ALBERT

Description: This study uses the high-resolution outputs of the recent CORDEX-Africa climate projections to investigate the future changes in different aspects of the hydrological cycle over West Africa. Over the twenty-first century, temperatures in West Africa are expected to increase at a faster rate (+0.5 ∘C per decade) than the global average (+0.3 ∘C per decade), and mean precipitation is expected to increase over the Guinea Coast (+0.03 mm d−1 per decade) but decrease over the Sahel (−0.005 mm d−1 per decade). In addition, precipitation is expected to become more intense (+0.2 mm d−1 per decade) and less frequent (−1.5 d per decade) over all of West Africa as a result of increasing regional temperature (precipitation intensity increases on average by +0.35 mm d−1 ∘C−1 and precipitation frequency decreases on average by −2.2 d ∘C−1). Over the Sahel, the average length of dry spells is also expected to increase with temperature (+4 % d ∘C−1), which increases the likelihood for droughts with warming in this subregion. Hence, the hydrological cycle is expected to increase throughout the twenty-first century over all of West Africa, on average by +11 % ∘C−1 over the Sahel as a result of increasing precipitation intensity and lengthening of dry spells, and on average by +3 % ∘C−1 over the Guinea Coast as a result of increasing precipitation intensity only.

Description: Daily travel distance (DTD), the distance an animal moves over the course of the day, is an important metric in movement ecology. It provides data with which to test hypotheses related to energetics and behaviour, e.g. impact of group size or food distribution on DTDs. The automated tracking of movements by applying GPS technology has become widely available and easy to implement. However, due to battery duration constraints, it is necessary to select a tracking-time resolution, which inevitably introduces an underestimation of the true underlying path distance. Here we give a quantification of this inherent systematic underestimation of DTDs for a terrestrial primate, the Guinea baboon. We show that sampling protocols with interval lengths from 1 to 120 min underestimate DTDs on average by 7 to 35 %. For longer time intervals (i.e. 60, 90, 120 min), the relative increase of deviation from the true trajectory is less pronounced than for shorter intervals. Our study provides first hints on the magnitude of error, which can be applied as a corrective when estimating absolute DTDs in calculations on travelling costs in terrestrial primates.

Description: Latent and sensible heat surface fluxes are key factors of the western African monsoon dynamics. However, few long-term observations of these land surface fluxes are available; these are needed to increase understanding of the underlying processes and assess their impacts on the energy and water cycles at the surface–atmosphere interface. This study analyzes turbulent fluxes of one full year, measured with the eddy covariance technique, over a cultivated area in northern Benin (western Africa). The study site is part of the long-term AMMA–CATCH (African Monsoon Multidisciplinary Analysis–Coupling of the Tropical Atmosphere and Hydrological Cycle) hydrological observatory. The flux partitioning was investigated through the evaporative fraction (EF) and the Bowen ratio (β) at both seasonal and daily scales. Finally, the surface conductance (Gs) and the decoupling coefficient (Ω) were calculated and compared with specific bare soil or canopy models. Four contrasting seasons were identified and characterized by their typical daily energy cycles. The results pointed out the contrasting seasonal variations of sensible and latent heat fluxes due to changing atmospheric and surface conditions. In the dry season, the sensible heat fluxes were largely dominant (β ~ 10) and a low but significant evapotranspiration was measured (EF = 0.08); this was attributed to a few neighboring bushes, possibly fed by the water table. During the wet season, after the monsoon onset, surface conditions barely affected the evaporative fraction (EF), which remained steady (EF = 0.75); the latent heat flux was dominant and the Bowen ration (β) was about 0.4. During the dry-to-wet and wet-to-dry transition seasons, both EF and β were highly variable, as they depended on the atmospheric forcing or the response to isolated rains. A complete surface–atmosphere decoupling was never observed in 2008 (0 〈 Ω 〈 0.6), which suggests a systematic mixing of the air within the canopy with the atmospheric surface layer, irrespective of the atmospheric conditions and the vegetation height. Modeling approaches showed a good agreement of soil resistance with the Sakaguchi bare soil model. Canopy conductance was also well reproduced with the Ball–Berry stomata model. We showed that the skin surface temperature had a large seasonal and daily amplitude, and played a major role in all the surface processes. Consequently, an accurate modeling of the surface temperature is crucial to represent correctly the energy and water budgets for this region.

