Climate Of West Africa Research Articles

Enhancing Understanding of the Impact of Climate Change on Malaria in West Africa Using the Vector-Borne Disease Community Model of the International Center for Theoretical Physics (VECTRI) and the Bias-Corrected Phase 6 Coupled Model Intercomparison Project Data (CMIP6)

In sub-Saharan Africa, temperatures are generally conducive to malaria transmission, and rainfall provides mosquitoes with optimal breeding conditions. The objective of this study is to assess the impact of future climate change on malaria transmission in West Africa using community-based vector-borne disease models, TRIeste (VECTRI). This VECTRI model, based on bias-corrected data from the Phase 6 Coupled Model Intercomparison Project (CMIP6), was used to simulate malaria parameters, such as the entomological inoculation rate (EIR). Due to the lack of data on confirmed malaria cases throughout West Africa, we first validated the forced VECTRI model with CMIP6 data in Senegal. This comparative study between observed malaria data from the National Malaria Control Program in Senegal (Programme National de Lutte contre le Paludisme, PNLP, PNLP) and malaria simulation data with the VECTRI (EIR) model has shown the ability of the biological model to simulate malaria transmission in Senegal. We then used the VECTRI model to reproduce the historical characteristics of malaria in West Africa and quantify the projected changes with two Shared Socio-economic Pathways (SSPs). The method adopted consists of first studying the climate in West Africa for a historical period (1950–2014), then evaluating the performance of VECTRI to simulate malaria over the same period (1950–2014), and finally studying the impact of projected climate change on malaria in a future period (2015–2100) according to the ssp245 ssp585 scenario. The results showed that low-latitude (southern) regions with abundant rainfall are the areas most affected by malaria transmission. Two transmission peaks are observed in June and October, with a period of high transmission extending from May to November. In contrast to regions with high latitudes in the north, semi-arid zones experience a relatively brief transmission period that occurs between August, September, and October, with the peak observed in September. Regarding projections based on the ssp585 scenario, the results indicate that, in general, malaria prevalence will gradually decrease in West Africa in the distant future. But the period of high transmission will tend to expand in the future. In addition, the shift of malaria prevalence from already affected areas to more suitable areas due to climate change is observed. Similar results were also observed with the ssp245 scenario regarding the projection of malaria prevalence. In contrast, the ssp245 scenario predicts an increase in malaria prevalence in the distant future, while the ssp585 scenario predicts a decrease. These findings are valuable for decision makers in developing public health initiatives in West Africa to mitigate the impact of this disease in the region in the context of climate change.

Water-smart farming: review of strategies, technologies, and practices for sustainable agricultural water management in a changing climate in West Africa

The global temperature increase of 1.1°C over the past decade and human population growth are already escalating competition for water in locations where water is scarce, adversely affecting agriculture. Water-smart farming was developed to address challenges associated with the availability, access and use of agricultural water. It complements practices and goals related to agricultural water in climate-smart agriculture. It encompasses several climate-responsive food cropping systems, water-saving methods, tools, and technologies to grow food sustainably. Effective implementation of water-smart farming can ameliorate the pressing burden of agricultural water competition. Therefore, the conceptual framework of water-smart farming and its principal elements or ideology are introduced in this review. The major pillars of water-smart cropping, which include conditioning and improving the soil’s capacity to capture and hold moisture, basic infrastructure for water harvesting, storage and use as nature-based cropping solutions were elaborated. Further examples of technologies and best practices in water-smart crop farming, such as breeding for drought-tolerant crop varieties, promoting dryland cropping systems, alternate wetting and drying technology, water drainage and control structures, rainwater harvesting, utilization of irrigation reservoirs, cover cropping and intercropping for enhancing soil moisture conservation were presented. The review also briefly shows the contribution of some of these best practices and adaptive technologies of water-smart cropping toward promoting water-saving methods used for growing rice, maize, legumes, cassava and peri-urban vegetables.

