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This paper examines intersectionality and climate vulnerability in Ghana's Upper West Region. Located within the southern fringe of the West African Sahel, and inhabited largely by smallholder farmers, the Upper West faces recurring climate extremes. This paper asks, how do the intersections between different inequalities and power relations shape vulnerability to climate extremes? Evidence for the paper comes from intensive qualitative fieldwork. Focusing especially on lived experiences from four case studies, the paper demonstrates the textured ways in which masculinity ideals, health status, religion, gender, age, marital status, and poverty intersect to deepen farmers' vulnerability to dry spells, flash floods, and after-storm recovery. Overall, the paper advances two interrelated arguments. Firstly, it argues that vulnerability analysis that focuses independently on gender, class, religion, and other characteristics, is insufficient because it risks homogenizing entire groups. Secondly, the paper argues that climate extremes do not always affect women more adversely than men. Indeed, dominant ideals of threatened masculinity can make men highly vulnerable during extreme climatic events. In the end, the paper concludes that if vulnerability analysis fails to unmask difference or move beyond binary gender categories, ensuing interventions may miss the real needs of countless individuals. [ABSTRACT FROM AUTHOR]

Extreme drought events from climate disturbances are weakening livelihood and limiting agriculture and livestock production in the Sahel region. The lack of relevant information to anticipate coping measures has exacerbated impacts leading to climate adaptation failure in most parts. In this regard, the current research paper has collected important datasets with an objective to assess the impact of extreme drought events on household's livelihoods for better understanding impacts, local people's perception, and the changes on vegetation cover in order to support a robust adaptation strategy to drought. The study conducted a household survey and collected satellite data for comparative analysis. The first survey was conducted in 2013 to collect data from 465 household heads through a structured questionnaire. Supplementary focus group discussions (FGDs) were also conducted in 2018 to collect qualitative information from targeted respondents such as village leaders and members of other key groups including women and youth. Descriptive statistics and correlation coefficient matrix were used to characterize the impact on households' main livelihoods and logistic regression to predict people's perception on pasture depletion over the last 20 years. Satellite data were used to derive spectral vegetation of land covers and unsupervised classification indexes. Both individual survey and focus group discussions identified drought as the main climate constraint which reduced crop production, water and pastures. The logistic analysis revealed that if the respondent's major occupation is livestock, the probability to perceive a depletion of pasture will increase by 28%. Concurrently, the satellite image observation in perfect agreement with the field survey showed 6.78% and 6.01% losses of water surface and vegetation cover respectively between 1986 and 2016 in the study area. These findings showed that logistic regression coupled with satellite information can inform on past and future impacts which are extremely crucial for sound adaptation planning in the Sahel region. [ABSTRACT FROM AUTHOR]

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]

This paper investigates the potential impact of 'idealized-but-realistic' land cover degradation on the late twentieth century Sahel drought using a regional climate model (RCM) driven with lateral boundary conditions (LBCs) from three different sources, including one re-analysis data and two global climate models (GCMs). The impact of land cover degradation is quantified based on a large number of control-and-experiment pairs of simulations, where the experiment features a degraded land cover relative to the control. Two different approaches of experimental design are tested: in the 1st approach, the RCM land cover degradation experiment shares the same LBCs as the corresponding RCM control, which can be derived from either reanalysis data or a GCM; with the 2nd approach, the LBCs for the RCM control are derived from a GCM control, and the LBCs for the RCM land cover degradation experiment are derived from a corresponding GCM land cover degradation experiment. When the 1st approach is used, results from the RCM driven with the three different sources of LBCs are generally consistent with each other, indicating robustness of the model response against LBCs; when the 2nd approach is used, the RCM results show strong sensitivity to the source of LBCs and the response in the RCM is dominated by the response of the driving GCMs. The spatiotemporal pattern of the precipitation response to land cover degradation as simulated by RCM using the 1st approach closely resembles that of the observed historical changes, while results from the GCMs and the RCM using the 2nd approach bear less similarity to observations. Compared with the 1st approach, the 2nd approach has the advantage of capturing the impact on large scale circulation, but has the disadvantage of being influenced by the GCMs' internal variability and any potential erroneous response of the driving GCMs to land degradation. The 2nd approach therefore requires a large ensemble to reduce the uncertainties derived from the driving GCMs. All RCM experiments based on the 1st approach produce a predominantly dry signal in West Africa throughout the year, with a dipole pattern found in the peak monsoon season that features a slight increase of precipitation over the Guinea Coast and strong decrease in the north; a similar spatiotemporal distribution is found for temperature changes, with warming (cooling) coinciding with precipitation decrease (increase). The model precipitation changes in West Africa are dominated by evapotranspiration changes in the north and by atmospheric moisture convergence changes in the south; in temperature changes, surface warming due to the decrease of evaporative cooling dominates over the albedo-induced radiative cooling. [ABSTRACT FROM AUTHOR]

