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The Republic of Djibouti is a small country (23 200 km²) in East Africa, characterised by an arid context coupled with a high variability of rainfall that generates flash floods causing severe damage to the population and infrastructure. The mechanisms controlling climate dynamics in Djibouti and the Eastern Africa region remain poorly understood. In this study, we document the atmospheric mechanisms associated with extreme rainfall events in the Republic of Djibouti. To that end, we use at the daily timescale rain-gauge data (a network of 36 stations on the period 2013-2020), satellite-based rainfall estimates (CHIRPS, IMERG, MSWEP and RFE) and atmospheric reanalyses (ERA5), selected over their common period 2001-2020.A multivariate Hierarchical Ascendant Classification of rainy days in Djibouti (≥ 10% of grid-points exceeding 1 mm.day-1, according to all four satellite products) reveal 4 clusters (intense rainfall, moderate rainfall, rainy in the southwest, rainy in the east) which differentiate from each other by the intensity and spatial extent of rainfall. These clusters show a non-homogeneous seasonal distribution, occurring mainly in the March-April-May (MAM) and July-August-September (JAS) seasons, and more rarely in October-November-December (OND). The atmospheric circulation anomaly patterns associated with the clusters are quite similar and highly season-dependent. In MAM most clusters display an anomalous trough over the Red Sea from 700 hPa to 200 hPa. In JAS, an anomalous low over the southern Red Sea drives a thicker than normal monsoon flow at 700 hPa (especially for the southwest cluster), while upper northerlies prevail at 200 hPa. In OND, most rainy events result from moisture advection from the Western Indian Ocean favoured by positive phases of the Indian Ocean Dipole. Some highly unusual atmospheric circulation patterns, which are not depicted by the above classification (e.g., associated with tropical cyclones), also result in intense rainfall events in the Republic of Djibouti.

Sub-Saharan Africa is affected by a high level of temporal and spatial climate variability, with large impacts on water resources, human lives, and economies, notably through hydrological extremes such as floods. Nevertheless, the key climatic factors driving interannual variability in flood frequency remain poorly documented and understood. To address this research gap, we first compile information on large-scale climate drivers that may potential affect sub-Saharan African hydroclimate (e.g., El Niño–Southern Oscillation, Atlantic Multidecadal Variability). Then, using a new 65-year long daily streamflow dataset of over 600 stations in sub-Saharan Africa, a bootstrapped stepwise regression and relative importance analysis is applied to quantify the relative contribution of different ocean basins to interannual variability in flood frequency between 1950 and 2014. Results show that interannual variations in the frequency of flood events are significantly linked to different modes of climate variability in the Pacific, Indian, and Atlantic Oceans. These modes of climate variability together explain around 60% of observed interannual variation in seasonal flood frequency. The relative influence of each ocean basin, however, differs from one region to another. The Indian and Pacific Oceans, for instance, have significant influences on interannual variations in the frequency of floods between December and May across much of southern and eastern Africa. In western Africa, the Mediterranean and Atlantic Oceans appear to have a dominant influence between September and November. In central Africa, the relative influence of different oceans basins is seasonally variable. Using the best combination of Sea-Surface Temperature predictors, we then examine projected future trends using a large ensemble of climate models from the CMIP6 experiments.

Western Equatorial Africa (WEA) has a record low sunshine duration during the June–September dry season due to the persistence of low clouds. This study examines the ability of two reanalysis products (ERA5 and MERRA-2) and 8 CMIP6 models (both coupled and atmosphere-only historical simulations) to reproduce the climatology of these low clouds, by comparing it with ground observations and a satellite product. All datasets show a reasonable representation of the regional distribution of low clouds over the Tropical Atlantic and the neighbouring African continent. However, CMIP6 models tend to underestimate the low cloud fraction, especially over WEA in the coupled simulations. This underestimation is partly due to an insufficient seasonal sea-surface temperature (SST) cooling over the Eastern Equatorial Atlantic from April to July in most models, which contributes to reducing the lower-tropospheric stability (LTS). ERA5 data, whose dry season WEA low-cloud fraction is close to observations, indicate that LTS strongly controls the interannual variability of WEA low clouds. Interestingly, the ability to reproduce the mean low cloud fraction does not necessarily scale with the SST biases of the CMIP6 models. The strong dependence of low clouds on interannual SST variations in ERA5 is captured by less than half of the CMIP6 models. Additional key drivers of interannual variations identified in this study, such as Bight of Bonny surface winds and mid-tropospheric temperatures, actually show up inconsistently in CMIP6. Further analyses are needed to disentangle the roles played by SST and independent atmospheric forcings on WEA low cloud formation.

