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A full understanding of the causes of the severe drought seen in the Sahel in the latter part of the twentieth-century remains elusive some 25 yr after the height of the event. Previous studies have suggested that this drying trend may be explained by either decadal modes of natural variability or by human-driven emissions (primarily aerosols), but these studies lacked a sufficiently large number of models to attribute one cause over the other. In this paper, signatures of both aerosol and greenhouse gas changes on Sahel rainfall are illustrated. These idealized responses are used to interpret the results of historical Sahel rainfall changes from two very large ensembles of fully coupled climate models, which both sample uncertainties arising from internal variability and model formulation. The sizes of these ensembles enable the relative role of human-driven changes and natural variability on historic Sahel rainfall to be assessed. The paper demonstrates that historic aerosol changes are likely to explain most of the underlying 1940-80 drying signal and a notable proportion of the more pronounced 1950-80 drying. [ABSTRACT FROM AUTHOR]

This paper investigates the relationship between large-scale dynamics, water vapor, and organized convection over West Africa. Making use of a simplified condensation hypothesis, a back-trajectory model fed by NCEP-analyzed winds is used to reconstruct the midtropospheric humidity field over Africa during July to August 1992. The approach documents both the moisture content and the origin of the air masses. Meteosat satellite infrared imagery is used to characterize the convective systems. A case study analysis reveals that very dry air patches (RH < 5%) are located in the immediate midtropospheric environment of a typical squall line. Such dry-air structures are shown to originate in the upper levels (200–250 hPa) on the anticyclonic side of the polar jet stream at 50°N. Focusing on the Sahel region, dry events are isolated using the time series of the 500-hPa relative humidity distribution during the monsoon period. These dry events are shown to be composed of extratropical air. Composite analysis of the convective activity indicator exhibits a strong positive association between dry intrusions and convection on the eastern side of the Sahelian region. Organized convective systems that are fast moving and long lasting are more likely over this region when a dry intrusion is present. This coincides with the well-established theory that midtropospheric dry air, when combined with sufficient wind shear, can maintain and intensify previously triggered deep convection through rain evaporation that feeds the cold pools, especially within squall lines. This paper suggests that the extratropical dry-air intrusions modulate the occurrence and duration of convective systems and, therefore, the mode of variability of rainfall over West Africa during the monsoon. [ABSTRACT FROM AUTHOR]

Beginning in response to the disastrous drought of 1968–73, considerable research and monitoring have focused on the characteristics, causes, predictability, and impacts of West African Soudano–Sahel (10°–18°N) rainfall variability and drought. While these efforts have generated substantial information on a range of these topics, very little is known of the extent to which communities, activities at risk, and policy makers are aware of, have access to, or use such information. This situation has prevailed despite Glantz's provocative BAMS paper on the use and value of seasonal forecasts for the Sahel more than a quarter century ago. We now provide a systematic reevaluation of these issues based on questionnaire responses of 566 participants (in 13 communities) and 26 organizations in Burkina Faso, Mali, Niger, and Nigeria. The results reveal that rural inhabitants have limited access to climate information, with nongovernmental organizations (NGOs) being the most important source. Moreover, the pathways for information flow are generally weakly connected and informal. As a result, utilization of the results of climate research is very low to nonexistent, even by organizations responsible for managing the effects of climate variability. Similarly, few people have access to seasonal climate forecasts, although the vast majority expressed a willingness to use such information when it becomes available. Those respondents with access expressed great enthusiasm and satisfaction with seasonal forecasts. The results suggest that inhabitants of the Soudano–Sahel savanna are keen for changes that improve their ability to cope with climate variability, but the lack of information on alternative courses of action is a major constraint. Our study, thus, essentially leaves unchanged both Glantz's negative “tentative conclusion” and more positive “preliminary assessment” of 25 years ago. Specifically, while many of the infrastructural deficiencies and socioeconomic impediments remain, the great yearning for climate information by Soudano–Sahalians suggests that the time is finally ripe for fostering increased use. Therefore, a simple model for improved dissemination of climate research and seasonal climate forecast information is proposed. The tragedy is that a quarter century has passed since Glantz's clarion call. [ABSTRACT FROM AUTHOR]

