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Data collection and management constitutes the foundation for all analysis and modeling tasks required for achieving water, food, energy, and environmental security. UN-Water identifies data and information as one of the five accelerators where efforts and investments should be concentrated to achieve the Sustainable Development Goal 6 (SDG 6). Availability of open and free data resources can greatly contribute to enhanced implementation capacity in water research. In this context, it is crucial that data are freely accessible, openly available, and reliable. Open datasets are available online, are accessible in machine-readable formats (i.e. not pdfs or reports), and are obtainable by the public. Free refers to the availability of datasets with licenses that allow sharing at no cost to individuals, researchers, institutions, or projects. However, such datasets are dispersed across various websites and repositories. This hampers visibility and findability, which in turn results in limited use for generation of new knowledge or transferability of insights. This highlights a critical need for improving visibility of existing hydrological datasets, dedicated projects, and repositories. Crowdsourced scientific data collection efforts enable collaborative work and provide potential solutions to these issues. In this talk, we present a crowdsourced hydrology initiative aimed at establishing a global catalog of open and free datasets. This inventory will provide a database of static links, data DOIs, description of datasets along with key references and supporting information. We report on our progress and challenges in designing such an initiative with the hope that others will benefit., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The oceans have a profound influence on the weather and climate of South Africa. Not only, most rainfall comes from condensation of water vapor originating from the flux of moisture from ocean to atmosphere but the temperature of the remote and surrounding oceans have an impact of the interannual variability of rainfall. A good example of this remote effect is the impact of the ocean during El Nino when the temperature of the Pacific and Indian Ocean is higher than normal. This creates more rainfall above the higher sea surface temperature thus modifying the global Walker and Hadley circulation. Air usually rises in the equatorial regions, especially above the ocean creating rainfall which increases further the ascending motion of air. The air is eventually pushed poleward and then cools and sinks in the subtropical region where Southern Africa sits, creating high pressure and subsidence which is not favorable to rainfall. This is one of the teleconnection mechanisms linking remote oceanic region to South Africa. The aim of the project was to better understand the role of the ocean on weather, climate and rivers of Southern Africa. Little work has been done to connect streamflow to the El Nino Southern Oscillation previous to that project. We also looked at the tropical Atlantic Ocean, which is closer to us although but smaller where a similar phenomenon to El Nino occurs, the Benguela Nino. Closer to South Africa, the Agulhas Current was known to impact the atmosphere above it due to high turbulent flux of moisture from sea to atmosphere. In the past, due to low resolution reanalyzed climate data that did not integrate the Agulhas Current, it was not possible to study the impact of the Agulhas Current on local climate with reanalyzed climate dataset such as ERA or NCEP. This project made some important advance to that matter using new high resolution reanalyzed climate data and a model and offer some sounds evidence on the impact of the Agulhas Current on coastal rainfall. A list of publication originating directly from the project and a list of student and thesis supported by the project is found at the end of the executive summary.

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-1). 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.

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 have 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 Intertropical Convergence Zone (ITCZ) is identified through an enhancement of northeasterly 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 midtroposphere 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. Key Points Zonal contrasts are detected in Sahel rainfall at quasi-decadal scale Quasi-decadal signals of the WAM are identified in both wet and dry periods Modifications of SST forcing are observed between wet and dry periods ©2013. American Geophysical Union. All Rights Reserved.