Ce travail présente la variabilité intra-saisonnière de la pluviométrie dans le Sud-ouest centrafricain. Les données pluviométriques journalières utilisées couvrent la période 1981–2017. Elles ont été extraites aux points de grilles les plus proches des quatre stations représentatives de la zone d'étude (Bangui, Berberati, Boukoko et Nola) sur la base de données CHIRPS (Climate Hazards Group InfraRed Precipitation with Station data). Un contrôle de qualité de ces données a été effectué à partir d'une inter-comparaison entre les produits d'estimations et les données in-situ sur différentes sous-périodes : 1998–2011 à Nola, 1998–2012 à Berberati, 1998–2014 à Bangui et 2002–2017 à Boukoko. Le coefficient de corrélation entre les données CHIRPS et les observations au pas annuel est faible à Bangui (r=0,46), moyen à Nola (r=0,57) et Berberati (r=0,60), et bon à Boukoko (r=0,73). Les dates de début de la saison de pluies varient entre le 13 février et le 2 avril et celles de fin entre le 31 octobre et le 4 décembre. Des épisodes secs et humides sont mis en évidence dans le Sud-ouest centrafricain. A Bangui et Berberati, la période du 15 au 22 mai 1999 a été sèche soit 23 jours sans pluie. A Boukoko et Nola, la période du 26 juin au 24 juillet de la même année était caractérisée par des épisodes secs. Ces résultats établissent que la répartition intra-saisonnière des pluies n'est pas uniforme dans le Sud-ouest centrafricain.

Drought is a meteorological and hydrological phenomenon affecting the environment, agriculture, and socioeconomic conditions, especially in arid and semi-arid regions. A better understanding of drought characteristics over short and long timescales is therefore crucial for drought mitigation and long-term strategies. For the first time, this study evaluates the occurrence, duration, and intensity of drought over the Republic of Djibouti by using a long-term (1961–2021) rainfall time series at Djibouti Airport, completed by the CHIRPS precipitation product and local records from 35 weather stations. The drought is examined based on the Standardized Precipitation–Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at 3-, 6-, 9-, 12-, and 24-month timescales, so as to document short-, medium-, and long-duration events. The SPEI and SPI showed a significant drying tendency for the indices computed over 12 and 24 months at Djibouti Airport. The eastern coastal region of the Republic of Djibouti was the most affected by the increased drought incidence in recent decades, with more than 80% of the extremely and severely dry events occurring within the period 2007–2017. In contrast, the western regions recorded a positive trend in their SPIs during the period 1981–2021, due to the dominance of the June–September (JJAS) rains, which tend to increase. However, in the last few decades, the whole country experienced the droughts of 2006/2007 and 2010/2011, which were the longest and most intense on record. Large-scale climate variability in the Indo-Pacific region partially affects drought in Djibouti. The SPI and SPEI are significantly positively correlated with the Indian Ocean Dipole during October–December (OND), while for JJAS the SPI and SPEI are negatively correlated with Nino3.4. The wet event in 2019 (OND) causing devastating floods in Djibouti city was linked with a positive IOD anomaly. This study provides essential information on the characteristics of drought in the Republic of Djibouti for decision-makers to better plan appropriate strategies for early warning systems to adapt and mitigate recurrent droughts that put the country’s agro-pastoral populations in a precarious situation.

During the long dry season (June-September) Western Central Africa (WCA) is a region with an extensive and persistent stratocumulus or stratus deck. Previous studies have shown that this extensive cloud cover is crucial for the existence of a biodiverse, light-deficient tropical rainforest ecosystem in Gabon. Yet, its climatological behaviour, the cloud genesis and lysis mechanisms, and thus its persistence under the ongoing climate change has not been intensively studied yet.In the present study, we created various climatologies of Low Cloud Cover (LCC) in the region based on (a) 3-hourly data from in-situ eye observations by synoptic stations, (b) on 15-minute data from the SEVIRI instrument aboard the METEOSAT Second Generation geostationary satellite, and (c) twice daily data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat’s Cloud Profiling Radar (CPR). To validate the satellite products against eye observations of cloudiness at synoptic stations, the latter are spatially weighted in the observer’s field of view and compared against co-located pixel averages of the satellites. To obtain an as accurate depiction of the diurnal cycle of LCC as possible, we combined the SAFNWC cloud type product during the day with the Night Microphysical Scheme during the night. Both product use various SEVIRI spectral channels. Vertical profiles of cloudiness at 250 m resolution are derived from the Calipso-CloudSat 2B-GEOPROF-LIDAR product.The mean climatology shows more clouds at the coast and the coastal plains, decreasing cloud occurrence frequency landward. The closer to the Congo basin, the less persistence is the LCC deck. The diurnal cycle of the LCC has a smaller amplitude on the coastal plains of Gabon, while at the leeward site of the Chaillu Mountains, an up to 1000 m high low mountain range in southern Gabon, a higher amplitude with substantial clearing in the afternoon and cloud formation in the night prevails. The persistence of the LCC on the windward side of the Chaillu Mountains might be related to upslope winds. Further, a change in the cloud genus from stratocumulus to cumulus is observed on the plateau during the afternoon in association with the clearing. It is speculated that this is associated with an increased boundary layer height. Another cause might be a Foehn-effect, dissipating the Low Cloud Cover behind the Chaillu Mountains and being responsible for the higher amplitude in the diurnal cycle above the plateau. On the windward side of the Chaillu Mountains, there is no transition observed from stratocumulus to cumulus clouds, presented with its persistence cloud deck.In summary, the present study constitutes the hitherto most comprehensive sub-daily station- and satellite-based, dry-season climatology of LCC over western equatorial Africa.

