Bastien Dieppois - Academia.edu (original) (raw)
Papers by Bastien Dieppois
Journal of Arid Environments, 2016
Using three daily measurements of wind speed and direction from synoptic weather station data in ... more 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 À1 ). 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 1969e1973 and 1983e1986 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.
Pluviometry variability at Maïné-Soroa station (SE Niger) shows correlation at 0.75 with the Cent... more Pluviometry variability at Maïné-Soroa station (SE Niger) shows correlation at 0.75 with the Central Sahel pluviometric index established by Lebel and Ali (2009) from 1950 to 2005. Three mains periods were indetified : wet (1950-1967), arid (1968-1999) and semi-arid (1988/89-2005). Continuous Wavelet Transform was used to evaluate forced (climatic) variability of pluviometric signal. An intensification of the 2-4 years and 5-8 years frequency bands is observed during wet and semiarid periods. Conversely, these bands become weaken and the 12-18 years band appears stronger during arid period. Moreover, Wavelet coherence and Phase are used to identify forcing climate indices. During the wet period, climate indices (NAO and Tropical North Atlantic SST) are coherent and in phase with pluviometry, while during the arid period, climate indices (NAO, Tropical North and South Atlantic SSTs) are coherent with pluviometry but out of phases.
Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis me... more Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Since the end of the 17th century, the observation of a long-term climate variability in NW Europ... more Since the end of the 17th century, the observation of a long-term climate variability in NW Europe shows a regional rainfall and a global temperature evolutions on the both sides of the Channel. Statistical relationships continuously changed fro interannual to decadal modes. On the contrary, the multidecadal modes reveal a link with AMO in Kew Garden and NAO in Paris during the rainfall positive anomalies (Kew Garden: 1733-1891; Paris: 1927-2010). The cold (end of the 17th century) and warm anomalies (beginnig of 19th century) are related to AMO. According to our study, at interannual and decadal scales, it is difficult to separate the anthropogenic impact from the natural variability. But at a multidecadal scale, between the end of 18th to the early of 19th century, the observed changes can be linked to an early anthropogenic impact.
Longtemps, le changement climatique a été caractérisé à travers les précipitations au Sahel. Cepe... more Longtemps, le changement climatique a été caractérisé à travers les précipitations au Sahel. Cependant, les précipitations ne sont observées que quelques dizaines de jours pendant 3 à 4 mois contrairement aux vents qui y sont permanents. Pour Nguigmi (partie occidentale du bassin du lac Tchad), nous avions étudié les mesures des vents et la visibilité horizontale à 6h, 12h et 18h entre 1950 et 1992. Les résultats obtenus montrent une modification des vitesses et directions des vents entre 1968-1973. De façon générale, la période 1973-1986 (aride) met en évidence une augmentation des vitesses des vents. En réponse, la visibilité horizontale a fortement baissé passant d’une moyenne de 24,5 km entre 1950-1969 à 16,3 km entre 1970 et 1992.
The study of current assessment of climate change on coastal zone of West Africa is of a great in... more The study of current assessment of climate change on coastal zone of West Africa is of a great interest. The proposed analysis aims to show initially the originality of this coastal desert climate. In a second step, the statistical analysis of time series of various parameters (rainfall, temperature, and wind) will identify the main changes that have marked more than fifty years of measurement. Finally, to determine the influence of the Atlantic Ocean on climate dynamics in this region, a research of a possible link between fluctuations in different climatic parameters temperature variability of ocean surface represented by the TASI index = NAT (Tropical North Atlantic) –– SAT (Southern North Atlantic) will be performed.
Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis me... more Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Des enregistrements sédimentaires de l’évolution du climat postérieure au dernier maximum glaciai... more Des enregistrements sédimentaires de l’évolution du climat postérieure au dernier maximum glaciaire ont été étudiés en trois régions du Niger : la vallée de l’Azawagh et le massif de Termit localisés sur la limite actuelle Sahara-Sahel (environ 100mm.an-1), la partie méridionale du Manga à la limite sud actuelle du Sahel (environ 400mm.an-1). A l’Holocène inférieur, dans un contexte climatique humide (« Sahara Vert »), on observe néanmoins l’existence d’une dynamique sédimentaire éolienne considérée comme liée à l’aridité. L’enregistrement de cette dynamique sédimentaire éolienne est variable selon la latitude (gradient climatique) mais aussi selon les caractéristiques propres à chaque géosystème (topographie, morphologie, lithologie, hydrogéologie). La présence constante de poussières éoliennes, même pendant le maximum humide, implique la permanence de zones où le couvert végétal était, au moins saisonnièrement, insuffisant pour assurer la protection du substratum. Dans l’Azawagh, ...