Since the industrial revolution, an anthropogenic forcing occurs over all time-scales of natural variability. This change resulted in significant variations of climate on the regional scale. Therefore, based on spectral analysis, this dissertation revisits the question of the spatiotemporal non-stationnarity of teleconnections between the global climatic variability in the Atlantic and the regional climates of NW Europe and West-Africa. The study of long-term climatic records back to the pre-industrial period reveals similar trends in temperatures of England and Northern France, contrary to trends of rainfalls. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperatures is marked by phase changes over the 19th century. For rainfall, coherence with AMO is observed for scales at around 30-60-year, whereas coherence with the North Atlantic Oscillation (NAO) is detected on 50-80-year and interdecadal 16-23-year scales. However, NAO/rainfall teleconnections are highly unstable due to spatial fluctuations of NAO patterns. After the Little Ice Age, the NAO pattern seems shifted south-westwards in winter and spring. Moreover, the results of an analysis of North Atlantic sea-level pressure (SLPs) at these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance. Over West-Africa, zonal contrasts are identified through a study of time-evolutions of Sahel rainfall variability. These patterns are more pronounced at quasi-decadal scale. The teleconnections with Atlantic sea-surface temperatures (SSTs), which depend on time-scales, also show east-west contrasts. During the 20th century, a synchronous Sahel rainfall teleconnection with North Atlantic and Tropical South Atlantic SSTs is rarely observed. In-phase relationship with North Atlantic SSTs only occurs in wet periods (e.g. 1950s) and across multi- and quasi-decadal scales. This teleconnection display a NW-SE pattern suggesting non-uniform modulations of meridional displacements of the ZCIT. The Tropical South Atlantic SSTs variability is often related to a dipole-type configuration between the in-phase rainfall along Gulf of Guinea and the phase opposition in the Sahel. Nevertheless, this teleconnection depends on the time-period, the time-scale, and the mean state of Pacific SSTs. The involved quasi-decadal anomalies of such zonal contrasts vary over time. In the 1950/60s, the western flank of West-Africa displays fluctuations of Harmattan flux, rising motions over sahelian region and of the subtropical sinking motions. In the 1970/80s, variations of the cross-equatorial Atlantic SST and pressure gradient can be related to modulations of monsoon fluxes, and more generally northward-southward shift of the ZCIT., L'étude des impacts du changement climatique actuel est soumise à plusieurs contraintes. Premièrement, si avant la révolution industrielle la variabilité du climat est uniquement conduite sur une large gamme d'échelle par des forçages naturels, cela n'est pas le cas au XXe siècle. Deuxièmement, les effets de ce changement montrent des variations significatives à l'échelle régionale. Ainsi, sur la base d'analyses spectrales, ces travaux réexaminent l'instabilité spatio-temporelle des téléconnexions entre la variabilité climatique globale du secteur Atlantique et les climats régionaux du NW Europe et de l'Afrique de l'Ouest. L'examen des longues séries climatiques de l'Angleterre et du Nord de la France révèle des tendances similaires pour les températures, mais désappariée pour les précipitations. Différente phase de renforcement des échelles de variabilité multi- et inter-décennale ont été détectée, ce qui suggère des " fluctuations " du control climatique global. Sur ces échelles, la relation statistique entre les températures et l'Oscillation Multi-décennale Atlantique (AMO) est marquée par un changement de phase au XIXe siècle. Les précipitations semblent cohérentes avec l'indice AMO à l'échelle 30-60 ans et avec l'Oscillation Nord Atlantique (NAO) aux échelles 50-80 et 16-23 ans. Toutefois, la relation NAO/pluie présente une grande instabilité en raison des fluctuations spatiales des régimes NAO. Après le Petit Age Glaciaire, les deux pôles semblent glisser vers le SW en hiver et printemps. De plus, une fraction de la variance du champ Nord-Atlantique de pression au niveau de la mer (SLPs), capturée par l'indice NAO, pourrait être associée à d'autres régimes atmosphériques. En Afrique de l'Ouest, l'étude des fluctuations temporelles de la variabilité des précipitations sahéliennes révèle une organisation zonale, plus particulièrement exacerbée à l'échelle quasi-décennale. Les téléconnexions avec les températures de surface (SSTs) de l'Atlantique, qui s'organise différemment suivant l'échelle de variabilité, présentent également des modifications d'Est en Ouest. Au cours du XXe siècle, les précipitations au Sahel semblent rarement s'établir simultanément avec les SSTs Atlantique Nord et Tropical Sud. Une relation en phase avec les SSTs de l'Atlantique Nord s'effectue uniquement lors des périodes humides (e.g. années 1950) aux échelles multi- ou quasi-décennale. Ces téléconnexions sont orientées selon un plan NW-SE, ce qui suggère des modulations non-uniformes du balancement de la ZCIT. Les téléconnexions avec les SSTs de l'Atlantique Tropical Sud sont associées à une structure dipolaire opposant les régions guinéenne et sahélienne. Cela s'accorde donc avec des modulations de la position latitudinales de la ZCIT. Néanmoins, celle-ci dépendrait de l'échelle, de la période étudiée et de l'état des SSTs du Pacifique. Les anomalies quasi-décennales impliquées dans ces contrastes zonaux varient au cours du temps. Dans les années 1950/60, le flanc occidental de l'Afrique de l'Ouest montre des variations du flux d'Harmattan, des mouvements ascendant sur la région sahélienne et des mouvements subsidents subtropicaux. Dans les années 1970/80, des fluctuations du gradient interhémisphérique de pression et température pourrait conduire des variations du flux de mousson, et plus généralement des décalages nord/sud de la ZCIT.

