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Abstract

Using three daily measurements of wind speed and direction from synoptic weather station data in SE Niger, we examined the diurnal, seasonal and interannual time-scale of Sahel climate variability between 1950 and 1992. The seasonal wind patterns are closely related to the temperatures and West African monsoon dynamics. The transitions between the two seasons are marked by an important increase in calms (wind speed <0.5 m.s-16 ). Such variations are related to meridional shifts of the Inter Tropical Discontinuity (ITD) and Inter Tropical Convergence Zone (ITCZ). Interannual fluctuations of annual wind speeds are consistent with Sahel rainfall variability. Dry years, such as in 1969-1973 and 1983-1986 periods, are associated with negative anomalies in wind speeds mainly due to an increase in calms and dry conditions. Nevertheless, we note several differences: the first period is associated with a yearly increase in the annual mean speed, while the second is associated with a decrease. Differences could be related to changes in atmospheric circulation, especially regarding the strength and latitudinal position of Tropical and African Easterly jets.

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... The Sahara and the Sahel are recognized as the first regions producing desert aerosols [2][3][4][5]. Several types of instruments have been used over time to quantify and identify the source areas [6][7][8][9]. Correlations between wind, temperature and precipitation during the African monsoon season have been established [10]. However, there are other parameters that can have a significant direct or indirect influence 2 of 19 on the lifting, transport and deposition of fine particles in the Sahelian zone. ...
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Thesis
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Space and time scales for analysis of the interannual variability of Sahelian rainfall are determined. Regionalizations of annual and monthly rainfall fields are performed in West Africa for the period 1948-78. Four coherent regions and a less coherent one are identified. Different type classifications derived from the regionalization results are built. The monthly type based on the rainfall anomaly signs north and south of 10°N suggests two major causes of the rainfall pattern variability, one resulting from an anomaly of rainfall amount and the other from a displacement of the intertropical convergence zone (ITCZ). The type based solely on the anomaly sign north of 10°N blends these factors and may give misleading analyses. The use of monthly rainfall fields over all of West Africa is then recommended.
Article
Based on a full-resolution Meteosat dataset, an extensive climatological study of the mesoscale convective systems (MCSs) observed by satellite over the Sahel leads to the definition of a subpopulation of MCSs-called organized convective systems (OCSs)-that represents only 12% of the total number of MCSs observed during 9 yr over the central Sahel while accounting for almost 80% of the total convective cloud cover defined at the 233-K threshold. Using a high-resolution rainfall dataset, it is shown that these OCSs are also the main source of rain in this region, accounting for about 90% of the seasonal rainfall, with a mean areal rainfall of 14.7 mm per system. All of the OCSs are associated with a rain event, and more than 90% of the major rain events are associated with an OCS. These figures are compared with those obtained for mesoscale convective complexes (MCCs). Each MCC produces more rainfall on average (19 mm per system) but there are only a few of them (1.2% of the total number of MCSs), and they consequently produce only 19% of the seasonal rainfall. The interannual rainfall variability is first determined by the year-to-year fluctuation of the number of events defined from satellite rather than by the fluctuations of their mean rain efficiency. In fact, the total rain yield of an OCS appears to be linked primarily to its duration (which itself is largely determined by its spatial extension) rather than to its average rain rate. The diurnal cycle over the region is also studied, and it is shown that it is largely conditioned by the propagative nature of the OCSs associated with orography-driven generations located a few hundred kilometers to the east of the validation area.