Marine Heatwaves, Upwelling, and Atmospheric Conditions during the Monsoon Period at the Northern Coast of the Gulf of Guinea

Ocean conditions influence the economies and climate of West Africa. Based on the 30-year daily Optimum Interpolation Sea Surface Temperature (OISST) dataset during May–October, upwelling surface variability and marine heatwaves (MHWs) at the northern coast of the Gulf of Guinea are investigated. The cooling surface decreases more rapidly around Cape Palmas than around Cape Three Points and extends eastward. MHWs variability exhibits a frequent occurrence of such events since 2015 that is consistent with the observed oceanic warming and the decrease in upwelling surface. The empirical orthogonal functions performed on the annual cumulated intensity of MHWs show four variability modes that include the whole northern coast, an east–west dipole between the two capes, a contrast between the northern coast at the two capes and the meridional section east of 5° E, and a north–south opposition. These patterns show 3-year, 6-year, and 8-year trends, and are related to coastal upwelling at the northern coast of the Gulf of Guinea. Similarly, surface ocean and atmospheric conditions are modified according to MHW periods. These changes take place before, during, and after MHW events. These results could be used to understand how this change influences the marine ecosystem, the local fisheries resources, and the extreme rainfall episodes in West Africa.

Effect of Plant Species on the Performance and Bacteria Density Profile in Vertical Flow Constructed Wetlands for Domestic Wastewater Treatment in a Tropical Climate

Bacteria are frequently studied due to their involvement in pollutants transformation processes during wastewater treatment. In this study, the treatment efficiency, bacteria densities and their vertical profile were investigated in pilot-scale vertical flow constructed wetlands (VFCW) planted with different plant species under a tropical climate in west Africa. Five beds were planted with local plant species, i.e., Andropogon gayanus, Chrysopogon zizanioides, Echinochloa pyramidalis, Pennisetum purpureum and Tripsacum laxum, while one bed remained unplanted. These species have been rarely used in CWs while some (e.g., T. laxum) are tested for the first time. After a 7-month trial, bacteria densities were measured in substrate samples separated into six layers along the bed depth. Plants presence enhanced the bacterial density and VFCW efficiency; the removal rates of organic matter (90.9–95.9%; COD and 95.2–98.5%; BOD5), nitrogen (74.3–84%; TN and 76–84%; NH4-N) and phosphorus (77.4–96.9%; PO4-P) were higher by 5.9–24.1% compared to the control bed, providing an overall excellent treatment performance for a single-stage VFCW system. Small numbers of anaerobic bacteria were obtained in the VFCWs, explaining the low-to-zero NO3-N removal, except for the VFCWs with T. laxum and P. purpureum. Aerobic bacteria decreased from the upper to bottom layers from 17.4 to 0.1 × 106 CFU/g in the planted beds, while anaerobic bacteria increased from 0.1 to 2.1 × 106 CFU/g. Anaerobic bacteria were more abundant in the unplanted than in the planted beds. The total bacteria count was dominated by aerobic bacteria, and decreased from the surface towards the bottom. Overall, the VFCW with P. purpureum demonstrated the highest efficiency, indicating that this design is an effective and sustainable nature-based solution for wastewater treatment in a tropical climate.

Testing the forest refuge hypothesis in sub-Saharan Africa using species distribution modeling for a key savannah tree species, Senegalia senegal (L.) Britton