In the Sahel field observations of wind-blown mass transport often show considerable spatial variation related to the spatial variation of the wind erosion controlling parameters, e.g. soil crust and vegetation cover. A model, used to predict spatial variation in wind erosion and deposition is a useful tool in the implementation of wind erosion control measures in the Sahel. The aim of this paper was to test two existing wind erosion models on spatial predictions of aeolian mass transport for Sahelian conditions. Field data from Burkina Faso were used to test an empirical (RWEQ) and a deterministic (WEPS) model.The revised wind erosion equation (RWEQ) poorly predicted maximum mass transport and so spatial predictions of mass transport were underestimated. Major constraints of RWEQ for application in the Sahel were the required non-eroding boundary and the fact that RWEQ assumes a more or less homogeneous field. It was concluded that RWEQ in its current state was not suitable for application in a Sahelian environment.With the correct roughness length (Z0), wind erosion prediction system (WEPS) correctly predicted friction velocity and initiation and cessation of mass transport. Furthermore, the model gave a reasonable prediction of the spatial distribution of mass transport at the research sites. It was concluded that WEPS in PCRaster is suitable for prediction of wind erosion in a Sahelian environment. A constraint of WEPS in PCRaster is that WEPS'' predictions of spatial variation in sediment transport are closely linked to the spatial variation in the input parameters. A good estimation of the spatial variation of the input parameters was required. Obtaining these might be an expensive exercise and could make its use in the Sahel difficult. [Copyright &y& Elsevier]

Since dust storm frequency in the African Sahel is considered an indicator of drought phases, this paper points out basic interrelations of annual and interannual dust storm, rainfall and temperature variability in the region, as well as some large-scale patterns of atmospheric control. The normal seasonal pattern is characterized by a temperature maximum in June/July, a rainfall maximum in August, and a dust storm maximum in April which is linked to a negative soil water budget towards the end of the dry season. However, the dust storm minimum is reached as late as October, i.e. after sufficient soil water infiltration and vegetation cover increase. Thus, seasonal parameter interrelation reflects seasonal shifts in regional atmospheric circulation.Interannual variability, however, seems to occur with periodicities around 2-4 years, 6 years, 11-12 years and 25 years, a result that applies when modelling temperature, rainfall and dust storm series. Further statistical analysis provides insight into the internal structure of the short-term oscillations. Positive springtime dust storm anomalies are preceded by one or two dry years with above-normal summer temperatures. In this case, the evolution of a secondary dust storm maximum in winter is most likely. Over a sequence of 2-3 dry years, dust storm anomalies increase from spring to the following winter.When considering possible atmospheric controls, it was observed that higher summer temperatures in a Sahelian dry year are followed by increased zonal flow in the mid-latitude circumpolar vortex, and colder winters in the Sahel itself. The summer and winter relationship implies these processes to be an effect of meridional transport of excess latent heat after suppression of the West African Walker cell in early summer, and an oversteepened pressure gradient with increased surface trades during the following winter. [ABSTRACT FROM PUBLISHER]

To account for the wet mid‐Holocene (ca. 6 ka) Sahara‐Sahel region, we exploit teleconnections from four large‐scale ocean‐atmosphere‐land forcings: El Niño–Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), hemispherical temperature difference, and net energy input at the equator; they offer theoretical bases for estimating effects of large‐scale forcing on mid‐Holocene Sahara‐Sahel rainfall. Together, these forcings explain 16–64% of historical rainfall variability, with the highest percentages in the Sahel region. With estimates for mid‐Holocene time, latitudes where rainfall today averages 1,000, 500, or 200 mm/year are reconstructed to have lain ~2.5° farther north. Calculated mid‐Holocene rainfall at 15°N, 17.5°N, and 20°N exceeds that today by ~250, 100, and 20 mm/year. These shifts account for more than half of paleoclimatic estimates of mid‐Holocene rainfall and are comparable with the General Circulation Model (GCM) runs that match paleorainfall estimates most closely. Mid‐Holocene radiation may have affected Sahel precipitation indirectly via teleconnections from oceanic regions. Key Points: More than half of paleoclimatic estimates of Mid‐Holocene Sahara‐Sahel rainfall can be explained by four large‐scale forcingsThese forcings are ENSO, AMO, hemispherical temperature difference, and the net energy input at the equatorWe obtained ~2.5° northward shifts of latitude bands along which rainfall today averages 1,000, 500, or 200 mm/year [ABSTRACT FROM AUTHOR]