Theoretical and Applied Climatology, 2014
ABSTRACT This study systematically examines teleconnections between Atlantic sea surface temperat... more ABSTRACT This study systematically examines teleconnections between Atlantic sea surface temperature (SST) and the west–east distribution of Sahel rainfall throughout the 20th century, taking non-stationarity into account. Sahel rainfall variability of six selected rain gauges displays three dominant time-scales: multi-decadal (&gt;20 year), quasi-decadal (8–18 year) and interannual (2–8 year). Regarding their patterns of low-frequency scales, three coherent Sahelian sub-regions can be identified: the Atlantic Coast (Dakar), western-central Sahel (Nioro and Mopti) and eastern Sahel (Niamey, Maradi, Maine-Soroa). Cross-analyses combining spectral and multivariate analysis of 20 station-based data and West-African gridded rainfall data statistically confirm dissimilarities between the western and eastern Sahel. Western and eastern Sahel rainfall data are correlated with SSTs from different regions of the Atlantic Ocean, especially in the North and tropical South Atlantic. As determined by wavelet coherence and phase, in phase relationship with North Atlantic SSTs only occurs in wet periods and at the multi- and quasi-decadal scales. This teleconnection depends on the time-period and the time-scale, displaying a NW-SE pattern, which suggests non-uniform modulations of meridional displacements of the ITCZ. Tropical South Atlantic SST variability is often related to opposite patterns between the Gulf of Guinean Coast (in phase) and Sahel region (out of phase).
Journal of Geophysical Research: Atmospheres, 2013
1] Knowledge of the variability of climate in the past is essential for understanding current cli... more 1] Knowledge of the variability of climate in the past is essential for understanding current climatic changes. Therefore, we investigated two temperature indices and seven rainfall time series of northwestern Europe since the 17th century. Trends and multidecadal to interdecadal variability are similar in England and northern France for temperature, whereas a strong regional contrast is evident between the two regions for rainfall. Multidecadal and interdecadal variability displays several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, temperatures in both England and France display phase opposition with the Atlantic Multidecadal Oscillation (AMO) before 1800, while they are in-phase afterward, as determined by wavelet coherence. On the other hand, the relationships between temperature and the North Atlantic Oscillation (NAO) are weak across multidecadal and interdecadal scales for the whole period under study. For rainfall, coherence with the AMO is observed for scales at around 30-60 years, whereas coherence with the NAO is detected on 50-80 year scales and interdecadal 16-23 year scales. However, relationships between rainfall variability and North Atlantic climate indices are highly contrasted depending on the region considered. Finally, the results of a mixed spectral/empirical orthogonal function analysis of mean sea level pressure on these co-oscillation time scales highlight not only NAO regimes but also other patterns, explaining a nonnegligible amount of variance during certain time periods.
In the present context of global changes, considerable efforts have been deployed by the hydrolog... more In the present context of global changes, considerable efforts have been deployed by the hydrological scientific community to improve our understanding of the impacts of climate fluctuations on water resources. Both obser-vational and modeling studies have been extensively employed to characterize hydrological changes and trends, assess the impact of climate variability or provide future scenarios of water resources. In the aim of a better un-derstanding of hydrological changes, it is of crucial importance to determine how and to what extent trends and long-term oscillations detectable in hydrological variables are linked to global climate oscillations. In this work, we develop an approach associating large-scale/local-scale correlation, enmpirical statistical downscaling and wavelet multiresolution decomposition of monthly precipitation and streamflow over the Seine river watershed, and the North Atlantic sea level pressure (SLP) in order to gain additional insights on the atmosphe...
West-African Sahel rainfall shows a large decadal signal, transitioning from wet conditions in th... more West-African Sahel rainfall shows a large decadal signal, transitioning from wet conditions in the 1950s to dry conditions in the 1970s/80s. Only the quasidecadal time-scale is significantly detected in the Sahel rainfall index during the 1970s and 1980s. This time-scale of rainfall variability appears to be related to two periods of dry conditions and one of relatively wet conditions in the mid-70s. The dominant quasidecadal anomalies of West-African Monsoon have been reconstructed using Fourier transform for the 70s-80s, and subsequently assessed by the differences in wet and dry states. A strengthening of cross-equatorial Atlantic SST and pressure gradients can be related to an enhancement of monsoon flow and the northward shift of the ITCZ.