Quaternary geographic range dynamics of savannah tree species are still not fully understood. The forest refuge hypothesis postulates that climatic and vegetational upheavals during the Pleistocene fragmented the previously continuous ranges of many species into isolated refuges that would have acted as shelters for rainforest taxa and allowed their survival through the Pleistocene cold stages. This hypothesis has recently been applied to studies of taxa in the African savannahs. We here test this hypothesis using the savannah tree species Senegalia senegal (L.) Britton., which is widely distributed across sub-Saharan Africa. Specifically, we ask the following questions: (i) Do we find evidence for savannah refugia during the last 130,000 before-present in sub-Saharan Africa? (ii) Would the climate in West Africa already have been suitable for S. senegal prior to the Last Glacial Maximum (LGM)? Using 1,132 occurrence records,we modeled the geographic distribution of S. senegal and projected the model into the past using climatic conditions from four time slices spanning the last 130,000 years bp. Our analyses show that the projected geographic extent of S. senegal was broader during the Last Interglacial, with a dramatic decline during the LGM and the subsequent recovery through the mid-Holocene to the present day. Our results indicate a range expansion at least from the mid-Holocene to the present and further show that S. senegal had similarly continuous distribution during the LGM as found today in sub-Saharan Africa. We also assessed the regional variation of environmental niche occupancy using a principal components analysis (PCA). The PCA reveals variation in the occupancy of environmental space across sub-Saharan Africa, a key indication of a wide ecological amplitude exhibited by the species. This study provides insights into the ancestral distribution and the temporal dynamics of a key savannah species that have shaped its current areas of occupancy.

The Radiative Forcing of Aerosols in a West Africa Sahelian Urban City: Case Study of Ouagadougou

This paper is an assessment of radiative forcing caused by atmospheric aerosols in an urban city in West Africa. It is carried out in Ouagadougou in Burkina Faso and is an illustration of the radiative impact in most of the large Sahelian urban cities which are under the same climatic influences and whose populations present similarities in their socio-economic aspects. Using the GAME code, the radiative forcing was calculated at the top of the atmosphere, in the atmospheric layer and at the earth’s surface. The results showed overall a cooling effect at the top of the atmosphere due to the backscattering in space of the incident radiation, a heating in the atmospheric layer due to the absorption effect and a surface cooling justified by the attenuation of radiation crossing the atmosphere. Using monthly average values of optical properties, vertical temperature and humidity profiles, daily temperatures and surface albedo, the simulation yielded forcing values ranging from -6.77 W/m2 to -2.56 W/m2 at the top of the atmosphere, from 15.8 W/m2 to 34.7 W/m2 in the atmospheric layer and from -41.00 W/m2 to -21.68 W/m2 at the earth’s surface. In addition, the warming was simulated in the first atmospheric layer (in contact with the surface), and the results show values ranging from 0.8°C to 1.8°C. The study of the annual variability of the results showed a strong correlation between the radiative forcing and the seasonal succession characteristic of the climate in West Africa with the extreme values in the month of March (characteristic of the dry and hot season) and in the month of August (characteristic of the rainy season).

Agricultural productivity in relation to climate and cropland management in West Africa

The climate of West Africa is expected to become more arid due to increased temperature and uncertain rainfall regimes, while its population is expected to grow faster than the rest of the world. As such, increased demand for food will likely coincide with declines in agricultural production in a region where severe undernutrition already occurs. Here, we attempt to discriminate between the impacts of climate and other factors (e.g. land management/degradation) on crop production across West Africa using satellite remote sensing. We identify trends in the land surface phenology and climate of West African croplands between 2000 and 2018. Using the combination of a an attribution framework and residual trend anlaysis, we discriminate between climate and other impacts on crop productivity. The combined effect of rainfall, land surface temperature and solar radiation explains approximately 40% of the variation in cropland productivity over West Africa at the 95% significance level. The largest proportions of croplands with greening trends were observed in Mali, Niger and Burkina Faso, and the largest proportions with browning trends were in Nigeria, The Gambia and Benin. Climate was responsible for 52% of the greening trends and 25% of the browning trends. Within the other driving factors, changes in phenology explained 18% of the greening and 37% of the browning trends across the region, the use of inputs and irrigation explained 30% of the greening trends and land degradation 38% of the browning trends. These findings have implications for adaptation policies as we map out areas in need of improved land management practices and those where it has proven to be successful.