Since the industrial revolution, an anthropogenic forcing occurs over all time-scales of natural variability. This change resulted in significant variations of climate on the regional scale. Therefore, based on spectral analysis, this dissertation revisits the question of the spatiotemporal non-stationnarity of teleconnections between the global climatic variability in the Atlantic and the regional climates of NW Europe and West-Africa. The study of long-term climatic records back to the pre-industrial period reveals similar trends in temperatures of England and Northern France, contrary to trends of rainfalls. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperatures is marked by phase changes over the 19th century. For rainfall, coherence with AMO is observed for scales at around 30-60-year, whereas coherence with the North Atlantic Oscillation (NAO) is detected on 50-80-year and interdecadal 16-23-year scales. However, NAO/rainfall teleconnections are highly unstable due to spatial fluctuations of NAO patterns. After the Little Ice Age, the NAO pattern seems shifted south-westwards in winter and spring. Moreover, the results of an analysis of North Atlantic sea-level pressure (SLPs) at these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance. Over West-Africa, zonal contrasts are identified through a study of time-evolutions of Sahel rainfall variability. These patterns are more pronounced at quasi-decadal scale. The teleconnections with Atlantic sea-surface temperatures (SSTs), which depend on time-scales, also show east-west contrasts. During the 20th century, a synchronous Sahel rainfall teleconnection with North Atlantic and Tropical South Atlantic SSTs is rarely observed. In-phase relationship with North Atlantic SSTs only occurs in wet periods (e.g. 1950s) and across multi- and quasi-decadal scales. This teleconnection display a NW-SE pattern suggesting non-uniform modulations of meridional displacements of the ZCIT. The Tropical South Atlantic SSTs variability is often related to a dipole-type configuration between the in-phase rainfall along Gulf of Guinea and the phase opposition in the Sahel. Nevertheless, this teleconnection depends on the time-period, the time-scale, and the mean state of Pacific SSTs. The involved quasi-decadal anomalies of such zonal contrasts vary over time. In the 1950/60s, the western flank of West-Africa displays fluctuations of Harmattan flux, rising motions over sahelian region and of the subtropical sinking motions. In the 1970/80s, variations of the cross-equatorial Atlantic SST and pressure gradient can be related to modulations of monsoon fluxes, and more generally northward-southward shift of the ZCIT.; L'étude des impacts du changement climatique actuel est soumise à plusieurs contraintes. Premièrement, si avant la révolution industrielle la variabilité du climat est uniquement conduite sur une large gamme d'échelle par des forçages naturels, cela n'est pas le cas au XXe siècle. Deuxièmement, les effets de ce changement montrent des variations significatives à l'échelle régionale. Ainsi, sur la base d'analyses spectrales, ces travaux réexaminent l'instabilité spatio-temporelle des téléconnexions entre la variabilité climatique globale du secteur Atlantique et les climats régionaux du NW Europe et de l'Afrique de l'Ouest. L'examen des longues séries climatiques de l'Angleterre et du Nord de la France révèle des tendances similaires pour les températures, mais désappariée pour les précipitations. Différente phase de renforcement des échelles de variabilité multi- et inter-décennale ont été détectée, ce qui suggère des " fluctuations " du control climatique global. Sur ces échelles, la relation statistique entre les températures et l'Oscillation Multi-décennale Atlantique (AMO) est marquée par un changement de phase au XIXe siècle. Les précipitations semblent cohérentes avec l'indice AMO à l'échelle 30-60 ans et avec l'Oscillation Nord Atlantique (NAO) aux échelles 50-80 et 16-23 ans. Toutefois, la relation NAO/pluie présente une grande instabilité en raison des fluctuations spatiales des régimes NAO. Après le Petit Age Glaciaire, les deux pôles semblent glisser vers le SW en hiver et printemps. De plus, une fraction de la variance du champ Nord-Atlantique de pression