En Afrique de l'Ouest sahélienne la fin des années 60 est marquée par une anomalie aride générali... more En Afrique de l'Ouest sahélienne la fin des années 60 est marquée par une anomalie aride généralisée sans précèdent au XXe siècle. Pourtant, en fonction de la proximité de l'Atlantique, les modes de variabilité des précipitations sont différents. Trois secteurs sahéliens peuvent être établis : Sahel atlantique (Dakar), Sahel central (Nioro du Sahel et Mopti) et Sahel oriental (Niamey, Maradi, Maïné-Soroa). Les relations observées entre les précipitations et la variabilité des températures de surface de l'océan (SST) Atlantique Nord (AMO) et Tropical Sud (TSA) pourraient expliquer une part de cette hétérogénéité spatiale. De plus, la répartition fréquentielle et surtout temporelle des relations observées montre qu'une situation de dipôle atlantique n'a pu s'établir que très rarement et localement. Ceci, de nouveau, remet en question le rôle du mode Inter-Hémisphérique des SST dans la conduite de la variabilité décennale à multi-décennale des précipitations sahéliennes.
ABSTRACT Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal an... more ABSTRACT Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Cette étude examine les téléconnexions entre les températures de surface de la mer (TSM) de l'océ... more Cette étude examine les téléconnexions entre les températures de surface de la mer (TSM) de l'océan Atlantique et la distribution zonale des précipitations au Sahel au cours du dernier siècle. Depuis la fin des années 60, les précipitations d'Afrique de l'Ouest présentent une tendance négative sur l'ensemble de la bande sahélienne. Pourtant, trois types de variabilité basse-fréquence des précipitations sahélienne sont observés : la côte atlantique (Dakar), le Sahel central (Nioro du Sahel et Mopti) et le Sahel oriental (Niamey, Maradi, Maïné-Soroa). Au cours du XX ème siècle, les précipitations sont rarement reliées simultanément avec les TSM de l'Atlantique Nord et Tropical Sud. Aux échelles multi-et quasi-décennale, dans les années 50 et depuis les années 90, un réchauffement des TSM de l
Journal of Geophysical Research: Atmospheres, 2013
ABSTRACT Sahel rainfall shows pronounced decadal variability and a negative trend between wet con... more ABSTRACT Sahel rainfall shows pronounced decadal variability and a negative trend between wet conditions in the 1950s–1960s and dry ones in the 1970s–1980s. Using continuous wavelet transform the quasi-decadal variability (QDV) of rainfall reveals zonal contrasts. The highest QDV is identified in the 1950s–1960s over western Sahel and in the 1970s–1980s over eastern Sahel. The quasi-decadal atmospheric anomalies has been reconstructed using Fourier transform for the 1950s–1960s and the 1970s–1980s, respectively, and assessed by the composite analysis of the QDV phases for the periods before and after 1968. Over western Sahel, the rainfall QDV in the 1950s–1960s is related to the north Atlantic sea-surface temperature (SST) variability, as highlighted by the wavelet coherence. A southward shift trend of the Inter-Tropical Convergence Zone (ITCZ) is identified through an enhancement of north-easterly fluxes and moisture convergence over the western part of West Africa. A decrease (increase) of southern (northern) subtropical sinking motions seems to be involved. In the 1970s–1980s, a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flow from lower troposphere up to the mid-troposphere and to the northward shift of the ITCZ, mainly over eastern Sahel. The Pacific SST influence is also identified, which involves changes in the global zonal circulation.
Comptes Rendus Geoscience, 2012
Indice de l'Oscillation Nord Atlantique Bassin versant de la Seine Fluctuations climatiques pluri... more Indice de l'Oscillation Nord Atlantique Bassin versant de la Seine Fluctuations climatiques pluriannuelles Variabilité des pré cipitations Analyse en ondelettes continue Segmentation
Journal of Arid Environments, 2016
Using three daily measurements of wind speed and direction from synoptic weather station data in ... more 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 À1 ). 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 1969e1973 and 1983e1986 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.