Malgré son importance bioclimatique, peu d'études se sont intéressées à la couverture nuageuse basse stratiforme de saison sèche (juin-septembre) de la façade Atlantique de l'Afrique Centrale. Grâce à des données in-situ documentant la période 1971-2019, nous montrons que cette couverture nuageuse est sous-estimée par les observations satellites SAFNWC et les modèles CMIP6. Nous identifions trois types de journées : claires, nuageuses la nuit et totalement nuageuses, ces dernières étant plus fréquentes sur la côte et l'intérieur, au vent des Monts de Cristal et de Chaillu. A l'échelle interannuelle, les anomalies de température dans l'Atlantique tropical Sud, le Pacifique équatorial et le bassin du Congo, en modulant les cellules méridienne et zonale de circulation jouent sur la stabilité des basses couches, le cisaillement vertical et in-fine la fréquence des journées les plus nuageuses.

20 pages; International audience; For the first time, the spatial and temporal variability of rainfall in the Republic of Djibouti is investigated using data from 14 weather stations over the period 1946–2017. Due to limited data availability, high-resolution long-term satellite rainfall products (CHIRPS, PERSIANN-CDR, TAMSATv3, ARC2) and ERA5 reanalysis also contribute to document time–space rainfall variability at monthly, seasonal and annual scales. Principal component analysis identifies two spatially coherent regions of rainfall variability in the east (coastal zone) and the west (inland zone) of the country. Annual rainfall amounts are everywhere very low (60–300 mm), but with contrasted regimes. At seasonal scale, the highest rainfall amounts in the eastern part of the country are found between October–December (OND) and March–May (MAM), while July–September is the wettest season in the western part. The monthly rainfall regimes are relatively well reproduced by most products. ERA5 displays the highest monthly correlations with observations, followed by PERSIANN-CDR and CHIRPS. Trend analysis since 1983 shows a significant decrease of rainfall during MAM which is in agreement with other parts of East Africa. On the other hand, nonsignificant decreasing trends are observed in January–February (JF) and OND. Only June–September (JJAS) revealed a nonsignificant increasing trend, but it follows a prior drying trend since the 1950s. The impact of large-scale background climate on rainfall variability is assessed with focus given on El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Rainfall variability during OND shows a significant correlation with IOD, while in JJAS it is significantly negatively correlated with ENSO. In general, ERA5, CHIRPS and PERSIANN datasets are best able to reproduce rainfall patterns in Djibouti and suitable for further analysis. The fact that the interannual and decadal-scale rainfall variations in Djibouti show large-scale teleconnections with global sea-surface temperature fields, as demonstrated in this study, provides good prospects for the prediction of rainfall variations at a range of different temporal scales.

International audience; The southern coastal belt of West Africa (SCWA) with its high population density and many major cities, combined to the low elevation and poor urban planning, is very vulnerable to floods resulting from extreme rainfall events. The aim of this paper is to analyze the characteristics of extreme rainfall in the SCWA during the 1981–2015 period, in terms of frequency, intensity, seasonality, and trends. Therefore, daily rainfall of 31 stations located in the southern part of Côte d’Ivoire, Ghana, Togo, and Benin and rainfall estimation products combining in situ observations and satellites rainfall estimation data have been used. For each station and pixel, the local 95th percentile (P95) computed on all rain days of at least 1 mm was used to define extreme rainfall events. Rainfall on the coastal belt is heavier than further inland, with P95 values reaching 82 and 52 mm/day for coastal and continental stations, respectively. Extreme rainfall along the coast occurs predominantly between May and July. Interannual variations of different indicators of extreme rainfall show a broad agreement between rain gauge data and rainfall estimates from CHIRPS (Climate Hazards Group InfraRed Precipitation with Station) data. In the southern part of Côte d’Ivoire and Togo/Benin, increase of number of extreme rainfall event (NP95) and stability number of days with rainfall less than P95 (NL95) are recorded, which induces an increase of total rainfall. But, in the southern part of Ghana, there is a stable total rainfall due to an increase in NP95 compensated by a decrease in NL95.

12 pages; International audience; This study interprets 11 years (2006 to 2016) and 6 months (March to August in 2017) of respectively monthly and daily isotopic (δD and δ18O) monitoring of rain at Douala (Cameroon), a humid tropical station in Western Africa. The main scope is to analyze the climate controls on precipitation isotopes at different timescales. Firstly, we examine the annual cycles of δ18O. Over the 11 years of survey, the annual cycle exhibits a W shape that is quite reproducible from year to year, with two minima in spring and autumn periods. Based on back trajectory calculations and remote sensing observations of water vapor isotopic composition, we show that the observed depletion in spring and autumn is due to strong convective activity along air mass trajectories. The same effect of convective activity can be observed at the daily timescale. At seasonal and daily time scales, the isotopic composition is also strongly tied to the convective organization and cloud types. More depleted precipitation is associated with larger areas of high clouds. Very low to low clouds are observed in July-August, mid-level to high clouds are dominant in June and high to very high clouds characterize March-April-May, thus explaining the enriched (depleted) values in summer (spring). Finally, this paper highlights the importance of large scale meteorological conditions controls on precipitation stable isotope composition in the Gulf of Guinea.