Partitioning uncertainty in projections of future climate change into contributions from internal variability, model response uncertainty and emissions scenarios has historically relied on making assumptions about forced changes in the mean and variability. With the advent of multiple single-model initial-condition large ensembles (SMILEs), these assumptions can be scrutinized, as they allow a more robust separation between sources of uncertainty. Here, the framework from Hawkins and Sutton (2009) for uncertainty partitioning is revisited for temperature and precipitation projections using seven SMILEs and the Coupled Model Intercomparison Project CMIP5 and CMIP6 archives. The original approach is shown to work well at global scales (potential method bias < 20 %), while at local to regional scales such as British Isles temperature or Sahel precipitation, there is a notable potential method bias (up to 50 %), and more accurate partitioning of uncertainty is achieved through the use of SMILEs. Whenever internal variability and forced changes therein are important, the need to evaluate and improve the representation of variability in models is evident. The available SMILEs are shown to be a good representation of the CMIP5 model diversity in many situations, making them a useful tool for interpreting CMIP5. CMIP6 often shows larger absolute and relative model uncertainty than CMIP5, although part of this difference can be reconciled with the higher average transient climate response in CMIP6. This study demonstrates the added value of a collection of SMILEs for quantifying and diagnosing uncertainty in climate projections. [ABSTRACT FROM AUTHOR]

Here, an evaluation of nine simulations of the regional climate models (RCM) performed within the frame of CORDEX-Africa Project to reproduce the Volta Basin climatology (1971–2005) is presented. Performance metrics of climatological means were computed from annual, seasonal, and monthly total values which detect differences in the spatial and temporal structures of the model outputs with observational datasets for precipitation and temperature evaluated over three sub-domains (the Guinea Coast, the Soudano-Sahel, and the Sahel) within the basin. The results reveal pronounced influence of the GCM-derived boundary conditions at homogeneous zones, for example, CNRM-induced model overestimations over the Sahel. A good replica of the spatial distribution of climatic parameters particularly for temperature with biases within 0.1 °C and obvious wet bias for precipitation up to 40% is detected predominantly over the Sahel with widespread differences in spatial extent and intensities. A better performance is obtained for summer and fall but worse in spring for precipitation. The analysis showed that the basic characteristics of the West African climate are reasonably reproduced by the models, including its variability in sign, amplitude, and spatial extent, but also, there are recognized deficiencies of the model simulations for some selected metrics, regions, and seasons. The study emphasizes on individual model skills than in ensemble performance, and CanESM2-RCA4 and NorESM1-RCA4 RCMs were realized to replicate best the observed dataset so these models are recommended for impact studies over the basin for most seasons. Results achieved will be valuable in the choice of GCMs/RCMs for impact study over the basin, and we conclude that this may also depend on the focus of interest, whether it is a study on variability and process representation or region of study or timescales. [ABSTRACT FROM AUTHOR]

This modeling study is conducted to examine the potential impact of the reforestation (greenbelt) location (either in Sahel or in Guinean region) on West African summer climate system. To this end, three simulations using the regional climate model RegCM4 driven by ERA-Interim reanalysis were performed at 50 km horizontal resolution over a West African domain for the period 2000–2011. The first experiment, namely the control (CTRL), uses the standard vegetation cover, while the two others incorporate throughout the model integration, a zonal reforestation band of evergreen broadleaf over different locations: (i) over a 13° N–17° N band latitudes in a Sahel-Sahara region (experiment hereafter referred to as GB15N) and (ii) between 8.5° N–11.5° N in the Guinea Coast region (experiment hereafter referred to as GB10N). A comparison of the CTRL experiment with observation reveals a faithful reproduction of the mean boreal and summer seasonal precipitation pattern, though substantial dry/wet biases remain, especially in the Atlantic Ocean. In addition, the seasonal cycle over sub-regions matches satisfactory the observed pattern. The GB15N reforestation leads to a precipitation increase in the range of 2–4 mm/day over the forested areas, whereas in the GB10N reforestation, precipitation increase is weaker and not necessarily located in the forested areas. Temperature cooling is observed over the reforested area and may be explained by a decrease of ground heat flux related to a reduction of the surface albedo. [ABSTRACT FROM AUTHOR]