Improving the Predictive Skill of a Distributed Hydrological Model by Calibration on Spatial Patterns With Multiple Satellite Data Sets

AbstractHydrological model calibration combining Earth observations and in situ measurements is a promising solution to overcome the limitations of the traditional streamflow‐only calibration. However, combining multiple data sources in model calibration requires a meaningful integration of the data sets, which should harness their most reliable contents to avoid accumulation of their uncertainties and mislead the parameter estimation procedure. This study analyzes the improvement of model parameter selection by using only the spatial patterns of satellite remote sensing data, thereby ignoring their absolute values. Although satellite products are characterized by uncertainties, their most reliable key feature is the representation of spatial patterns, which is a unique and relevant source of information for distributed hydrological models. We propose a novel multivariate calibration framework exploiting spatial patterns and simultaneously incorporating streamflow and three satellite products (i.e., Global Land Evaporation Amsterdam Model [GLEAM] evaporation, European Space Agency Climate Change Initiative [ESA CCI] soil moisture, and Gravity Recovery and Climate Experiment [GRACE] terrestrial water storage). The Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature data set is used for model evaluation. A bias‐insensitive and multicomponent spatial pattern matching metric is developed to formulate a multiobjective function. The proposed multivariate calibration framework is tested with the mesoscale Hydrologic Model (mHM) and applied to the poorly gauged Volta River basin located in a predominantly semiarid climate in West Africa. Results of the multivariate calibration show that the decrease in performance for streamflow (−7%) and terrestrial water storage (−6%) is counterbalanced with an increase in performance for soil moisture (+105%) and evaporation (+26%). These results demonstrate that there are benefits in using satellite data sets, when suitably integrated in a robust model parametrization scheme.

Fuzzy measure-based multicriteria land assessment for rainfed maize in West Africa for the current and a range of plausible future climates

Crop production in West Africa is largely under rainfed conditions, making the system vulnerable to the impacts of climate change. However, the impact of future climate on the geographic range of many crops in West Africa is still uncertain. This is exacerbated by considerable uncertainty in projecting future West African climate by global circulation models (GCMs). In this paper, the current land suitability for rainfed maize (Zea mays L.) production in West Africa and how it would change in response to a range of future climate changes were assessed. A novel non-additive approach involving the integration of a fuzzy measure modelling procedure and the Choquet integral to deal with non-linear relationships among criteria was introduced. The supervised and unsupervised variants of the approach and the Ordered Weighted Averaging (OWA) aggregators were evaluated and compared using data on the current climate, soil characteristics, and actual maize yield estimates at some locations across the region. The supervised fuzzy measure modelling outperformed the unsupervised variant and the OWA operators, demonstrating the importance of considering interaction among criteria and incorporating human reasoning in multicriteria analysis. Consequently, the supervised modelling approach was used to estimate suitability scores for rainfed maize for the current and a range of plausible future climates which were generated from unique combinations of temperature and rainfall percentile values (5th, 50th and 95th) from 25 and 32 GCMs projections under the RCPs 2.6 and 8.5 scenarios, respectively. Under the current climate in West Africa, the combined suitable/highly suitable areas for rainfed maize production are in the humid savanna zones. In contrast, the forest region to the south exhibited moderate suitability with the semi-arid savanna zones being marginally suitable. Relative to the current climate in which 91.3% of West Africa is at least moderately suitable for maize production, suitability for maize will change in at least 43% of West Africa in response to future climate change. Across all scenarios and years, future suitability in the semi-arid region of the Sudano-Sahelian savannas shows strong sensitivity to uncertainty in rainfall projections. However, regardless of the trajectory of future rainfall projection, the 95th percentile temperature projection will decrease suitability in about 15% of the West African region by 2080 under the RCP8.5 scenario. Because of the incorporation of uncertainties of future climate projections in this analysis, the results from this study can be used in the development of flexible climate adaptation strategies.