Apple tree bloom onset in Germany has advanced by 2 days/decade in 1951-2014 and by 3 days/decade in 1988-2014, behaving similarly in respect to its evolution since 1951 and its sensitivity to temperature to other species’ phenological spring phases. The evolution however was not linear; by conducting a split moving-window dissimilarity analysis (SMWDA) we were able to detect the “break-period” 1987-1989 which coincides with a breakpoint that has been identified in the phases of the North Atlantic Oscillation (NAO). We observed distinct spatial patterns with apple bloom advancing from southwest to northeast and, most interestingly, a longitudinal gradient in the trend of apple bloom onset revealed by a probabilistic principal components analysis (PPCA). In the period of 1951-2014, plants located in the east displayed a much stronger trend (‑16.53 days on average) than those in the western part of the country (‑6.74 days on average). This pattern seems to be linked to patterns in temperature which is highly correlated to apple bloom onset (best one predictor model: mean temperature March to May, R² = 0.82, ‑6 days/°C): the coldest regions exhibit the strongest warming trends and the greatest advances in apple bloom onset., Les dates de floraison des pommiers en Allemagne ont en moyenne avancé de 2 jours/décade de 1951 à 2014 et de 3 jours/décade de 1988 à 2014, en accord avec l’évolution des phases phénologiques d’autres espèces. Cependant, cette évolution n’a pas été linéaire : en réalisant une analyse de dissimilarité d’une fenêtre glissante fractionnée (SMWDA), nous avons détecté une période de rupture entre 1987 et 1989 qui coïncide avec un point de rupture qui a été identifié dans les phases de l’oscillation nord-atlantique (ONA). Nous avons également pu constater une structuration spatiale : la floraison des pommiers progresse du sud-ouest du pays au nord-est ; en termes de variabilité interannuelle des dates de floraison, il existe, superposé à un mode principal commun à l’ensemble du pays, un gradient longitudinal qui a été révélé par une analyse probabiliste en composantes principales (PPCA). Entre 1951 et 2014, la tendance à la précocité est plus marquée à l’est du pays (en moyenne ‑16,53 jours) qu’à l’ouest (en moyenne ‑6,74 jours) et la floraison des pommiers est très étroitement liée à la variabilité des températures moyennes de mars à mai (R² = 0,82 ; ‑6 jours/°C). Les régions les plus froides montrent les tendances les plus importantes au réchauffement et à la précocité de la floraison.

Many African countries whose economies are largely based on weather/climate sensitive sectors are vulnerable to long-term changes in weather and climate. This study is aimed at assessing whether the recent decades have observed any significant trend in the intraseasonal descriptors (ISDs) of wet and dry spells at local and sub-regional levels at seasonal and monthly timescales over equatorial eastern Africa (EEA). Daily rainfall observations over 36 stations and spanning a period of 51 years (1962–2012) were used. The study has expanded on previous results that showed contrasting trends on seasonal totals between the two rainfall seasons by demonstrating that this also affects the ISDs. At the local level, it was observed that during the long rainfall season, a given ISD would have a significant trend over several neighbouring locations, which was not the case during the short rainfall season. Secondly, for the short rainfall season, a given location would have significant trend in several ISDs. Finally, when a given ISD had a significant trend at seasonal timescale during the long rainfall season, the same ISD would have significant trends in the second and third months of the season and rarely in the first month. Such a feature was not observed for the short rainfall season. Binomial probability distribution assessment confirmed that the significant trends in the various ISDs during the long rainfall season did not occur by mere chance.

Mean diurnal rainfall regimes over Eastern Africa (also referred to as the Greater Horn of Africa) are studied based on 3-hourly data from the TRMM 3B42 data set, averaged over a 17-year period (1998–2014). The consistency with long-term mean raingauge data, available for partly independent periods, varies from good (Sudan, Ethiopia, Eritrea and Somalia) to very good (Kenya, Tanzania and Uganda). Over sea (Indian Ocean and Red Sea), the diurnal rainfall distribution is quite uniform; however, a morning peak dominates and there is evidence of offshore phase propagation south of the equator. Over land, both rainfall frequency and rainfall amounts show a dominant afternoon maximum (1500–1800 East African Time, i.e. GMT + 3). However, many inland regions show a delayed rainfall maximum (evening, night-time or morning). The evening to night-time maximum found over some land areas is associated with a phase propagation from areas showing an afternoon peak. This occurs west of high ground areas (Sudan and parts of the Great Lakes region) and in belts parallel to the seashores (Eritrea, northeastern Ethiopia, Somalia and eastern Kenya). The latter provide indirect evidence that sea breeze effects can be detected at unexpectedly great distances from the coast (up to 300–400 km) in parts of Eastern Africa. A remarkable ring of early afternoon (1500) maxima is found around most lakes, although some east–west asymmetries occur. Over the lakes, a morning or late night maximum is mostly found. It is generally inversely related to the distance to the shorelines for the larger lakes, but over the mid-size lakes it is replaced by or competes with a late afternoon to evening maximum.