Description: Latent and sensible heat fluxes are known as key factors in the West African monsoon dynamics. However, few long-term observations of these land surface fluxes are available to document their impact in the climate variability of this region. The present study took advantage of the Sudanian site of the AMMA-CATCH (African Monsoon Multidisciplinary Analysis – Coupling the Tropical Atmosphere and Hydrological Cycle) observatory where turbulent fluxes were measured using the eddy covariance technique. One full year of data of energy budget over a cultivated site located in northern Benin was examined. Four contrasted seasons were identified and detailed focusing on their corresponding daily cycles. The flux partitioning was investigated through the evaporative fraction (EF) and the Bowen ratio (β) at both seasonal and daily scales. Finally, the surface conductance (Gs) and the decoupling coefficient (Ω) were calculated and confronted with specific bare soil or canopy models to identify the main processes for each season. The results pointed out the contrasted seasonal variations of sensible and latent heat fluxes due to changing atmospheric and surface conditions. During the wet season, surface conditions barely affected EF, which remained in steady regime (EF = 0.75), while latent heat flux was dominant and β was about 0.4. During the transitional periods, both EF and β were highly variable. A low but significant evapotranspiration was measured in the dry season (EF = 0.08) attributed to few scattered bushes, distributed on a bare area, possibly fed by the water table. Nevertheless, sensible heat fluxes were largely dominant (β ~ 10) during dry season. Moreover, β revealed the ligneous vegetation flowering dynamics during the dry season. The results also showed a strong surface atmosphere coupling, which suggests a systematic mixing of the flow within the canopy with the atmospheric surface layer whatever the atmospheric conditions and vegetation height. Modeling approaches showed the good agreement of soil evaporation with the Sakaguchi bare soil model. Transpiration was also well reproduced with the Collatz stomata model. Finally, skin surface temperature had large seasonal and daily amplitude and played a major role for all surface processes. As a consequence, the modeling of surface temperature is crucial to represent correctly energy and water budget for this region.

Description: The main objective of this work is to investigate at regional scale the variability in burned areas over the savannahs of West Africa and their links with the rainfall and the large-scale climatic indexes such as the Southern Oscillation Index (SOI), Multivariate ENSO Index (MEI), North Atlantic Oscillation (NAO) and sea surface temperature gradient (SSTG). Daily satellite products (L3JRC) of burned areas from the SPOT Vegetation sensor at a moderate spatial resolution of 1 km x 1 km between 2000 and 2007 were analyzed over the West African savannah in this paper. Results from seasonal analysis revealed a large increase in burned areas from November to February, with consistent peaks in December at the regional scale. In addition, about 30% of the pixels are burned at least four times within the 7-year period. Positive correlations were found between burned areas and rainfall values obtained from the TRMM satellite over savannahs located above 8° N, meaning that a wet rainfall season over these regions was favorable to biomass availability in the next dry season and therefore may induce an increase in burned areas in this region. Moreover, our results showed a nonlinear relationship between the large-scale climatic indexes SOI, MEI, NAO and SSTG and burned-area anomalies. Positive (negative) correlations between burned areas and SOI (MEI) were consistent over the Sahel and Sudano-Sahelian areas. Negative correlations with Atlantic SSTG were significant over the Guinea subregion. Correlations between burned areas over Sudano-Guinean subregion and all the large-scale indexes were weak and may be explained by the fact that this subregion had a mean rainfall greater than 800 mm yr−1 with permanent biomass availability and an optimal amount of soil moisture favorable to fire practice irrespective of the climate conditions. The teleconnection with NAO was not clear and needed to be investigated further.

Description: Bushfires are recognized as environmental processes that affect the atmosphere by the gases and particles emitted and have an ecological and climatic impact. However, there are still numerous uncertainties, particularly about the variability of fire occurrence on the intra- and inter-annual scale. Our objective was to distinguish the space-time variability of fires in West Africa through analysis of burned areas using SPOT VEGETATION from L3JRC (1 April 2000 to 31 March 2007) which were obtained from the modification in the algorithm of cells from the GBA 2000 project. We also analyzed the influence of several large scale factors such as the ENSO, SOI, NAO and the north-south Atlantic temperature gradient factor (SSTG) on the variability of the span of burned areas. Based on areas burned monthly, we calculated the frequency of fire passage on the same pixel. This helped characterize the activity of pixels and distinguish the most vulnerable zones (with a lot of fire activity) from the least vulnerable ones (with less activity). Using a correlation calculation, we also found the influence of the quality of precipitations during preceding rainy seasons on burned areas during the dry season along with climate factors such as MEI, SOI, NAO and the SSTG.