Exploring the challenges confronting the west Africa climate system

The peculiar location of West Africa on the World map suggests that the region is prone to be under severe atmospheric influences from other regions. With a population of over 300 million people, the human activity and its corresponding impact on the climate system of the region is worth noting. The meteorological exploration of funded projects like AERONET, AMMA, DACCIWA etc. in West Africa shows the peculiarity of the danger that life-forms in West Africa might be facing in the next two decades. The documentation of salient discoveries over the West Africa Monsoon and whose challenges may be the possible answers to questions on the West Africa climate system. A re-view on the satellite exploration within 2002-2006 shows the anomalies which require intense concern by all and sundry.

Analysis of the Performance of Base Flow Separation Methods Using Chemistry and Statistics in Sudano-Sahelian Watershed, Burkina Faso

This research aims to determine appropriate methods for base flow separation under Sudano-Sahelian climate in West Africa. Four Recursive Digital Filtering (RDF) methods and the Conductivity Mass Balance (CMB) method were used. Daily streamflow of the Mouhoun River (formerly Black Volta River) at Samendeni gauge station has been separated into base flow and runoff. The RDF methods are easy to use but cannot take into account the actual hydrological processes within the watershed, while CMB method does it well. But, given that regular discharge measurement is rarely coupled with Electrical Conductivity measurements, it is therefore not possible to apply CMB method over time at each outlet. The CMB method is frequently used on a short time as a reference to assess the performance or to calibrate RDF methods. In the present study, CMB method was used for the year 2017, especially during the rainy season (from July to October) to produce more realistic base flow values. Statistical analysis was used to compare RDF and CMB methods. It was found that all the four RDF methods used are significantly different from the CMB method in the study area. Among the RDF methods, the Eckhardt method which is two parameters filtering method was successfully calibrated using CMB method Base Flow Index (BFI) as constraint. With the calibration process, the parameter BFImax of the Eckhardt method was adjusted to 0.32 in the study area context. The achievements of this study can have several implications such as adequate base flow estimation over time at Samendeni gauge station and at other similar gauges of the Mouhoun watershed which will be particularly beneficial to the critical issue of assessment of climate change impact on base flow in the study area.

Land-Surface Characteristics and Climate in West Africa: Models’ Biases and Impacts of Historical Anthropogenically-Induced Deforestation

Land Use Land-Cover Change (LULCC), such as deforestation, affects the climate system and land-atmosphere interactions. Using simulations carried out within the LUCID (Land Use and Climate, IDentification of robust Impacts) project framework, we first quantify the role of historical land-cover change induced by human activities on surface climate in West Africa. Focusing on two contrasted African regions, we find that climate responses of land-use changes are small but they are still statistically significant. In Western Sahel, a statistically significant near-surface atmospheric cooling and a decrease in water recycling are simulated in summer in response to LULCC. Over the Guinean zone, models simulate a significant decrease in precipitation and water recycling in autumn in response to LULCC. This signal is comparable in magnitude with the effect induced by the increase in greenhouse gases. Simulated climate changes due to historical LULCC could however be underestimated because: (i) the prescribed LULCC can be underestimated in those regions; (ii) the climate models underestimate the coupling strength between West African surface climate and leaf area index (LAI) and (iii) the lack of interactive LAI in some models. Finally, our study reveals indirect atmospheric processes triggered by LULCC. Over the Western Sahel, models reveal that a significant decrease in solar reflection tend to cool down the surface and thus counteract the atmospheric feedback. Conversely, over the Guinea zone, models reveal that the indirect atmospheric processes and turbulent heat fluxes dominate the climatic responses over the direct effects of LULCC.