Daily rainfall in southern West Africa (4&ndash, 8&deg, N, 7&deg, W&ndash, 3&deg, E) is analyzed with the aim of documenting the intense rainfall events which occur in coastal Ivory Coast, Ghana, Togo, and Benin. The daily 99th percentile (P99) shows that the coastline experiences higher intensity rainfall than inland areas. Using Tropical Rainfall Measuring Mission (TRMM) rainfall data for 1998&ndash, 2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8&deg, latitude-longitude and five days centered on the event. A total 39,680 events (62 at each location) are classified into three major types, mainly found over the oceanic regions south of 5&deg, N, the Bight of Benin, and the inland regions respectively. These types display quite distinct rainfall patterns, propagation features, and seasonal occurrence. Three inland subtypes are also defined. The atmospheric circulation anomalies associated with each type are examined from ERA-interim reanalysis data. Intense rainfall events over the continent are mainly a result of westward propagating disturbances. Over the Gulf of Guinea, many intense events occur as a combination of atmospheric disturbances propagating westward (mid-tropospheric easterly waves or cyclonic vortices) and eastward (lower tropospheric zonal wind and moisture anomalies hypothesized to reflect Kelvin waves). Along the coast, there is a mixture of different types of rainfall events, often associated with interacting eastward- and westward-moving disturbances, which complicates the monitoring of heavy precipitation.

This study characterizes the future changes in extreme rainfall and air temperature in the Mono river basin where the main economic activity is weather dependent and local populations are highly vulnerable to natural hazards, including flood inundations. Daily precipitation and temperature from observational datasets and Regional Climate Models (RCMs) output from REMO, RegCM, HadRM3, and RCA were used to analyze climatic variations in space and time, and fit a GEV model to investigate the extreme rainfalls and their return periods. The results indicate that the realism of the simulated climate in this domain is mainly controlled by the choice of the RCMs. These RCMs projected a 1 to 1.5 &deg, C temperature increase by 2050 while the projected trends for cumulated precipitation are null or very moderate and diverge among models. Contrasting results were obtained for the intense rainfall events, with RegCM and HadRM3 pointing to a significant increase in the intensity of extreme rainfall events. The GEV model is well suited for the prediction of heavy rainfall events although there are uncertainties beyond the 90th percentile. The annual maxima of daily precipitation will also increase by 2050 and could be of benefit to the ecosystem services and socioeconomic activities in the Mono river basin but could also be a threat.

13 pages; International audience; Dry spells are natural hazards which have strong impacts on the environment and on many socioeconomic sectors. Their evolution in the Mediterranean Basin is a major issue because it is one of the most extreme hot spots of climate change world-wide. Very long dry spells are defined as climatic events characterized by their location, spatial extent, duration, and recurrence. This study investigates the evolution of these events during the wet season and for regional future climate projections of the Mediterranean and European Coordinated Downscaling Experiment initiatives (called Med-CORDEX and EURO-CORDEX). All simulations were performed with different regional climate models and various climate change scenarios. Main results are (i) an increase in the number of very long dry spells (between + 3 and + 31 events) and (ii) an increase of their mean duration and spatial extent. This increase in the severity of very long dry spells may have strong socioeconomic impacts in particular for the most vulnerable areas.

24 pages; International audience; The modulation of the rainfall diurnal cycle at intraseasonal and interannual time scales is examined using gridded rainfall data over Eastern Africa and the nearby Indian Ocean for the period 1998–2014. Our focus is on the October‐December season which is strongly impacted by both the Madden‐Julian Oscillation (MJO) and the Indian Ocean Dipole Mode (IODM). The effect of the MJO, which is not synchronized across the whole region, is mainly through changes in the amplitude of the diurnal cycle, especially an enhancement of the late afternoon peak in active phases of the MJO. The rainfall phase only shows minor variations between the active and quiescent phases of the MJO, except over the drier regions and the Indian Ocean where the overall diurnal rainfall signal is more uneven. The effect of the positive phase of the IODM (warm western Indian Ocean) on Eastern Africa rainfall is found at any time of the day, but the enhancement is more prominent during the wettest part of the day (generally the afternoon). Substantial changes in the diurnal phase of the rains are found in some land areas away from the coast or from mountain ranges, where the dominant afternoon peak during the negative IODM phase is replaced by a nighttime maximum during the positive phase. Cross‐sections and a cluster analysis of diurnal rainfall patterns over Kenya and southern Somalia suggest that this feature may be associated with the development of longer‐lived rainfall systems propagating inland from the coast during the IODM positive phase.