Article
This study examines selected dynamical factors associated with wet and dry years in the West African Sahel. The approach is to evaluate the temperature, wind, and moisture fields and the dynamic instabilities for a 4-yr ''wet'' composite (1958-61) and a 4-yr ''dry'' composite (1982-85). The analysis, limited to the months of June through September, is carried out using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) 40-Year Reanalysis Project dataset. Two upper-air datasets are also evaluated to confirm trends apparent in the NCEP-NCAR data. This study confirms some of the results of earlier studies, such as the weaker African easterly jet (AEJ) and stronger tropical easterly jet (TEJ) during wet years, but suggests a different interpretation of the wet-dry contrasts. In the Sahel, the most important characteristic of the AEJ appears to be its latitudinal location rather than its intensity. This governs the instability mechanisms. The AEJ is displaced northward during the wet years, thereby enhancing both the horizontal and vertical wind shear over the Sahel. Baroclinic instability is probably the dominant mechanism here. South of the Sahel, both the location and intensity of the AEJ appear to be important, and the dominant instability mechanism appears to be barotropic. Results presented here further suggest that equatorial westerlies significantly modulate inter- annual variability. These serve to displace the jet and associated disturbances poleward and to enhance convective activity. The TEJ may also play an important role by promoting ascent in the lower troposphere on the cyclonic side of the AEJ, where the disturbances develop. Contrasts in the moist layer between wet and dry years may be consequences of these other changes. On an interannual basis, the rainfall maximum and the surface position of the ITCZ are effectively decoupled, with the ITCZ keeping a relatively stable location from year to year despite large latitudinal shifts in the rain belt. As a consequence, the length of the rainy season does not change markedly. Instead, wet years in the Sahel are characterized by more intense rainfall (mm month 21). This is linked to a northward shift in the belt of maximum rainfall, which is coincident with the northward shift in the AEJ.
Article
Climatic and environmental changes in Africa during the last 2 centuries have been examined, using both systematic rainfall records and proxy information concerning lakes and rivers and the occurrence of famine and drought. The rainfall records provide excellent detail for the 20th century. The proxy data have been used to produce a semi-quantitative data set spanning most of the continent and having an annual time resolution. These provide an overview of conditions during the 19th century. Various issues related to the causes of these 2 centuries of variability are also considered: atmospheric and oceanic processes, desertification, surface albedo, mineral dust and hydrological feedbacks. The most significant climatic change that has occurred has been a long-term reduction in rainfall in the semi-arid regions of West Africa. This was on the order of 20 to 40% in parts of the Sahel. There have been 3 decades of protracted aridity. Nearly all of Africa has been affected by increased aridity, particularly since the 1980s. Few changes in temperature have been demonstrated. These have occurred on a much smaller scale and are of considerably lower magni- tude than those over the continents. The rainfall conditions over Africa during the last 2 to 3 decades are not unprecedented. A similar dry episode prevailed during most of the first half of the 19th cen- tury. By mid-century, conditions more typical of the 'normal' for the current century again prevailed. Thus, the 3 decades of dry conditions evidenced in the Sahel are not in themselves evidence of irre- versible global change. On the other hand, the processes controlling rainfall over most of the conti- nent are now reasonably well understood. One of the most important factors, particularly in the Sahel, is sea-surface temperatures. It has been hypothesized that anthropogenic changes in the land surface, particularly land use change and desertification, have contributed significantly to the decline in rainfall. Current evidence suggests that if changes in the land surface (e.g., vegetation cover, sur- face albedo, soil moisture) signficantly impact climate, they are much more strongly controlled by natural climate variations, such as the recent decline in rainfall, than by human-induced land-use change or degradation. Unfortunately, we still do not have any accurate large-scale assessments of the extent, nature and degree of such changes. The dreaded 'desertification' process appears to be confined to relatively small scales. However, there is still cause for concern because the net effect of the various feedback processes involved in land degradation appears to be destabilization of ecosys- tems. Thus, a priority must be large-scale monitoring of the land surface and estimates of the degree of change.
Article
This study investigates the origin and structure of easterly waves that form in the lower troposphere of North Africa and have a periodicity of 3-5 days,. From June to early October these waves propagate across the Atlantic and occasionally reach the eastern Pacific. Although only a few of these disturbances actually intensify after reaching the Atlantic, they account for approximately half of the tropical cyclones that form in the Atlantic.Spectral analysis of five years of upper air data shows that African waves produce a spectral peak of the meridional wind at periods of 3-5 days with a maximum amplitude of 1-2 m sec1 near 700 mb. These waves normally originate between Khartoum (32E) and Ft. Lamy (I5E) and affect a greater depth of the atmosphere as they propagate westward.Wind statistics at stations flanking the mountains in Ethiopia indicate that airflow over these mountains is not the cause of the easterly waves. This study shows that the African waves are directly related to the mid-tropospheric easterly jet that is found within the baroclinic zone to the south of the Sahara. During the same season that the waves are observed, the gradient of the monthly mean potential vorticity vanishes along the isentropic surfaces. Charney and Stem have shown that this is a necessary condition for instability of the jet provided that the amplitude of the waves is negligible at the ground. Results show that the horizontal and vertical shear of the mean zonal wind are acting as nearly equal sources of energy for the perturbations. The role of convection in the origin of these waves has not yet been determined.