In this issue of Weather

This month, we publish four papers that were written following presentations at last year’s RMetS Student Conference. They cover a wide range of subjects, demonstrating the health of meteorological science in the UK and the importance of weather data. The students were each the lead authors of their articles.We begin with ‘The role of cyclone clustering during the stormy winter of 2013/2014’ by Matthew Priestley, Joaquim Pinto, Helen Dacre and Len Shaffrey on p. 187. This paper outlines the evidence that depressions may deepen rapidly in association with Rossby‐wave breaking. This was seen in particular during the winter of 2013/14, when many depressions became storms, having crossed the Atlantic along a similar course to enter the left exit of a persistent strong high‐level jet stream. Some of these deep depressions bring storm‐force winds into Europe and so may be associated with significant damage and possible loss of life.Next, we move south. In ‘Does high‐resolution modelling improve the spatial analysis of föhn flow over the Larsen C Ice Shelf?’ on p. 192, J V Turton, A Kirchgaessner, A N Ross and J C King examine the advances made as model resolution improves, as well as the remaining shortcomings. The Larsen C Ice Shelf lies to the east of the mountainous Antarctic Peninsula and is often subject to the warm dry winds associated with lee flow. Föhn winds aid the break‐up of the ice shelf and improvements in our knowledge can be important for the operations of scientists in the area – not least the British Antarctic Survey.Next, we move to the effects of the monsoon‐dominated climate of West Africa. Preparedness allows farmers to take measures to protect against drought. Matt Brown, Emily Black, Dagmawi Asfaw and Fred Otu‐Larbi examine a method to predict drought during the growing season in the relatively dry Sahel zone in ‘Monitoring drought in Ghana using TAMSAT‐ALERT: a new decision support system’ on p. 201. They set out its increasing value as this prediction tool is developed, but also investigate its shortcomings. TAMSAT‐ALERT relies on satellite data, surface observations and ensemble output from the Met Office Unified Model, as well as a knowledge of evolution of soil moisture under a range of weather scenarios.The last of our Special Issue papers is on p. 206: ‘The local response of El Niño events and changing disease distribution in Tanzania’ by Rachael Reynolds, Gina Cavan and Mark Cresswell. One of the challenges presented by global climate change is the possible spread of diseases responsive to moisture and temperature, although some diseases may become restricted in a more humid world. El Niño years bring at least some of the weather likely to be seen in an anthropogenically warmed world, so this study compares disease data from recent El Niño years with those in other years.

Impacts of boundary condition changes on regional climate projections over West Africa

AbstractFuture projections using regional climate models (RCMs) are driven with boundary conditions (BCs) typically derived from global climate models. Understanding the impact of the various BCs on regional climate projections is critical for characterizing their robustness and uncertainties. In this study, the International Center for Theoretical Physics Regional Climate Model Version 4 (RegCM4) is used to investigate the impact of different aspects of boundary conditions, including lateral BCs and sea surface temperature (SST), on projected future changes of regional climate in West Africa, and BCs from the coupled European Community‐Hamburg Atmospheric Model 5/Max Planck Institute Ocean Model are used as an example. Historical, future, and several sensitivity experiments are conducted with various combinations of BCs and CO2 concentration, and differences among the experiments are compared to identify the most important drivers for RCMs. When driven by changes in all factors, the RegCM4‐produced future climate changes include significantly drier conditions in Sahel and wetter conditions along the Guinean coast. Changes in CO2 concentration within the RCM domain alone or changes in wind vectors at the domain boundaries alone have minor impact on projected future climate changes. Changes in the atmospheric humidity alone at the domain boundaries lead to a wetter Sahel due to the northward migration of rain belts during summer. This impact, although significant, is offset and dominated by changes of other BC factors (primarily temperature) that cause a drying signal. Future changes of atmospheric temperature at the domain boundaries combined with SST changes over oceans are sufficient to cause a future climate that closely resembles the projection that accounts for all factors combined. Therefore, climate variability and changes simulated by RCMs depend primarily on the variability and change of temperature aspects of the RCM BCs. Moreover, it is found that the response of the RCM climate to different climate change factors is roughly linear in that the projected changes driven by combined factors are close to the sum of projected changes due to each individual factor alone at least for long‐term averages. Findings from this study are important for understanding the source(s) of uncertainties in regional climate projections and for designing innovative approaches to climate downscaling and impact assessment.

