or extreme precipitation, are of major concern in Africa, a region in which the economy and population are highly vulnerable to climate hazards. However, recent trends in climate events are not often documented in this poorly surveyed continent. This study makes use of a large set of daily rain gauge data covering Southern West Africa (extending from 10° W to 10° E and from 4° N to 12° N) from 1950 to 2014. The evolution of the number and the intensity of daily rainfall events, especially the most extremes, were analyzed at the annual and seasonal scales. During the first rainy season (April–July), mean annual rainfall is observed to have a minor trend due to less frequent but more intense rainfall mainly along the coast of Southern West Africa (SWA) over the last two decades. The north–south seasonal changes exhibit an increase in mean annual rainfall over the last decade during the second rainy season (September–November) linked by both an increase in the frequency of occurrence of rainy days as well as an increase in the mean intensity and extreme events over the last decade. The study also provides evidence of a disparity that exists between the west and east of SWA, with the east recording a stronger increase in the mean intensity of wet days and extreme rainfall during the second rainy season (September–November).

Description: Management of hydroelectric dams is an aspect of sustainability that comes with resolving problems locally. The use of global indicators has not been a sustainable solution, thus the need for local indicators. Besides, current sustainability assessment tools lack the integration of climate, making assessments in a climate change context impossible. In this paper, we present management and sustainability assessment in a climate change context using sustainability indicators. We modeled a change in the climate using normal, moderate, and extreme climate conditions defined by Standardized Precipitation Indices (SPI) values. Out of 36 years analyzed, 24 years fall in the near-normal climate regime, and the remaining 12 years in moderate and extreme conditions, making near-normal climate regime the basis for managing the Taabo Dam. The impact of climate, techno-economic, and socio-environmental indicators on sustainability were investigated, and the results were analyzed according to scenarios. Climate adaptation shows higher sustainability indices than techno-economic and socio-environmental scenarios. Probability matrices show high and low values, respectively, for environmental and flooding indicators. Risk matrices, on the other hand, show that even with small probability values, risks still exist, and such small probabilities should not be taken as an absence of risk. The study reveals that sustainability can be improved by integrating climate into existing assessment methods.

Description: Traffic source emission inventories for the rapidly growing West African urban cities are necessary for better characterization of local vehicle emissions released into the atmosphere of these cities. This study is based on local field measurements in Yopougon (Abidjan, Côte d’Ivoire) in 2016; a site representative of anthropogenic activities in West African cities. The measurements provided data on vehicle type and age, traveling time, fuel type, and estimated amount of fuel consumption. The data revealed high traffic flow of personal cars on highways, boulevards, and backstreets, whereas high flows of intra-communal sedan taxis were observed on main and secondary roads. In addition, the highest daily fuel consumption value of 56 L·day−1 was recorded for heavy vehicles, while the lowest value of 15 L·day−1 was recorded for personal cars using gasoline. This study is important for the improvement of uncertainties related to the different databases used to estimate emissions either in national or international reports. This work provides useful information for future studies on urban air quality, climate, and health impact assessments in African cities. It may also be useful for policy makers to support implementation of emission reduction policies in West African cities.

Description: West African basins play a vital role in the socio-economic development of the region. They are mostly trans-boundary and sources of different land use practices. This work attempts to assess the spatio-temporal land use and land cover changes over three South Western African basins (Volta, Mono and Sassandra basins) and their influence on discharge. The land use and land cover maps of each basin were developed for 1988, 2002 and 2016. The results show that all the studied basins present an increase in water bodies, built-up, agricultural land and a decline in vegetative areas. These increases in water bodies and land use are as a result of an increase in small reservoirs, of dugouts and of dam constructions. However, the decline in some vegetative clusters could be attributed to the demographic and socio-economic growth as expressed by the expansion of agriculture and urbanization. The basic statistical analysis of precipitation and discharge data reveals that the mean annual discharge varies much more than the total annual precipitation at the three basins. For instance, in the entire Volta basin, the annual precipitation coefficient of variation (CV) is 10% while the annual discharge CV of Nawuni, Saboba and Bui are 43.6%, 36.51% and 47.43%, respectively. In Mono basin, the annual precipitation CV is 11.5% while the Nangbeto and Athieme annual discharge CV are 37.15% and 46.60%, respectively. The annual precipitation CV in Sassandra basin is 7.64% while the annual discharge CV of Soubre and Dakpadou are 29.41% and 37%, respectively. The discharge varies at least three times much more than the precipitation in the studied basins. The same conclusion was found for all months except the driest months (December and January). We showed that this great variation in discharge is mainly due to land use and land cover changes. Beside the hydrological modification of the land use and land cover changes, the climate of the region as well as the water quality and availability and the hydropower generation may be impacted by these changes in land surfaces conditions. Therefore, these impacts should be further assessed to implement appropriate climate services and measures for a sustainable land use and water management.

Description: This study characterizes the future changes in extreme rainfall and air temperature in the Mono river basin where the main economic activity is weather dependent and local populations are highly vulnerable to natural hazards, including flood inundations. Daily precipitation and temperature from observational datasets and Regional Climate Models (RCMs) output from REMO, RegCM, HadRM3, and RCA were used to analyze climatic variations in space and time, and fit a GEV model to investigate the extreme rainfalls and their return periods. The results indicate that the realism of the simulated climate in this domain is mainly controlled by the choice of the RCMs. These RCMs projected a 1 to 1.5 °C temperature increase by 2050 while the projected trends for cumulated precipitation are null or very moderate and diverge among models. Contrasting results were obtained for the intense rainfall events, with RegCM and HadRM3 pointing to a significant increase in the intensity of extreme rainfall events. The GEV model is well suited for the prediction of heavy rainfall events although there are uncertainties beyond the 90th percentile. The annual maxima of daily precipitation will also increase by 2050 and could be of benefit to the ecosystem services and socioeconomic activities in the Mono river basin but could also be a threat.