Article
In the second part of this study, the variability of surface and airmass characteristics associated with West African rainfall fields is investigated. Ship observations over the tropical Atlantic and rawinsonde records over West Africa are used for the months of July, August, and September from 1948 to 1978.A composite analysis based on the sign of monthly rainfall departures north of 10°N indicates that the Sahelian drought may result from a more southern than normal position of the intertropical convergence zone (ITCZ) over the tropical Atlantic.A composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10°N. It shows two types of Sahelian drought. The first is linked to an enhanced surface atmospheric circulation over the northern tropical Atlantic, higher sea surface temperatures in the Gulf of Guinea, a convergent moisture flux anomaly south of 10°N along the western African coast, and a lower (larger) water content in the monsoon flux north (south) of 10°N. It may be associated with a southward location of the ITCZ and of the Hadley cells. The second is related to positive sea level pressure and surface easterly circulation anomalies over the northern tropical Atlantic and weaker convection inside the ITCZ over West Africa. It may be associated with an enhancement and/or an eastward and equatorward displacement of the subsiding branch of the Atlantic Walker cell.
Article
The annual and monthly rainfall variability in north tropical Africa are analysed by principal component analysis in order to detect the main coherent modes. The Sahel constitutes the first mode, although it is divided close to 10°W into an Atlantic section and a Continental section. The Guinean zone is less coherent, and is divided into distinct modes. The subequatorial area never formed a distinct mode. Rainfall variability for the coherent areas is constructed from rainfall anomaly indices (RAI), based on the coherent areas, and a rainfall typology taking into account the spatial pattern of the rainfall anomaly fields and the intensity of the anomaly. Their evolution (mainly since 1950) is synchronous on the Sahelian and the Guinean zones with the opposition between the 1950s and 1960s (wetter than normal) and the 1970s and the 1980s (drier than normal). In particular, the area of drought, which has plagued the Sahel since 1968, extends to Guinean Africa, mainly after the mid-70s. The typology allows us to investigate the relationships at both the annual and the monthly scale.
Article
African free air wind velocity data for wet and dry periods in the Sahel have been analysed objectively by the Cressman scheme and show that the lower troposphere easterly jet over west Africa is stronger in the dry period whereas the analogous easterly jet to the south of the equator exhibits greater vertical shear in the wet period. Two possible explanations, based on modulations of the meridional temperature gradients across the jet, are offered.
Article
This study examines the nature of the tropical rainbelt over west Africa in 4 years corresponding to the four most common patterns of rainfall anomalies in the region. These include 2 years that were anomalously wet in the Sahel, 1955 and 1950, and 2 years, 1983 and 1984, that were anomalously dry. The year 1955 is ubiquitously wet, while a rainfall dipole prevails in 1950, with abnormally high rainfall in the Sahelian latitudes but drought conditions in the Guinea Coast latitudes to the south. The year 1983 is ubiquitously dry and 1984 is a year with a rainfall dipole but opposite in phase (i.e. Sahel dry, Guinea Coast wet). The main results of the study are that the contrast between wet and dry years is associated with a weakening and contraction of the tropical rainbelt. In addition, a north/south displacement of the rainbelt is associated with the dipole patterns, with a northward/southward shift bringing good rains to the Sahel/Guinea Coast. These shifts are associated with north/south shifts in the latitude of the African Easterly Jet (AEJ) over west Africa. The latitudinal position and extent of the rainbelt correspond to the latitudinal position and extent of an immense ‘tongue’ of strong ascending motion. The core of the ascending motion is bound by the axes of the AEJ on the north and the Tropical Easterly Jet (TEJ) in the south. This importance of these two jets in determining the character of the summer rainy season implies that the Sahelian region provides the link between the Sahel and global scale phenomena. Copyright © 2008 Royal Meteorological Society
Article
The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. © 1998 Royal Meteorological Society.