Saharan dust detection using multi-sensor satellite measurements.

Contemporary scientists have vested interest in trying to understand the climatology of the North Atlantic Basin since this region is considered as the genesis for hurricane formation that eventually get shipped to the tropical Atlantic region and the Caribbean. The effects of atmospheric water cycle and the climate of West Africa and the Atlantic basin are hugely impacted by the radiative forcing of Saharan dust. The focus area in this paper would be to improve the dust detection schemes by employing the use of multi sensor measurements in the thermal emissive wavelengths using legacy sensors such as Terra (T) and Aqua (A) MODerate-resolution Imaging Spectroradiometer (MODIS), fusing with Ozone Monitoring Instrument (OMI). Previous work by Hao and Qu (2007) had considered a limited number of thermal infrared channels which led to a correlation coefficient R2 value of 0.765 between the Aerosol Optical Thickness (AOT) at 550 nm and the modeled dust index. In this work, we extend the thermal infrared based dust detection by employing additional channels: the 8.55 μm which has shown high sensitivity to the Saharan dust, along with water vapor channel of 7.1 μm and cloud top channel of 13.1 μm. Also, the dust pixels were clearly identified using the OMI based aerosol types. The dust pixels were cleanly segregated from the other aerosol types such as sulfates, biomass, and other carbonaceous aerosols. These improvements led to a much higher correlation coefficient R2 value of 0.85 between the modified dust index and the AOT in comparison to the previous work. The key limitations from the current AOT products based on MODIS and were put to test by validating the improved dust detection algorithm. Two improvements were noted. First, the dust measurement radiometry using MODIS is significantly improved by at least an order of 2. Second the spatial measurements are enhanced by a factor of at least 10.

Modeling the impact of changes in Atlantic sea surface temperature on the climate of West Africa

This study assesses the impacts of warming/cooling of the Atlantic sea surface temperature (SST) on the climate of West Africa using Version 4.4 of Regional Climate Model (RegCM4.4) of International Center for Theoretical Physics, Trieste, Italy. The 1–2 K cooling and warming of the Atlantic SST both result in tripole temperature and precipitation change structure, having a northwest–southeast orientation over West Africa. Findings reveal that the responses of precipitation and temperature to the Atlantic SST cooling are opposite to those for the Atlantic SST warming and these responses intensify with increased warming/cooling of the Atlantic SST. The structure of the change in climate is attributed to the response of atmospheric/soil moisture gradient and orientation of orography in West Africa.

The possible role of local air pollution in climate change in West Africa

Here it is argued that air pollution over West African cities needs greater consideration. The effects of aerosol pollution on clouds and solar and thermal radiation can be expected to alter regional climate and impact human health and food security. The climate of West Africa is characterized by a sensitive monsoon system that is associated with marked natural precipitation variability. This region has been and is projected to be subject to substantial global and regional-scale changes including greenhouse-gas-induced warming and sea-level rise, land-use and land-cover change, and substantial biomass burning. We argue that more attention should be paid to rapidly increasing air pollution over the explosively growing cities of West Africa, as experiences from other regions suggest that this can alter regional climate through the influences of aerosols on clouds and radiation, and will also affect human health and food security. We need better observations and models to quantify the magnitude and characteristics of these impacts.