Description: This study uses the high resolution outputs of the recent CORDEX-AFRICA climate projections to investigate the future changes in different aspects of the hydrological cycle over West Africa. Over the twenty-first century, temperatures in West Africa are expected to increase at a faster rate (+ 0.5 °C per decade) than the global average (+ 0.3 °C per decade), and mean precipitation is expected to increase over the Guinea Coast (+ 0.03 mm/day per decade) but decrease over the Sahel (− 0.005 mm/day per decade). In addition, precipitation is expected to become more intense (+ 0.2 mm/day per decade) and less frequent (− 1.5 days per decade) over the entire West Africa as a results of increasing regional temperature (precipitation intensity increases on average by + 0.35 mm/day per °C and precipitation frequency decreases on average by − 2.2 days per °C). Over the Sahel, the average length of dry spells is also expected to increase with temperature (+ 4 % days per °C), which increases the likelihood for droughts with warming in this sub-region. Hence, the hydrological cycle is expected to increase throughout the twenty-first century over the entire West Africa, on average by + 11 % per °C over the Sahel as a result of increasing precipitation intensity and lengthening of dry spells, and on average by + 3 % per °C over the Guinea Coast as a result of increasing precipitation intensity only.

Description: A multi-platform field measurement campaign involving aircraft and balloons took place over West Africa between 26 July and 25 August 2006, in the frame of the concomitant AMMA Special Observing Period and SCOUT-O3 African tropical activities. Specifically aiming at sampling the upper troposphere and lower stratosphere, the high-altitude research aircraft M55 Geophysica was deployed in Ouagadougou (12.3° N, 1.7° W), Burkina Faso, in conjunction with the German D-20 Falcon, while a series of stratospheric balloon and sonde flights were conducted from Niamey (13.5° N, 2.0° E), Niger. The stratospheric aircraft and balloon flights intended to gather experimental evidence for a better understanding of large scale transport, assessing the effect of lightning on NOx production, and studying the impact of intense mesoscale convective systems on water, aerosol, dust and chemical species in the upper troposphere and lower stratosphere. The M55 Geophysica carried out five local and four transfer flights between southern Europe and the Sahel and back, while eight stratospheric balloons and twenty-nine sondes were flown from Niamey. These experiments allowed a characterization of the tropopause and lower stratosphere of the region. We provide here an overview of the campaign activities together with a description of the general meteorological situation during the flights and a summary of the observations accomplished.

Description: The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated anomalies in the atmospheric general circulation. This sensitivity study was done with the Météo-France climate model: ARPEGE. The sensitivity to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensitivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahel years observed since 1970. For each case, the spatiotemporal variability of the simulated rainfall anomaly and changes in the modelled tropical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant changes in the simulated TEJ and an enhancement of the AEJ are found; at the surface, the energy budget and the hydrological cycle are substantially modified. On the other hand, SST anomalies induce a negative rainfall anomaly over the Sahel and a positive rainfall anomaly to the south of this area. The rainfall deficit due to those anomalies is consistent with previous diagnostic and sensitivity studies. The TEJ is weaker and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated rainfall anomaly is similar to the observed mean African drought patterns. This work suggests that large-scale variations of surface conditions may have a substantial influence on Sahel rainfall and shows the importance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainfall.

Description: The latest version of RegCM4 with CLM4.5 as land surface scheme was used to assess the performance and the sensitivity of the simulated West African climate system to different convection schemes. The sensitivity studies were performed over the West Africa domain from November 2002 to December 2004, at spatial resolution of 50km×50km and involved five (5) convective schemes: (i) Emanuel; (ii) Grell; (iii) Emanuel over land and Grell over ocean (Mix1); (iv) Grell over land and Emanuel over ocean (Mix2); and (v) Tiedtke. All simulations were forced with ERA-Interim data. Validation of surface temperature at 2m and precipitation were conducted using respectively data from the Climate Research Unit (CRU) and Global Precipitation Climatology Project (GPCP) during June to September (rainy season). Quantitative assessment of the sensitivity tests were carried out using the mean bias, the pattern correlation coefficient, the root mean square difference, the probability density function of the temperature bias and the Taylor diagram. Results revealed a better performance of the configuration with Emanuel convection scheme to simulate the spatial and temporal variability of the temperature and the precipitation. Therefore, the configuration of RegCM4 with CLM4.5 as land surface model and implementing Emanuel convective scheme is recommended for the study of the West African climate system.