Pluviometry variability at Maïné-Soroa station (SE Niger) shows correlation at 0.75 with the Cent... more Pluviometry variability at Maïné-Soroa station (SE Niger) shows correlation at 0.75 with the Central Sahel pluviometric index established by Lebel and Ali (2009) from 1950 to 2005. Three mains periods were indetified : wet (1950-1967), arid (1968-1999) and semi-arid (1988/89-2005). Continuous Wavelet Transform was used to evaluate forced (climatic) variability of pluviometric signal. An intensification of the 2-4 years and 5-8 years frequency bands is observed during wet and semiarid periods. Conversely, these bands become weaken and the 12-18 years band appears stronger during arid period. Moreover, Wavelet coherence and Phase are used to identify forcing climate indices. During the wet period, climate indices (NAO and Tropical North Atlantic SST) are coherent and in phase with pluviometry, while during the arid period, climate indices (NAO, Tropical North and South Atlantic SSTs) are coherent with pluviometry but out of phases.
Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis me... more Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Since the end of the 17th century, the observation of a long-term climate variability in NW Europ... more Since the end of the 17th century, the observation of a long-term climate variability in NW Europe shows a regional rainfall and a global temperature evolutions on the both sides of the Channel. Statistical relationships continuously changed fro interannual to decadal modes. On the contrary, the multidecadal modes reveal a link with AMO in Kew Garden and NAO in Paris during the rainfall positive anomalies (Kew Garden: 1733-1891; Paris: 1927-2010). The cold (end of the 17th century) and warm anomalies (beginnig of 19th century) are related to AMO. According to our study, at interannual and decadal scales, it is difficult to separate the anthropogenic impact from the natural variability. But at a multidecadal scale, between the end of 18th to the early of 19th century, the observed changes can be linked to an early anthropogenic impact.
Longtemps, le changement climatique a été caractérisé à travers les précipitations au Sahel. Cepe... more Longtemps, le changement climatique a été caractérisé à travers les précipitations au Sahel. Cependant, les précipitations ne sont observées que quelques dizaines de jours pendant 3 à 4 mois contrairement aux vents qui y sont permanents. Pour Nguigmi (partie occidentale du bassin du lac Tchad), nous avions étudié les mesures des vents et la visibilité horizontale à 6h, 12h et 18h entre 1950 et 1992. Les résultats obtenus montrent une modification des vitesses et directions des vents entre 1968-1973. De façon générale, la période 1973-1986 (aride) met en évidence une augmentation des vitesses des vents. En réponse, la visibilité horizontale a fortement baissé passant d’une moyenne de 24,5 km entre 1950-1969 à 16,3 km entre 1970 et 1992.
The study of current assessment of climate change on coastal zone of West Africa is of a great in... more The study of current assessment of climate change on coastal zone of West Africa is of a great interest. The proposed analysis aims to show initially the originality of this coastal desert climate. In a second step, the statistical analysis of time series of various parameters (rainfall, temperature, and wind) will identify the main changes that have marked more than fifty years of measurement. Finally, to determine the influence of the Atlantic Ocean on climate dynamics in this region, a research of a possible link between fluctuations in different climatic parameters temperature variability of ocean surface represented by the TASI index = NAT (Tropical North Atlantic) –– SAT (Southern North Atlantic) will be performed.
Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis me... more Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Des enregistrements sédimentaires de l’évolution du climat postérieure au dernier maximum glaciai... more Des enregistrements sédimentaires de l’évolution du climat postérieure au dernier maximum glaciaire ont été étudiés en trois régions du Niger : la vallée de l’Azawagh et le massif de Termit localisés sur la limite actuelle Sahara-Sahel (environ 100mm.an-1), la partie méridionale du Manga à la limite sud actuelle du Sahel (environ 400mm.an-1). A l’Holocène inférieur, dans un contexte climatique humide (« Sahara Vert »), on observe néanmoins l’existence d’une dynamique sédimentaire éolienne considérée comme liée à l’aridité. L’enregistrement de cette dynamique sédimentaire éolienne est variable selon la latitude (gradient climatique) mais aussi selon les caractéristiques propres à chaque géosystème (topographie, morphologie, lithologie, hydrogéologie). La présence constante de poussières éoliennes, même pendant le maximum humide, implique la permanence de zones où le couvert végétal était, au moins saisonnièrement, insuffisant pour assurer la protection du substratum. Dans l’Azawagh, ...