Abstract: Concern about climate and its impact on human populations in the Sahel since the 1970s was an immediate response to the most recent of recurrent drought periods. Understanding the relative impact of this drought on rural life in the Sahel is, however, not straightforward. This is due to the fact that climate is only one of many factors influencing local adaptation strategies to environmental changes. Another explanation could be that climate in many rural communities in the Sahel is simply no longer the primary worry. The argument presented in this paper, supported by data from a small village in northern Burkina Faso, is that the villagers there are ‘beyond climate’ as their current livelihood strategies are increasingly climate independent. People have over the past decades engaged in livelihood diversification in order to negate the negative impact of climate variability on agriculture. In order to analyse the temporal perspective of climate–livelihood interaction, the paper employs human–environmental timelines. The results document the multiplicity of exposures shaping decisions in the village. While significant correlation exists between recent livelihood diversifications and major climatic events, it is equally obvious that recent political developments and the economic flow of project activities are crucial factors of change. [Copyright &y& Elsevier]

Recent increases in the accuracy of climate models have enhanced the possibilities for analyzing the impacts of climate change on society. This paper explores how the local, economic impacts of climate change can be modeled for a specific eco-region, the Western Sahel. The people in the Sahel are highly dependent on their natural resource base, and these resources are highly vulnerable to climate change, in particular to changes in rainfall. Climate models project substantial changes in rainfall in the Sahel in the coming 50 years, with most models predicting a reduction in rainfall. To connect climate change to changes in ecosystem productivity and local income, we construct an ecological–economic model that incorporates rangeland dynamics, grazing and livestock prices. The model shows that decreased rainfall in the Sahel will considerably reduce local incomes, in particular if combined with increases in rainfall variability. Adaptation to these climate change projections is possible if reductions in rainfall are followed by destocking to reach efficient grazing levels. However, while such a strategy is optimal from the perspective of society, the stocking rate is determined by individual pastoralists that face few incentives to destock. [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]

Climate extremes pose threats to lives and properties. It is therefore critical to assess the dynamics of climate extremes in the future. Knowledge of the appropriate models to be used for projection helps reduces uncertainties in model projections. The study assesses the evolution of 8 extreme temperature indices over the Volta Basin for the 1985–2014 period. Consequently, the ability of 40 Coupled Model Intercomparison Project Phase 6 (CMIP6) models in reproducing these extremes is assessed. The biases and the M-scores of the annual spatial distribution of indices are determined. It is observed that the temperature extremes are frequent and profound in the Sahel, indicating that the Sahel is susceptible to extreme temperature changes. The warm indices showed positive trends, while the cold indices mainly showed negative trends. Most of the models had difficulty reproducing the climatologies of the indices. However, the CMCC-ESM2, MPI-ESM-1-2-HAM, MIROC6, and AWI-CM-1-1-MR reproduced the TNN, TNX, TXN, and TXX well while the MIROC6, MRI-ESM2-0, INM-CM4-8, and MIROC6 showed robust performance in reproducing the TN10P, TN90P, TX10P, and TX90P respectively. Poor performance of models may be due to poor simulation of sub-grid features such as vegetation cover, and clouds as a result of coarse grid sizes. Warming of the basin poses a significant impact on agriculture, drought occurrence, and seasonal to annual scale precipitation distribution. Southern sections of the basin may be vulnerable to temperature-related extremes as a result of deforestation and urbanization. [ABSTRACT FROM AUTHOR]