EnglishAntoine d'Abbadie, eminent scientific personality of the nineteenth century, known for his work in astronomy, geodesy, linguistics and for his exploration of Ethiopia, also took a strong interest in meteorology, which made him contribute to the founding of the French Meteorological Society in 1852. His stay in Ethiopia, between 1838 and 1848, allowed him to collect a wide range of meteorological observations, particularly on the dynamics and climatology of rainfall and thunderstorms. This article traces the path of the scientist and presents his observations by putting them in the double perspective of past and present knowledge francaisAntoine d'Abbadie, personnalite scientifique eminente du XIXe siecle, connu pour ses travaux en astronomie, geodesie, linguistique et pour son exploration de l'Ethiopie, a egalement porte un fort interet a la meteorologie, contribuant a la fondation de la Societe meteorologique de France en 1852. Son sejour ethiopien, entre 1838 et 1848, lui a permis de collecter un large eventail d'observations meteorologiques, notamment sur la dynamique et la climatologie des pluies et des orages. Cet article retrace le parcours du scientifique et presente ses observations en les mettant dans la double perspective des connaissances de l'epoque et actuelles.

24 pages; International audience; An intercomparison of seven gridded rainfall products incorporating satellite data (ARC, CHIRPS, CMORPH, PERSIANN, TAPEER, TARCAT, TMPA) is carried out over Central Africa, by evaluating them against three observed datasets: (a) the WaTFor database, consisting of 293 (monthly records) and 154 (daily records) rain‐gauge stations collected from global datasets, national meteorological services and monitoring projects, (b) the WorldClim v2 gridded database, and (c) a set of stations expanded from the FAOCLIM network, these two latter sets describing climate normals. All products fairly well reproduce the mean rainfall regimes and the spatial patterns of mean annual rainfall, although with some discrepancies in the east–west gradient. A systematic positive bias is found in the CMORPH product. Despite its lower spatial resolution, TAPEER shows reasonable skills. When considering daily rainfall amounts, TMPA shows best skills, followed by CMORPH, but over the central part of the Democratic Republic of the Congo, TARCAT is amongst the best products. Skills ranking is however different at the interannual time‐scale, with CHIRPS and TMPA performing best, though PERSIANN has comparable skills when only fully independent stations are used as reference. A preliminary study of Southern Hemisphere dry season variability, from the example of Kinshasa, shows that it is a difficult variable to capture with satellite‐based rainfall products. Users should still be careful when using any product in the most data‐sparse regions, especially for trend assessment.

The E-OBS precipitation gridded data set v.10.0 is used to detect very long dry spell (VLDS) events over the entire Mediterranean Basin for the 1957–2013 period, during the wet season (September to April). The main objective is to characterize these events as climatic objects, in terms of location, spatial extent, duration, and temporal variability. In this study, 76 VLDS events were detected in the Mediterranean Basin and grouped into four spatial patterns: scattered localized (with 25 events), northeast Mediterranean (11 events), West Mediterranean (15 events), and southeast Mediterranean (25 events). Each pattern shows seasonality in events. Most of the scattered localized, northeast, and southeast events begin in September, at the beginning of the wet season. In contrast, most of the West Mediterranean events begin in the middle of the wet season. Over time, there is a slight but not significant increase in the cumulative number of days per season affected by VLDS events.

The sunlightconditions prevailing in Central Africa, and their potential impact on forests traits and functioning, have neverbeen fully explored. Using satellite estimates documenting mostly the period 2005-2013, and accounting forscale interactions between the diurnal and annual cycles, we demonstrate that the seasonality of sunlight levelsstrongly vary across Central Africa forests. Gabon stands out with a main dry season strongly light-deficient,cool and moist. An original map of terra firme forest types developed by CIRAD shows that most of theevergreen forests of Central Africa develop in Gabon. We postulate that despite a mean annual precipitationamount below 2000 mm/yr, the Gabonese climate can harbor most of the evergreen forests of Central Africabecause of a heavy cloudiness which reduces the water demand and decreases the direct to diffuse light ratio.These findings and the methodology developed pave the way for further analyses of the past and future changesin the light-deficient climates of Western Central Africa and the vulnerability of the evergreen forests to thesechanges., Les conditions d’éclairement prévalant en Afrique Centrale et leur impact potentiel sur les traits et lefonctionnement des forêts ont été peu étudiés. S’appuyant sur des estimations satellitaires horaires derayonnement solaire incident, et tenant compte des interactions d’échelles entre cycle diurne et cycle saisonnier,nous montrons que le Gabon se démarque des autres parties d’Afrique Centrale par une saison sèche principaletrès déficitaire en lumière. En parallèle, une carte des forêts développée par le CIRAD montre que la plupart desforêts sempervirentes d’Afrique Centrale se localisent au Gabon. Nous postulons que le climat du Gabon peutabriter ces forêts en raison de la forte nébulosité qui s’y développe, réduisant la demande en eau mais aussi leratio rayonnement direct / rayonnement diffus, favorable à la photosynthèse. Ces résultats et la méthodologiedéveloppée ouvrent la voie à de futures analyses sur les changements passés et futurs des climats sombres de lafaçade Atlantique de l’Afrique Centrale, et la vulnérabilité des forêts sempervirentes à ces changements.