Theoretical and Applied Climatology, 2014
ABSTRACT This study systematically examines teleconnections between Atlantic sea surface temperat... more ABSTRACT This study systematically examines teleconnections between Atlantic sea surface temperature (SST) and the west–east distribution of Sahel rainfall throughout the 20th century, taking non-stationarity into account. Sahel rainfall variability of six selected rain gauges displays three dominant time-scales: multi-decadal (&gt;20 year), quasi-decadal (8–18 year) and interannual (2–8 year). Regarding their patterns of low-frequency scales, three coherent Sahelian sub-regions can be identified: the Atlantic Coast (Dakar), western-central Sahel (Nioro and Mopti) and eastern Sahel (Niamey, Maradi, Maine-Soroa). Cross-analyses combining spectral and multivariate analysis of 20 station-based data and West-African gridded rainfall data statistically confirm dissimilarities between the western and eastern Sahel. Western and eastern Sahel rainfall data are correlated with SSTs from different regions of the Atlantic Ocean, especially in the North and tropical South Atlantic. As determined by wavelet coherence and phase, in phase relationship with North Atlantic SSTs only occurs in wet periods and at the multi- and quasi-decadal scales. This teleconnection depends on the time-period and the time-scale, displaying a NW-SE pattern, which suggests non-uniform modulations of meridional displacements of the ITCZ. Tropical South Atlantic SST variability is often related to opposite patterns between the Gulf of Guinean Coast (in phase) and Sahel region (out of phase).
Journal of Geophysical Research: Atmospheres, 2013
1] Knowledge of the variability of climate in the past is essential for understanding current cli... more 1] Knowledge of the variability of climate in the past is essential for understanding current climatic changes. Therefore, we investigated two temperature indices and seven rainfall time series of northwestern Europe since the 17th century. Trends and multidecadal to interdecadal variability are similar in England and northern France for temperature, whereas a strong regional contrast is evident between the two regions for rainfall. Multidecadal and interdecadal variability displays several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, temperatures in both England and France display phase opposition with the Atlantic Multidecadal Oscillation (AMO) before 1800, while they are in-phase afterward, as determined by wavelet coherence. On the other hand, the relationships between temperature and the North Atlantic Oscillation (NAO) are weak across multidecadal and interdecadal scales for the whole period under study. For rainfall, coherence with the AMO is observed for scales at around 30-60 years, whereas coherence with the NAO is detected on 50-80 year scales and interdecadal 16-23 year scales. However, relationships between rainfall variability and North Atlantic climate indices are highly contrasted depending on the region considered. Finally, the results of a mixed spectral/empirical orthogonal function analysis of mean sea level pressure on these co-oscillation time scales highlight not only NAO regimes but also other patterns, explaining a nonnegligible amount of variance during certain time periods.
In the present context of global changes, considerable efforts have been deployed by the hydrolog... more In the present context of global changes, considerable efforts have been deployed by the hydrological scientific community to improve our understanding of the impacts of climate fluctuations on water resources. Both obser-vational and modeling studies have been extensively employed to characterize hydrological changes and trends, assess the impact of climate variability or provide future scenarios of water resources. In the aim of a better un-derstanding of hydrological changes, it is of crucial importance to determine how and to what extent trends and long-term oscillations detectable in hydrological variables are linked to global climate oscillations. In this work, we develop an approach associating large-scale/local-scale correlation, enmpirical statistical downscaling and wavelet multiresolution decomposition of monthly precipitation and streamflow over the Seine river watershed, and the North Atlantic sea level pressure (SLP) in order to gain additional insights on the atmosphe...
West-African Sahel rainfall shows a large decadal signal, transitioning from wet conditions in th... more West-African Sahel rainfall shows a large decadal signal, transitioning from wet conditions in the 1950s to dry conditions in the 1970s/80s. Only the quasidecadal time-scale is significantly detected in the Sahel rainfall index during the 1970s and 1980s. This time-scale of rainfall variability appears to be related to two periods of dry conditions and one of relatively wet conditions in the mid-70s. The dominant quasidecadal anomalies of West-African Monsoon have been reconstructed using Fourier transform for the 70s-80s, and subsequently assessed by the differences in wet and dry states. A strengthening of cross-equatorial Atlantic SST and pressure gradients can be related to an enhancement of monsoon flow and the northward shift of the ITCZ.