Article
A major factor in rainfall variability over Sahelian West Africa is the latitudinal location of the tropical rainbelt. When it is displaced abnormally far northward, the Sahel experiences a wet year. An anomalous southward displacement results in drought. In this paper we examine the question of what controls the location during the boreal summer, hypothesizing that inertial instability plays a role. An analysis of surface pressure and temperature fields, wind fields, divergence and vertical motion show that the criteria for inertial instability are satisfied in wet Augusts but not in dry ones. The key determinant appears to be the surface pressure gradient between the continent and the equatorial Atlantic. When this is large, inertial instability results in the development of a low-level westerly jet. The presence of this jet enhances the horizontal and vertical shear, and displaces the African Easterly Jet northwestward. Associated with this situation is strong vertical motion over the Sahel and subsidence over the Guinea Coast, producing dry conditions over the latter. The result is a rainfall dipole, one of two major modes of variability over West Africa. Important factors include sea surface temperatures (SSTs) in the equatorial Atlantic and pressure in the South Atlantic. The first of these factors suggests a link with the Atlantic Niño mode of tropical Atlantic variability; while the second suggests a possible link with the Pacific and the extratropical South Atlantic. Overall, our study relates the well-known SST influence on Sahel rainfall to atmospheric dynamics over the continent. Copyright © 2007 Royal Meteorological Society
Article
Time series analysis is an important issue in the earth science-related engineering applications such as hydrology, meteorology and environmetrics. Inconsistency and nonhomogeneity that might arise in a time series yield segments with different statistical characteristics. In this study, an algorithm based on the first order statistical moment (average) of a time series is developed and applied on five time series with length ranging from 84 items to nearly 1,300. Comparison to the existing segmentation algorithms proves the applicability and usefulness of the proposed algorithm in long hydrometeorological and geophysical time series analysis.
Article
In the Sahel, rainfall is the single most important factor conditioning the hydrology and the climate, but comprehensive statistical analyses of the rainfall climatology in the region are rare. Yet, even though in the Sahel rainfall data are scarce by the standards of the temperate countries, it is shown here that it is possible to obtain a reasonably good idea of what the rainfall has been over Sahelian Niger for the past 40 years, both in terms of interannual variability and spatial distribution. To that aim a statistical model is used, which decomposes the space-time fluctuations of long-term rainfall averages into the fluctuations of the mean event rainfall on the one hand, and of the mean number of rainfall events over any period of accumulation, on the other hand. This model is first applied to the analysis of monthly rainfall data over the whole of Niger. It is shown that the lasting drought which has affected Niger for more than 20 years is associated with a decrease in the number of rainy events, rather than to a decrease of the mean event rainfall, and that this decrease is more pronounced for the core of the rainy season. Because these fluctuations are not homogeneous over Niger, a 5° × 4° zone centred on the HAPEX-Sahel 1° × 1° square is selected in order to characterise more accurately the rainfall climatology of the HAPEX-Sahel area between 1950 and 1990. In comparison with what it was between 1950 and 1970, the average length of the rainy season has not changed significantly during the dry period 1970–1990. Rather, it is the decrease of rainfall in July and August that explains most of the diminution of the total annual rainfall over this part of the Sahel since 1970. The average number of rainy events in August was reduced by about 30%, while the mean event rainfall remained roughly constant. Finally, the analysis of the daily rainfall series for Niamey (which constitutes the longes record available in Niger, starting in 1905) enables the comparison of four periods of 20 years between 1910 and 1990. The period 1970–1989 appears to be by far the longest and most severe dry spell of the past century. Almost 90% of the annual rainfall decrease over this period is explained by the decrease of the mean number of rainfall events during July and August, while both the length of the rainy season and the mean event rainfall remained stable.