au niveau de la mer (SLPs), capturée par l'indice NAO, pourrait être associée à d'autres régimes atmosphériques. En Afrique de l'Ouest, l'étude des fluctuations temporelles de la variabilité des précipitations sahéliennes révèle une organisation zonale, plus particulièrement exacerbée à l'échelle quasi-décennale. Les téléconnexions avec les températures de surface (SSTs) de l'Atlantique, qui s'organise différemment suivant l'échelle de variabilité, présentent également des modifications d'Est en Ouest. Au cours du XXe siècle, les précipitations au Sahel semblent rarement s'établir simultanément avec les SSTs Atlantique Nord et Tropical Sud. Une relation en phase avec les SSTs de l'Atlantique Nord s'effectue uniquement lors des périodes humides (e.g. années 1950) aux échelles multi- ou quasi-décennale. Ces téléconnexions sont orientées selon un plan NW-SE, ce qui suggère des modulations non-uniformes du balancement de la ZCIT. Les téléconnexions avec les SSTs de l'Atlantique Tropical Sud sont associées à une structure dipolaire opposant les régions guinéenne et sahélienne. Cela s'accorde donc avec des modulations de la position latitudinales de la ZCIT. Néanmoins, celle-ci dépendrait de l'échelle, de la période étudiée et de l'état des SSTs du Pacifique. Les anomalies quasi-décennales impliquées dans ces contrastes zonaux varient au cours du temps. Dans les années 1950/60, le flanc occidental de l'Afrique de l'Ouest montre des variations du flux d'Harmattan, des mouvements ascendant sur la région sahélienne et des mouvements subsidents subtropicaux. Dans les années 1970/80, des fluctuations du gradient interhémisphérique de pression et température pourrait conduire des variations du flux de mousson, et plus généralement des décalages nord/sud de la ZCIT.

6 pages; International audience; 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 XXème 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'Atlantique Nord (AMO+ : Atlantic Multidecadal Oscillation) contribue à l'augmentation des précipitations sahéliennes de la côte Atlantique à Mopti ou Niamey (6°W/2°E). L'impact des TSM de l'Atlantique Tropical Sud dépend de la période d'observation et de l'échelle de variabilité.

International audience; Au Sahel, les études sur le changement climatique portent principalement sur les précipitations. Pourtant, ces précipitations sont peu représentatives des conditions locales permanentes car elles sont concentrées sur une partie de l'année (saison des pluies) et ne concernent que quelques dizaines de jours seulement. Au contraire les jours sans vent sont rares. Cette étude a donc pour objectif de savoir si le changement climatique peut être appréhendé à partir des paramètres du vent. Elle concerne la station de Maïné-Soroa (Niger oriental) entre 1950 et 1989. Elle porte sur les chroniques de 3 mesures journalières instantanées (à 6h, 12h et 18h) de la vitesse, de la direction du vent et de la visibilité horizontale liée aux poussières transportées par le vent. Les variations de vitesse et direction du vent permettent aisément de retrouver le changement majeur de la période 1967-1972 aussi bien que les fortes anomalies, tant aride (1984-1986) qu'humide (1951- 1954). On observe une diminution très importante de la visibilité horizontale après 1969. Studies on climate change in the Sahel mainly concerns precipitations. However, these precipitations are not very representative of the permanent local conditions because they are concentrated during a part of the year (rainy season) and only during a few tens of days. On the contrary, the days without wind are rare. The aim of this study is thus to know if the climate change can be appraised by an analysis of wind parameters. The study concerns the meteorological station of Maïné- Soroa (Eastern Niger) between 1950 and 1989. It relates to the high frequency chronicle (3 measurements each day at 6:00, 12:00 and 18:00) of the speed and direction of the wind and the horizontal visibility related to the dust transported by the wind. The variations of speed and direction show the well-known major change during the period 1967-1972 as well as the strong anomalies, arid (1984-1986) and wet (1951-1954). Since 1969, we also observe a very important reduction in the horizontal visibility.