En Afrique de l'Ouest sahélienne la fin des années 60 est marquée par une anomalie aride générali... more En Afrique de l'Ouest sahélienne la fin des années 60 est marquée par une anomalie aride généralisée sans précèdent au XXe siècle. Pourtant, en fonction de la proximité de l'Atlantique, les modes de variabilité des précipitations sont différents. Trois secteurs sahéliens peuvent être établis : Sahel atlantique (Dakar), Sahel central (Nioro du Sahel et Mopti) et Sahel oriental (Niamey, Maradi, Maïné-Soroa). Les relations observées entre les précipitations et la variabilité des températures de surface de l'océan (SST) Atlantique Nord (AMO) et Tropical Sud (TSA) pourraient expliquer une part de cette hétérogénéité spatiale. De plus, la répartition fréquentielle et surtout temporelle des relations observées montre qu'une situation de dipôle atlantique n'a pu s'établir que très rarement et localement. Ceci, de nouveau, remet en question le rôle du mode Inter-Hémisphérique des SST dans la conduite de la variabilité décennale à multi-décennale des précipitations sahéliennes.
ABSTRACT Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal an... more ABSTRACT Maine-Soroa and N’Guigmi are located in SE Niger, in the Lake Chad basin. Some signal analysis methods used in hydrolgy were tested on the rainfalls, the wind speed and the horizontal visibility data. Using a local polynomial fitting reveals a long-term trend change of regim in the late 1960s. The Continuous Wavelets Analysis reveals some coherents frequency modes at seasonnal and quasidecadal scales, but the rainfall-visibility and the wind speed-visibiliy relationship are more complex. Hysteresis and cross-correlation function of mean daily regims were used for study the intra-seasonal and seasonal scales. We notice that, even if the rainfall increases the visibility, this one doesn’t modulate instantaneously the regional dust contributions. On the contrary, these contributions are linked to the wind speed.
Cette étude examine les téléconnexions entre les températures de surface de la mer (TSM) de l'océ... more Cette étude examine les téléconnexions entre les températures de surface de la mer (TSM) de l'océan Atlantique et la distribution zonale des précipitations au Sahel au cours du dernier siècle. Depuis la fin des années 60, les précipitations d'Afrique de l'Ouest présentent une tendance négative sur l'ensemble de la bande sahélienne. Pourtant, trois types de variabilité basse-fréquence des précipitations sahélienne sont observés : la côte atlantique (Dakar), le Sahel central (Nioro du Sahel et Mopti) et le Sahel oriental (Niamey, Maradi, Maïné-Soroa). Au cours du XX ème siècle, les précipitations sont rarement reliées simultanément avec les TSM de l'Atlantique Nord et Tropical Sud. Aux échelles multi-et quasi-décennale, dans les années 50 et depuis les années 90, un réchauffement des TSM de l
Journal of Geophysical Research: Atmospheres, 2013
ABSTRACT Sahel rainfall shows pronounced decadal variability and a negative trend between wet con... more ABSTRACT Sahel rainfall shows pronounced decadal variability and a negative trend between wet conditions in the 1950s–1960s and dry ones in the 1970s–1980s. Using continuous wavelet transform the quasi-decadal variability (QDV) of rainfall reveals zonal contrasts. The highest QDV is identified in the 1950s–1960s over western Sahel and in the 1970s–1980s over eastern Sahel. The quasi-decadal atmospheric anomalies has been reconstructed using Fourier transform for the 1950s–1960s and the 1970s–1980s, respectively, and assessed by the composite analysis of the QDV phases for the periods before and after 1968. Over western Sahel, the rainfall QDV in the 1950s–1960s is related to the north Atlantic sea-surface temperature (SST) variability, as highlighted by the wavelet coherence. A southward shift trend of the Inter-Tropical Convergence Zone (ITCZ) is identified through an enhancement of north-easterly fluxes and moisture convergence over the western part of West Africa. A decrease (increase) of southern (northern) subtropical sinking motions seems to be involved. In the 1970s–1980s, a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flow from lower troposphere up to the mid-troposphere and to the northward shift of the ITCZ, mainly over eastern Sahel. The Pacific SST influence is also identified, which involves changes in the global zonal circulation.
Comptes Rendus Geoscience, 2012
Indice de l'Oscillation Nord Atlantique Bassin versant de la Seine Fluctuations climatiques pluri... more Indice de l'Oscillation Nord Atlantique Bassin versant de la Seine Fluctuations climatiques pluriannuelles Variabilité des pré cipitations Analyse en ondelettes continue Segmentation