Article
The distributions of frequency of occurrence (FOO) of ‘thick dust haze’ (TDH) and of ‘light dust haze’ (LDH) with respect to latitude, longitude, elevation and distance from source during Harmattan season (from November to February) are investigated, using 30-year visibility data collected from 27 synoptic stations located in Nigeria between the West African Sahel and the coasts of Gulf of Guinea. This region lies along the Harmattan season trajectory of Saharan dust. Also investigated is the temporal variability of TDH and LDH on intra-seasonal and inter-annual timescales spanning three decades from 1971 to 2000. Dust haze distribution over the region is found to correlate with the four geographical variables to different extents, with latitude, distance from source and elevation showing strong correlations with both TDH and LDH. The investigation also shows that while FOO of TDH days increases with latitude (correlation coefficient, rlat,FOO_TDH=0.88), FOO of LDH days decreases with latitude over the region (correlation coefficient, rlat,FOO_LDH=−0.74). The correlation coefficients with longitude over the region are rlon,FOO_TDH=0.44 and rlon,FOO_LDH=−0.25, respectively, indicating weaker dependence of TDH and LDH on longitude. The average number of TDH days per month during the Harmattan season in the region ranges from about 0.5 in the humid coastal zone near the Gulf of Guinea in the south, to approximately 6 in the dry semi-arid zone near the West African Sahel. An empirical equation which shows that FOO of TDH increases exponentially with latitude over the region is derived. The FOO of TDH is found to be most variable in or near the Sahel zone and decreases southwards towards the Gulf of Guinea. The average of the standard deviations is 1.13 for the six northernmost synoptic stations and decreases to 0.51 over the six coastal and southernmost locations. In contrast, the FOO of LDH is most variable in the south (with standard deviation of 2.11 (over the six southernmost and coastal stations) and decreases to an average of 0.30 (over six of the extreme northern stations). The observed patterns of TDH and LDH distribution are attributed to distance from dust source region, decrease in dust particle size and wind speed from north to south, increase in atmospheric humidity and vegetation cover from the Sahel in the north to the coastal zone in the south, as well as inter-annual variability of Sahel rainfall.
Article
An original segmentation procedure hydrometeorology series is setailed. From the beginning of this century, the results of rainfall and discharge series analysis exhibit a West African climatological evolution in successive stages. These stages, separated by jumps, come within more and more arid general tendency. The length of the sequences between the jumps are 9 to more than 19 years long. These climatological sequences, computed from various time series are concomitant and then have a regional significance.RésuméUne procédure originale de segmentation des séries hydrométéorologiques est présentée. Elle permet de mettre en évidence, aussi bien pour la pluviométrie que pour le débit de grands fleuves, une évolution climatique de l'Afrique de l'Ouest, depuis le début siècle, en séquence successives qui s'inscrivent dans une tendance générale d'aggravation de l'aridit′e. Ces séquences longues de 9 à plus de 19 ans, déterminées à partir de séries de localisation et de nature variées, sont concomittantes et ont donc une valeur régionale.
Article
Thesis (doctoral)--Dijon. Bibliography: p. 1356-1413.
Pluviom etrie journali ere et types de pluies au Sahel central : Manga et Bornou (Niger et Nigeria)
  • S Mugnier
Mugnier, S., 1995. Pluviom etrie journali ere et types de pluies au Sahel central : Manga et Bornou (Niger et Nigeria). Evolution de 1950 a 1990. M emoire DEA Univ. Bourgogne, Dijon, p. 48.
Les lithom et eores en r egion sah elienne: un indicateur climatique de la d esertification
  • P Ozer
Ozer, P., 2000. Les lithom et eores en r egion sah elienne: un indicateur climatique de la d esertification. GEO-ECO-TROP 24, 1e317.
M ethodologie pour une meilleure repr esentation spatiotemporelle des fluctuations pluviom etriques observ ees au Niger depuis 1905
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  • M Erpicum
Ozer, P., Erpicum, M., 1995. M ethodologie pour une meilleure repr esentation spatiotemporelle des fluctuations pluviom etriques observ ees au Niger depuis 1905. S echeresse 6 (1), 103e108.
Erosion eolienne dans le Damagaram Est
  • A Tidjani
Tidjani, A., 2008. Erosion eolienne dans le Damagaram Est (Sud-Est du Niger) :
Analysis of a Sahelian annual rainfall index from 1896 to 2000, the drought continues
  • T L'hote
  • G Mah E
  • B Some
  • J P Triboulet
L'Hote, T., Mah e, G., Some, B., Triboulet, J.P., 2002. Analysis of a Sahelian annual rainfall index from 1896 to 2000, the drought continues. Hydrol. Sci. J. 47 (4), 563e572.