The West Africa rainfall regime constitutes a considerable challenge for Regional Climate Models (RCMs) due to the complexity of dynamical and physical processes that characterise the West African Monsoon. In this paper, daily precipitation statistics are evaluated from the contributions to the AFRICA-CORDEX experiment from two ERA-Interim driven Canadian RCMs: CanRCM4, developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) and CRCM5, developed at the University of Québec at Montréal. These modelled precipitation statistics are evaluated against three gridded observed datasets-the Global Precipitation Climatology Project (GPCP), the Tropical Rainfall Measuring Mission (TRMM), and the Africa Rainfall Climatology (ARC2)-and four reanalysis products (ECMWF ERA-Interim, NCEP/DOE Reanalysis II, NASA MERRA and NOAA-CIRES Twentieth Century Reanalysis). The two RCMs share the same dynamics from the Environment Canada GEM forecast model, but have two different physics' packages: CanRCM4 obtains its physics from CCCma's global atmospheric model (CanAM4), while CRCM5 shares a number of its physics modules with the limited-area version of GEM forecast model. The evaluation is focused on various daily precipitation statistics (maximum number of consecutive wet days, number of moderate and very heavy precipitation events, precipitation frequency distribution) and on the monsoon onset and retreat over the Sahel region. We find that the CRCM5 has a good representation of daily precipitation statistics over the southern Sahel, with spatial distributions close to GPCP dataset. Some differences are observed in the northern part of the Sahel, where the model is characterised by a dry bias. CanRCM4 and the ERA-Interim and MERRA reanalysis products overestimate the number of wet days over Sahel with a shift in the frequency distribution toward smaller daily precipitation amounts than in observations. Both RCMs and reanalyses have difficulties in reproducing the local onset date over the Sahel region. Nevertheless, the large-scale features of the monsoon precipitation evolution over West Africa are well reproduced by the RCMs, whereas the northern limit of the rainy bands is less accurately reproduced. Both RCMs exhibit an overall good representation of the local retreat index over the Sahel region. [ABSTRACT FROM AUTHOR]

Over the recent decades, extreme precipitation events (EPEs) have become more frequent over the Sahel. Their properties, however, have so far received little attention. In this study the spatial distribution, intensity, seasonality, and interannual variability of EPEs are examined, using both a reference dataset based on a high-density rain gauge network over Burkina Faso and 24 precipitation gridded datasets. The gridded datasets are evaluated in depth over Burkina Faso while their commonalities are used to document the EPE properties over the Sahel. EPEs are defined as the occurrence of daily accumulated precipitation exceeding the all-day 99th percentile over a 1° × 1° pixel. Over Burkina Faso, this percentile ranges between 21 and 33 mm day−1. The reference dataset show that EPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. These results are consistent among the gridded datasets over Burkina Faso but also over the wider Sahel. The gridded datasets exhibit a wide diversity of skills when compared to the Burkinabe reference. The Global Precipitation Climatology Centre Full Data Daily version 1 (GPCC-FDDv1) and the Global Satellite Mapping of Precipitation Gauge Reanalysis version 6.0 (GSMaP-gauge-RNL v6.0) are the only products that properly reproduce all of the EPE features examined in this work. The datasets using a combination of microwave and infrared measurements are prone to overestimate the EPE intensity, while infrared-only products generally underestimate it. Their calibrated versions perform better than their uncalibrated (near-real-time) versions. This study finally emphasizes that the lack of rain gauge data availability over the whole Sahel strongly impedes our ability to gain insights in EPE properties. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the development of a system dynamic model to simulate and analyze desertification. The human–environment coupled model integrates socio-economic drivers with bio-physical drivers of biomass production, land degradation and desertification. It is based on the UN and GEF definitions of desertification. It illustrates the concept of desertification through differential equation and simulation output graphics. It is supplemented with a causal loop diagram demonstrating the existing feed-back mechanisms. The model relates population pressure and dynamics over time to the growth and availability of biomass resources. The human population stock is described as a function of growth rate, death rate and resources dependent in and out migration of people. The relative growth rate of the stock of resources is modeled as a function of climate and exploitation pressure affecting soil erosion and water availability. The model is applied for the Sahelian desertification syndrome using input data to illustrate and simulate a 150 years period (1900–2050) in Kordofan, Sudan. The model indicates that it is difficult to generate irreversible desertification. [Copyright &y& Elsevier]

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]

In the south-west of Mauritania, the present provinces of Brakna and Trarza stem from the history of the pre-colonial emirates of the same names. This paper quotes the few but various data at our disposal concerning the landscape history and vegetation and climate evolution during the last centuries. We propose a diachronic approach of this area, to identify the part of the heritage in the current landscape and usefully enlighten environmental and development issues. We can thus identify the main features of the precolonial landscapes of this region as well as the historical roots of the production systems and put into perspective their recent evolutions. Beyond its historical interest, this knowledge can also be one of the grounds of the current environmental approaches. [ABSTRACT FROM AUTHOR]

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]