International audience; Une régression polynomiale non-paramétrique (méthode LOESS) appliquée aux précipitations annuelles de Maïné-Soroa (SE Niger) montre trois périodes : humide (1950-1967), aride (1968-1993), semi-aride (1994-2005). L'utilisation de la transformée en ondelettes continues permet de séparer la variabilité interne (haute fréquence) et la variabilité forcée (basse fréquence). Nous avons pu mettre en évidence sept modes de fréquence localisés dans le temps : deux modes intra-saisonniers (4-9jrs, 16-20jrs), deux modes saisonniers (6 mois, 1an), deux modes pluriannuels (2-4ans et 5-8ans) et un mode quasi-décennal (12-18ans). Lors du régime humide, on retrouve une forte variance des modes saisonniers liés à la ZCIT et des modes pluriannuels se succédant dans le temps (5-8ans puis 2-4ans) en liaison avec les températures de l'Atlantique Tropical Nord (TNA) puis avec l'Anticyclone des Açores. Ces modes basses fréquences sont synchrones avec une activité convective mature (mode 4-9jrs) et l'apparition d'une perturbation (mode 16-20jrs) lors de la transition entre la phase océanique et la phase continentale de la mousson. Au contraire, lors des années arides on retrouve le mode quasi-décennal, en lien avec les températures de l'Atlantique Tropical Sud (TSA), forçant une activité convective peu développée et perturbée au coeur de la phase continentale. A non-parametric polynomial regression applied to the Maine-Soroa annual rainfall timeseries reveals three periods: wet (1950-1967), arid (1968-1993) and semi-arid (1994-2005). Continuous Wavelet Transform allows crossing internal variability (high frequency) and forcing variability (low frequency). We could reveal seven frequency modes highly localized in time: two intraseasonal modes (4-9 days, 16-20 days), two seasonal modes (6mo, 1yr), two interannuals modes (2- 4yrs, 5-8yrs) and one quasidecadal mode (12-18yrs). During wet period, we notice a strong variance across seasonal modes in relation to ITCZ and the two interannual modes succeeding in time (5-8yrs then 2-4yrs) in connection with the Tropical North Atlantic temperatures (TNA) then Azores high. At the same time, we notice great convective activity (4-9days) and the oceanic/continental transition phase disturbation (16-20days). During the arid period, we notice the quasidecadal mode in relation with the Tropical South Atlantic temperatures (TSA), forcing a weak convective activity and disturbed in the medium of the continental phase.

25 pages; International audience; This study examines for the first time the changing characteristics of summer and winter southern African rainfall and their teleconnections with large-scale climate through the dominant time scales of variability. As determined by wavelet analysis, the austral summer and winter rainfall indices exhibit three significant time scales of variability over the twentieth century: interdecadal (15–28 years), quasi-decadal (8–13 years), and interannual (2–8 years). Teleconnections with global sea surface temperature and atmospheric circulation anomalies are established here but are different for each time scale. Tropical/subtropical teleconnections emerge as the main driver of austral summer rainfall variability. Thus, shifts in the Walker circulation are linked to the El Niño–Southern Oscillation (ENSO) and, at decadal time scales, to decadal ENSO-like patterns related to the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation. These global changes in the upper zonal circulation interact with asymmetric ocean-atmospheric conditions between the South Atlantic and South Indian Oceans; together, these lead to a shift in the South Indian Convergence Zone and a modulation of the development of convective rain-bearing systems over southern Africa in summer. Such regional changes, embedded in quasi-annular geopotential patterns, consist of easterly moisture fluxes from the South Indian High, which dominate southerly moisture fluxes from the South Atlantic High. Austral winter rainfall variability is more influenced by midlatitude atmospheric variability, in particular the Southern Annular Mode. The rainfall changes in the southwestern regions of southern Africa are determined by asymmetrical changes in the midlatitude westerlies between the Atlantic and Indian Oceans.