Potential climate effect of mineral aerosols over West Africa. Part I: model validation and contemporary climate evaluation

Mineral dusts present in the atmosphere can play an important role in climate over West Africa and surrounding regions. However, current understanding regarding how dust aerosols influence climate of West Africa is very limited. In this study, a regional climate model is used to investigate the potential climatic impacts of dust aerosols. Two sets of simulations driven by reanalysis and Earth System Model boundary conditions are performed with and without the representation of dust processes. The model, regardless of the boundary forcing, captures the spatial and temporal variability of the aerosol optical depth and surface concentration. The shortwave radiative forcing of dust is negative at the surface and positive in the atmosphere, with greater changes in the spring and summer. The presence of mineral dusts causes surface cooling and lower troposphere heating, resulting in a stabilization effect and reduction in precipitation in the northern portion of the monsoon close to the dust emissions region. This results in an enhancement of precipitation to the south. While dusts cause the lower troposphere to stabilize, upper tropospheric cooling makes the region more prone to intense deep convection as is evident by a simulated increase in extreme precipitation. In a companion paper, the impacts of dust emissions on future West African climate are investigated.

Vulnerability of native savanna trees and exotic Khaya senegalensis to seasonal drought.

Seasonally dry ecosystems present a challenge to plants to maintain water relations. While native vegetation in seasonally dry ecosystems have evolved specific adaptations to the long dry season, there are risks to introduced exotic species. African mahogany, Khaya senegalensis Desr. (A. Juss.), is an exotic plantation species that has been introduced widely in Asia and northern Australia, but it is unknown if it has the physiological or phenotypic plasticity to cope with the strongly seasonal patterns of water availability in the tropical savanna climate of northern Australia. We investigated the gas exchange and water relations traits and adjustments to seasonal drought in K. senegalensis and native eucalypts (Eucalyptus tetrodonta F. Muell. and Corymbia latifolia F. Muell.) in a savanna ecosystem in northern Australia. The native eucalypts did not exhibit any signs of drought stress after 3 months of no rainfall and probably had access to deeper soil moisture late into the dry season. Leaf water potential, stomatal conductance, transpiration and photosynthesis all remained high in the dry season but osmotic adjustment was not observed. Overstorey leaf area index (LAI) was 0.6 in the native eucalypt savanna and did not change between wet and dry seasons. In contrast, the K. senegalensis plantation in the wet season was characterized by a high water potential, high stomatal conductance and transpiration and a high LAI of 2.4. In the dry season, K. senegalensis experienced mild drought stress with a predawn water potential -0.6 MPa. Overstorey LAI was halved, and stomatal conductance and transpiration drastically reduced, while minimum leaf water potentials did not change (-2 MPa) and no osmotic adjustment occurred. Khaya senegalensis exhibited an isohydric behaviour and also had a lower hydraulic vulnerability to cavitation in leaves, with a P50 of -2.3 MPa. The native eucalypts had twice the maximum leaf hydraulic conductance but a much higher P50 of -1.5 MPa. Khaya senegalensis has evolved in a wet-dry tropical climate in West Africa (600-800 mm) and appears to be well suited to the seasonal savanna climate of northern Australia. The species exhibited a large phenotypic plasticity through leaf area adjustments and conservative isohydric behaviour in the 6 months dry season while operating well above its critical hydraulic threshold.

Practising historical climatology in West Africa: a climatic periodisation 1750–1800

This paper presents the first interdecadal and interannual periodisation of the climate in West Africa between 1750 and 1800. The climatic synthesis is founded on previously unexploited British documents written at Cape Coast Castle, Cape Coast, Ghana and travel journals, which are combined with previous research on the Sahel. By combining historical evidence with the most recently identified rainfall patterns in the Sahel and the Guinea coast, the climatic periodisation distinguishes between wetter and drier periods. The results show that there was great interannual and interdecadal variability in the West African rainfall regime in the 18th century. The historical evidence suggests that the climate in the Sahel and the Guinea coastal interior alternated between five wet and dry periods, whereas the coastal dry zone alternated between three wet and dry periods. The most pronounced changes in the climate are found in the 1780s and 1790s. The Guinea coastal interior and the Sahel experienced a wet period in the 1780s and a dry period in the 1790s, whereas the coastal dry zone was drier in the 1780s and wetter in the 1790s. The investigation shows that the secondary rainy season on the Gold coast was weak or non-existent.