At decadal time scales, the capability of state-of-the-art atmosphere-ocean coupled climate models in predicting the precipitation in Sahel is assessed. A set of 14 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) is selected and two experiments are analysed, namely initialized decadal hindcasts and forced historical simulations. Considering the strong linkage of the atmospheric circulation signatures over West Africa with the rainfall variability, this study aims to investigate the potential of using wind fields for decadal predictions. Namely, a West African monsoon index (WAMI) is defined, based on the coherence of low (925 hPa) and high (200 hPa) troposphere wind fields, which accounts for the intensity of the monsoonal circulation. A combined empirical orthogonal functions analysis is applied to explore the wind fields' covariance modes, and a set of indices is defined on the basis of the identified patterns. The WAMI predictive skill is assessed by comparing WAMI from coupled models with WAMI from reanalysis products and with a standardized precipitation index (SPI) from observations. Results suggest that the predictive skill is highly model dependent and it is strongly related to the WAMI definition. In addition, hindcasts are more skilful than historical simulations in both deterministic and probability forecasts, which suggests an added value of initialization for decadal predictability. Moreover, coupled models are more skilful in predicting the observed SPI than the WAMI obtained from reanalysis. WAMI performance is also compared with decadal predictions from CMIP5 models based on a Sahelian precipitation index, and an improvement in predictive skill is observed in some models when WAMI is used. Therefore, we conclude that dynamics-based indices are potentially more effective for decadal prediction of precipitation in Sahel than precipitation-based indices for those models in which Sahel rainfall variability is not well simulated. We thus recommend a two-fold approach when testing the performance of models in predicting Sahel rainfall, based not only on rainfall but also on the dynamics of the West African monsoon. [ABSTRACT FROM AUTHOR]

The actual dynamics and physical mechanisms affecting the Sahel precipitation pattern and amplitude in the climate models remain under debate due to the inconsistent drying and rainfall variability/pattern among them. We diagnose the boreal summer rainfall pattern in the Sahel and its possible causes using short-range ensemble hindcasts based on NCAR community atmospheric model with the local ensemble transform Kalman filter (CAM-LETKF) data assimilation. The CAM-LETKF assimilation was conducted using 64 ensemble members with an assimilation cycle of 6-h. By comparing the superior and inferior groups within these 64 ensembles, we confirmed the influence of the Atlantic in the West Sahel rainfall (a robust feature in the ensembles) and a severe model bias resulting from erroneously modeled locations and magnitudes of low-level Sahara heat low (SHL) and African easterly jet (AEJ). This bias is highly related to atmospheric jet dynamics as shown in recent studies and local wave instability triggered mainly by the boundary-layer temperature gradient and amplified by land-atmosphere interactions. In particular, our results demonstrated that more accurate divergence and convergence fields resulting from improved SHL and AEJ in the superior groups enabled more accurate rainbelt patterns to be discerned, thus improving the ensemble mean model hindcast prediction by more than 25 % in precipitation and 16 % in temperature. We concluded that the use of low-resolution climate models to project future rainfall in the Sahel requires caution because the model hindcasts may quickly diverge even the same boundary conditions and forcings are applied. The model bias may easily grow up within a few months in the short-range CAM-LETKF hindcast, let along the free model centennial simulations. Unconstrained future climate model projections for the Sahel must more effectively capture the short-term key boundary-layer dynamics in the boreal summer to be credible regardless model dynamics and physics. [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 article presents a study which aims to clarify the usefulness and relevance of the Standard Precipitation Index (SPI) in measuring the Sahelian rainfall, as well as to determine the needed precautions for interpreting it. It discusses the robustness of SPI value in method deviations, and the significance of a global value of Sahelian rainfall despite its spatial variability. It also looks into the updates on the climatological conditions observed over the Sahel in the 1950-2006 period.

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]

Mobility is the most important response by the inhabitants of the Sahel to climatic adversity. This 'condition sahélienne', characterized by unstable climatic circumstances, irregular rainfall patterns and periods of drought, has an important influence on people's decision-making processes regarding their livelihood. Migration studies mainly focus on labour migration to urban areas. Although mobility is part of the repertoire of Sahelian people, the form it takes varies considerably between social groups and individuals, and over time. In this article we focus on a neglected and almost invisible category of rural-rural migrants in the Sahel, more specifically on Fulbe pastoral people and their developments over the last three decades in the Sahel and the Sudan zone of West Africa and the economic and social conditions in which they find themselves. It concludes that these rural-rural migrations are deeply engrained in cultural patterns in West Africa, exemplified by specific institutions for dealing with hosts and strangers. However, mobility is often not a planned process, and all kinds of survival strategies are used in a very flexible manner. The phenomenon has given rise to a specific character of cultural dynamics and ways of defining identity for the people involved. [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]

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]

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]