16 pages; International audience; Exceptional dry spells, in this study referred to as very long dry spells (VLDS), are natural hazards to which the Mediterranean region is extremely vulnerable, with socio‐economic and environmental impacts. In this study, they are characterized in terms of location, spatial extent, duration, temporal variability and associated atmospheric circulations. The main objective is to assess the performance of five HyMeX/Med‐CORDEX regional climate simulations to detect and reproduce VLDS in comparison with the E‐OBS observed daily gridded data. Models accurately reproduce the occurrence of precipitation around the Mediterranean Basin, and therefore the occurrence of VLDS, with at least 51% of the E‐OBS VLDS reproduced by the regional simulations. They also accurately simulate the spatio‐temporal characteristics. A second objective is to identify the synoptic atmospheric patterns associated with VLDS. A clustering analysis shows that all models accurately reproduce the main VLDS spatial patterns, associated with anticyclonic conditions above the affected regions. The simulated VLDS occurrence is strongly related to the amplitude of the sea level pressure and 500‐hPa geopotential height anomalies as well as the location of the maximum anomalies. All VLDS events are associated with anticyclonic conditions except those occurring in the eastern Mediterranean, where they are generally not associated with a specific meteorological event but with the usual summer weather regime.

17 pages; International audience; Very long dry spell events occurring during winter are natural hazards to which the Mediterranean region is extremely vulnerable, because they can lead numerous impacts for environment and society. Four dry spell patterns have been identified in a previous work. Identifying the main associated atmospheric conditions controlling the dry spell patterns is key to better understand their dynamics and their evolution in a changing climate. Except for the Levant region, the dry spells are generally associated with anticyclonic blocking conditions located about 1000 km to the Northwest of the affected area. These anticyclonic conditions are favourable to dry spell occurrence as they are associated with subsidence of cold and dry air coming from boreal latitudes which bring low amount of water vapour and non saturated air masses, leading to clear sky and absence of precipitation. These extreme dry spells are also partly related to the classical four Euro-Atlantic weather regimes are: the two phases of the North Atlantic Oscillation, the Scandinavian “blocking” or “East-Atlantic”, and the “Atlantic ridge”. Only the The “East-Atlantic”, “Atlantic ridge” and the positive phase of the North Atlantic Oscillation are frequently associated with extremes dry spells over the Mediterranean basin but they do not impact the four dry spell patterns equally. Finally long sequences of those weather regimes are more favourable to extreme dry spells than short sequences. These long sequences are associated with the favourable prolonged and reinforced anticyclonic conditions.

Mount Kenya is an equatorial mountain whose climatic setting is fairly simple (two rainy seasons in March–May, the Long Rains, and October–December, the Short Rains) though concealing significant spatial variations related to elevation and aspect (part I, Camberlin et al., 2014). This part II is dedicated to the sensitivity of sorghum yields to climate variability in space and time, with a focus on the intra-seasonal characteristics of the rainy seasons. To that aim we use the crop model SARRA-H calibrated for the region and fed with rainfall, temperature, wind speed, humidity and solar radiation data over the period 1973–2001 at three stations located on the eastern slopes of Mount Kenya. The crop model is run independently for the two rainy seasons. Four groups of simulations are conducted by varying the initialization date of the simulation, the sowing dates and the type of soil, in order to test sorghum sensitivity to water availability. Evidence is found that potential sorghum yields are dominantly controlled by variations in seasonal rainfall amounts: mean yields are higher at higher and wetter locations, and are higher during the wettest rainy season and years. However, beyond this apparent simplicity, more complex aspects emerge of the crop–climate relationships. First, the yield–elevation relationship is altered at high elevation due to lower temperature. Second, despite a strong link with the seasonal rainfall amounts, we evidence an underlying role of some intra-seasonal rainfall characteristics such as the number of rainy days (itself mainly determined by the rainy season duration) or the occurrence of long dry spells. Third, unseasonal rains occurring after the end of the rainy season, especially after the Short Rains, play a role in final crop yield. Fourth, variations of climate variables such as solar radiation by modulating the potential evapotranspiration concur to yield variations at the wettest locations.

