Can daily meteorological measurement of near-surface wind detect climate changes in the Sahel (SE Niger, 1950–1992)?
Introduction
During the 20th century, studies on climate variability in the Sahel are most often based on interannual or decadal fluctuations of rainfall, which are the most important from both an economic and social point of view. All studies have indicated that this region experienced a dramatic change a series of unprecedented droughts since the end of the 1960s (e.g., Janicot and Fontaine, 1993, Nicholson, 2001, L'Hote et al., 2002). These droughts led to a disruption in balance of the fragile Sahelian ecosystem (UNEP, 1992). In particular, we have witnessed a remobilization of dunes (e.g., Courrel and . Chamard, 1987, Tidjani, 2008) and an increase in dust suspension in the atmosphere (Middleton, 1985), which reduced horizontal visibility at the local level (e.g., Ozer, 2000, Anuforom, 2007).
Rainfall over this region appears to be linked to the timing and amplitude of the Inter-Tropical Convergence Zone (ITCZ) shifting, as well as the intensity of the atmospheric flows (e.g., Nicholson, 2008); however, rainfall is not representative of the permanent local conditions because it is only observed for a few days during the rainy season (i.e., when the ITCZ is in its northernmost position). In contrast, windless days are unusual (Leroux, 1983). Near-surface wind speeds and directions recorded by meteorological stations are likely to describe the circulation patterns of the air masses throughout the year. Therefore, the question arises as to whether it is possible to understand climate variability from wind dynamics in semi-arid areas.
The objective is to examine, on a local scale, the fluctuations of wind dynamics and compare them to observed changes in rainfall between 1950 and 1992. The spatial resolution of these observations is considerably lower than that offered by reanalysis fields (NCEP/NCAR-1 [NNR-1], 20th century reanalysis [20CR], ≥2° × 2°), which are only representative of the regional scale. This local approach to wind dynamics is the first step to improve our understanding of soil erosion and dust transport in relation to environmental changes following dry years. Three daily measurements of wind (direction and speed at 6:00 am, 12:00 noon and 6:00 pm) from two synoptic stations in SE Niger were used: Maïné-Soroa (sahelian zone) and Nguigmi (northern sahelian zone, near the Sahara boundary).
After a discussion of the study area and its climatic setting in Section 2, we present the data and methods in Section 3. Next, we examine the local rainfall variability in Section 4, prior to an analysis of fluctuations in wind speed and direction in Section 5. Finally, the characteristics of wind patterns during droughts are described in Section 6.
Section snippets
Geographical, geomorphologic and climatic settings
The Maïné-Soroa and Nguigmi weather stations are located in the Sahel, to the west of Lake Chad in the administrative region of Diffa (Eastern Niger, Fig. 1). This region consists of flat areas with altitudes ranging from 400 m in the NW to 280 m in the SE. No topographic obstacles opposed the dominant NE–SW winds that have shaped the landscape of the Chad Basin during the Upper Quaternary (Mainguet, 1984, Durand, 1995).
The Sahel region is a semi-arid area lying just to the south of the Sahara
Data and methods
Two weather stations located in the Central Sahel were selected (Fig. 2): Maïné-Soroa (13°13′ 48″ N, 11°58′48″ E) at a 338 m elevation and Nguigmi (14°15′ N, 13°7′12″ E; located near the boundary of the Sahara) at a 287 m elevation. These stations were established in 1936 (Maïné Soroa) and 1921 (Nguigmi). Since 1961, these stations have been used for the synoptic meteorological observation network.
Data from 1950 to 1992 were collected from the Niger Republic National Direction of Meteorology
Seasonal variations
Rainy season occurs from May to October with a peak in August (Fig. 3A1 and B1). The seasonal total precipitation distributions differ between the two stations because of their latitudinal locations (cf sect. 2). At the Maïné-Soroa station (Fig. 3B1), where the rainy season lasts longer, rainfall amounts exceed 20 mm month−1 during the transition periods between the dry and wet seasons (i.e., June and September). Monthly rainfall amounts are much less in Nguigmi (Fig. 3B1). After the 1960s,
Seasonal variations
Forty-three years of three daily measurements of wind speed and direction were analyzed (6:00 am, 12:00 noon and 6:00 pm) (Fig. 4 and Table 1). Of the 47,118 observations per station, 82.7% and 73.7% showed a significant wind (speed ≥ 0.5 m s−1) at the Maïné-Soroa (Fig. 4A1) and the Nguigmi weather stations (Fig. 4B1), respectively. Winds were most frequent at 12:00 noon at both stations: 31.9% of the 15,030 observations (27% at 6:00 am and 23.8% at 6:00 pm) occurred at Maïné-Soroa and 31% of
Wind vs droughts
At Maïné-Soroa, the dry periods were concomitant with an increasing number of calms (Fig. 8, Fig. 9). Furthermore, throughout the period of 1950–1992, the number of calms was more important at Nguigmi (26.3%) than at Maïné-Soroa (17.3%). Climatologically, the Nguigmi weather station was however, much dryer compared to Maïné-Soroa (Fig. 3); which thus raises the question of a possible relationship between the number of calms and dry conditions.
The time-evolution of the monthly percentage of
Conclusions
At daily and seasonal scales, the observations are good illustrations of the current knowledge regarding tropical meteorology. At the daily scale, the average wind speeds emphasize the role of thermal turbulence (i.e., greater speeds at 12:00 pm than at 6:00 am and 6:00 pm). At the intra-annual scale, through wind direction and speed, it is possible to define the seasons that are closely related to rainfall. Thus, the dry season is characterized by the predominance of the NE, E and N wind
Acknowledgments
This study was supported by the AIRD (Agence Inter-établissements de Recherche pour le Développement) through the CORUS2 project, titled “Impact de la pression anthropique et du Changement Global sur les flux sédimentaires en zone sahélienne” (grant no. 6116). It is also a contribution to the AMEDE action (Analyse Multi-Echelle de la Dynamique Eolienne au Sahel), which is funded by the FED 4116 SCALE (ESTER project) with support from the Institut de Recherche pour le Développement (IRD) and the
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