When drought hit the marginally habitable land along the southern edge of the Sahara in the 1970s and '80s, the people of the Sahel suffered greatly. Soon some researchers were hypothesizing that intensive use of the land might have altered the surface of the Sahel enough to dry it out. In a paper published online, climate dynamicist Alessandra Giannini of the International Research Institute for Climate Prediction in Palisades, New York and her colleagues report results from the first climate model to simulate accurately the history of Sahel rainfall. In their model, the Indian Ocean's warming over the decades was the dominant driver of the drying of the Sahel. The model's greenhouse gas amounts were not changed over the 70 years, nor was the vegetation or any other surface property in or around the Sahel. Tropical Pacific temperatures accounted for much of the year-to-year variability of Sahel rain but not much of the long-term trend. The warming of the tropical Atlantic did play some role in long-term drying, but it was the Indian Ocean that drove most of the drying in the model. The link between the Indian Ocean and Sahel rainfall is only the latest ocean-drought connection.

In the sub-Saharan Sahel, energy and water cycling at the land surface is pivotal for the regional climate, water resources and land productivity, yet it is still very poorly documented. As a step towards a comprehensive climatological description of surface fluxes in this area, this study provides estimates of long-term average annual budgets and seasonal cycles for two main land use types of the cultivated Sahelian belt: rainfed millet crop and fallow bush. These estimates build on the combination of a 7-year field data set from two typical plots in southwestern Niger with detailed physically based soil-plant-atmosphere modeling, yielding a continuous, comprehensive set of water and energy flux and storage variables over this multiyear period. In the present case in particular, blending field data with mechanistic modeling makes the best use of available data and knowledge for the construction of the multivariate time series. Rather than using the model only to gap-fill observations into a composite series, model-data integration is generalized homogeneously over time by generating the whole series with the entire data-constrained model simulation. Climatological averages of all water and energy variables, with associated sampling uncertainty, are derived at annual to subseasonal scales from the time series produced. Similarities and differences in the two ecosystem behaviors are highlighted. Mean annual evapotranspiration is found to represent ~82-85% of rainfall for both systems, but with different soil evaporation/plant transpiration partitioning and different seasonal distribution. The remainder consists entirely of runoff for the fallow, whereas drainage and runoff stand in a 40-60% proportion for the millet field. These results should provide a robust reference for the surface energy- and water-related studies needed in this region. Their significance and the benefits they gain from the innovative data-model integration approach are thoroughly discussed. The model developed in this context has the potential for reliable simulations outside the reported conditions, including changing climate and land cover. [ABSTRACT FROM AUTHOR]

The observed abrupt latitudinal shift of maximum precipitation from the Guinean coast into the Sahel region in June, known as the West African monsoon jump, is studied using a regional climate model. Moisture, momentum, and energy budget analyses are used to better understand the physical processes that lead to the jump. Because of the distribution of albedo and surface moisture, a sensible heating maximum is in place over the Sahel region throughout the spring. In early May, this sensible heating drives a shallow meridional circulation and moisture convergence at the latitude of the sensible heating maximum, and this moisture is transported upward into the lower free troposphere where it diverges. During the second half of May, the supply of moisture from the boundary layer exceeds the divergence, resulting in a net supply of moisture and condensational heating into the lower troposphere. The resulting pressure gradient introduces an inertial instability, which abruptly shifts the midtropospheric meridional wind convergence maximum from the coast into the continental interior at the end of May. This in turn introduces a net total moisture convergence, net upward moisture flux and condensation in the upper troposphere, and an enhancement of precipitation in the continental interior through June. Because of the shift of the meridional convergence into the continent, condensation and precipitation along the coast gradually decline. The West African monsoon jump is an example of multiscale interaction in the climate system, in which an intraseasonal-scale event is triggered by the smooth seasonal evolution of SSTs and the solar forcing in the presence of land–sea contrast. [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of the 200–850-hPa vertical wind shear over the tropical Atlantic is examined for a period of 46 yr. This work extends and updates past studies by considering a longer data record as well as a tropospheric-deep measure of vertical wind shear. Composite fields are constructed to illustrate the spatial pattern of the large-scale circulation associated with the mean and extreme cases of vertical shear within the tropical Atlantic. The contemporaneous relationship of vertical shear with El Niño–Southern Oscillation (ENSO) and Sahel precipitation are also examined. While the ENSO–shear correlation appears to have slightly strengthened during the past decade, the Sahel–shear correlation has become significantly degraded. A combined empirical orthogonal function (EOF) analysis of the zonal and meridional components of the vertical shear reveals interannual and multidecadal modes. The leading EOF exhibits mainly interannual variability and is highly correlated with ENSO. The second EOF is associated with a multidecadal temporal evolution and is correlated with Sahel precipitation. Both EOFs correlate at the same level with tropical cyclones in the main development region of the tropical Atlantic. [ABSTRACT FROM AUTHOR]

The "Great Green Wall" of trees (GGW) is an emblematic Pan‐African initiative of re‐greening the Sahel through afforestation and assisted natural regeneration of trees in order to tackle desertification, soil degradation and to mitigate greenhouse gases. This study investigates (i.e., The Sahel Greenbelt) the potential impacts of the GGW and other assisted natural regeneration of trees on the frequency and intensity of extreme climate events over the Sahel and West Africa using the regional climate model (RegCM version 4.3). Our investigation shows that the Sahel greenbelt would increase significantly the number of rainy days (+9%) and the intensity of heavy rain events over the Sahel while extreme dry spells decrease (−4%). Important shifts appear in the modes of variability of all precipitation indices. These probability distribution shapes reveal tremendous intra‐seasonal variability as the new land use land cover (LULC) changes affect the regional climate. Changes in atmospheric circulation including increase of the moisture convergence and evapotranspiration appeared to be the main drivers of heavy rainfall changes. For temperature extremes, the maximum temperature shows significant decrease around the GGW area during summer and an increase in other seasons while the diurnal temperature range increases significantly without an evident change in temperature trends. Intra‐seasonal distributions of temperature extremes show less obvious changes compared to precipitation extremes. This investigation highlights the role of the planned and implemented re‐greening policies (i.e., afforestation by the GGW project and policies of assisted natural regeneration of trees) in affecting the frequency and the amplitude of some climatic extreme events (e.g., heavy rain events, maximum temperatures, etc.). These planned LULC policies need to be accounted for in the diagnostics and future projections of climate extremes over the region. [ABSTRACT FROM AUTHOR]

The summertime West African Sahel has the worldwide highest degree of thunderstorm organisation into long-lived, several hundred-kilometre elongated, fast propagating systems that contribute 90% to the annual rainfall. All current global weather prediction and climate models represent thunderstorms using simplified parameterisation schemes which deteriorates the modelled distribution of rainfall from individual storms and the entire West African monsoon circulation. It is unclear how this misrepresentation of Sahelian convection affects forecasts globally. Our study is the first to demonstrate how a computationally feasible increase of model resolution over West Africa – allowing to avoid convection parameterisation – yields a better representation of organised convection in the Sahel and of moisture within the monsoon system, ultimately improving 5–8-day tropical and mid-latitude weather forecasts. We advocate an operational use of a modelling strategy similar to the one presented here for a cost-effective improvement of global weather prediction and potentially even (sub-)seasonal and climate simulations. Thunderstorms are commonly represented through simplified parametrizations in weather and climate models. Here it is shown that an increase in model resolution over West Africa, enabling the explicit modeling of Sahelian convective systems, can improve 5–8 day tropical and mid-latitude weather forecasts. [ABSTRACT FROM AUTHOR]

In Africa, adaptation will be crucial to offset expected negative climate change impacts on food security and agriculture development. In this study, we combine meteorological data from 18 local stations, field surveys on agricultural practices and agronomic information on the growth of millet to demonstrate the crop suitability to the present climate and the ability of Senegalese farmers to adapt their practices to climate variability, and to disseminate them. From data collected in both 665 villages and 1061 farmers, our study provides quantitative evidence of the responsive adaptation of farmers in the Sahel where the recent resumption of rainfall has provided new agricultural opportunities. Statistical models and cropping simulations show that these farmers innovate by reintroducing and disseminating a long cycle millet cultivar—more suitable for wet environments. We note that although this adaptation is a clear response to recent changes in quantity and distribution of rainfall, its adoption remains limited (50% of the villages visited and 25% of the surveyed agricultural producers have cultivated the new millet variety) and varies strongly within the same climatic context and by characteristics of farmers (willing and capacity), indicating different agricultural strategies (diversification, market exchanges). If land access and development of cash crops are hindrances to the adoption of sanio, poverty is clearly not a barrier and adaptation is not a lever for wealth creation. Such adaptative capacities, together with government incentives for farmers to sustainably adapt to climate change, can be important in reducing climate risks in the coming years. [ABSTRACT FROM AUTHOR]