24 pages; International audience; The relationships between precipitation and temperature in the central Sudano-Sahelian belt are investigated by analyzing 50 years (1959–2008) of observed temperature (Tx and Tn) and rainfall variations. At daily time-scale, both Tx and Tn show a marked decrease as a response to rainfall occurrence, with a strongest departure from normal 1 day after the rainfall event (−0.5 to −2.5 °C depending on the month). The cooling is slightly larger when heavy rainfall events (>5 mm) are considered. The temperature anomalies weaken after the rainfall event, but are still significant several days later. The physical mechanisms accounting for the temperature response to precipitation are analysed. The Tx drop is accounted for by reduced incoming solar radiation associated with increased cloud cover and increased surface evaporation following surface moistening. The effect of evaporation becomes dominant a few days after the rainfall event. The reduced daytime heat storage and the subsequent sensible heat flux result in a later negative Tn anomaly. The effect of rainfall variations on temperature is significant for long-term warming trends. The rainfall decrease experienced between 1959 and 2008 accounts for a rainy season Tx increase of 0.15 to 0.3 °C, out of a total Tx increase of 1.3 to 1.5 °C. These results have strong implications on the assessment of future temperature changes. The dampening or amplifying effects of precipitation are determined by the sign of future precipitation trends. Confidence on temperature changes under global warming partly depend on the robustness of precipitation projections.

Eastern Africa, classically presented as a major dry climate anomaly region in the otherwise wet equatorial belt, is a transition zone between the monsoon domains of West Africa and the Indian Ocean. Its complex terrain, unequaled in the rest of Africa, results in a huge diversity of climatic conditions that steer a wide range of vegetation landscapes, biodiversity and human occupations. Meridional rainfall gradients dominate in the west along the Nile valley and its surroundings, where a single boreal summer peak is mostly observed. Bimodal regimes (generally peaking in April and November) prevail in the east, gradually shifting to a single austral summer peak to the south. The swift seasonal shift of the Intertropical Convergence Zone and its replacement in January–February and June–September by strong meridional, generally diverging low-level winds (e.g., the Somali Jet), account for the low rainfall. These large-scale flows interact with topography and lakes, which have their own local circulation in the form of mountain and lake breezes. This results in complex rainfall patterns, with a strong diurnal component, and a frequent asymmetry in the rainfall distribution with respect to the major relief features. Whereas highly organized rain-producing systems are uncommon, convection is partly modulated at intra-seasonal (about 30–60-day) timescales. Interannual variability shows a fair level of spatial coherence in the region, at least in July–September in the west (Ethiopia and Nile Valley) and October–December in the east along the Indian Ocean. This is associated with a strong forcing from sea-surface temperatures in the Pacific and Indian Oceans, and to a lesser extent the Atlantic Ocean. As a result, Eastern Africa shows some of the largest interannual rainfall variations in the world. Some decadal-scale variations are also found, including a drying trend of the March–May rainy season since the 1980s in the eastern part of the region. Eastern Africa overall mean temperature increased by 0.7 to 1 °C from 1973 to 2013, depending on the season. The strong, sometimes non-linear altitudinal gradients of temperature and moisture regimes, also contribute to the climate diversity of Eastern Africa.

La reconstitution de données de pluies manquantes au pas de temps quotidien et à l’échelle stationnelle est parfois nécessaire pour l’étude de changements climatiques ou des travaux de climatologie appliquée. C’est le cas, en domaine tropical, de l’analyse des variations des caractéristiques des saisons des pluies (dates de démarrage et de fin, durée, etc.). A partir d’une approche méthodologique classique fondée sur une régression linéaire multiple pas à pas, mais appliquée pour tirer le meilleur parti possible des données disponibles et incluant une correction des biais, les performances de la reconstitution des pluies obtenues sont évaluées en considérant différents descripteurs de la structure temporelle des saisons des pluies est-africaines. L’Afrique de l’Est, par sa topographie contrastée et ses deux saisons des pluies forcées par des mécanismes d’échelles spatiales différentes, constitue une zone pertinente pour tester la méthodologie de reconstitution. Les résultats montrent que l’occurrence des jours de pluies ainsi que les quantités précipitées sont reproduites correctement dans les espaces où les précipitations sont les plus abondantes et où le nombre de pluviomètres est plus important. Si au pas de temps journalier, les performances des reconstitutions restent parfois modestes, elles sont bien plus satisfaisantes lorsqu’on les évalue à partir des caractéristiques saisonnières ou intra-saisonnières comme le cumul saisonnier, le nombre de jours de pluie, ou les dates de début et de fin des saisons des pluies. L’impact du comblement des lacunes est directement associé à la proportion de lacunes dans les séries temporelles de pluie, sans qu’il soit possible d’identifier un pourcentage de lacunes au-delà duquel les performances de la méthode de reconstitution se détériorent significativement. La distribution temporelle des valeurs manquantes a également un impact non négligeable sur la qualité de la reconstitution de la saison des pluies : ainsi, lorsque l’absence de données de pluie concerne de longues périodes continues, les caractéristiques de la saison des pluies sont mal reproduites.

Les inondations des dernières années en Afrique de l'Ouest rendent nécessaire la prévision hydro-logique pour réduire la vulnérabilité des populations. Nous avons étudié la dynamique des crues dans le bassin du Mono en amont du barrage de Nangbéto (Togo et Bénin) par des méthodes statistiques et déterministes. L'augmentation des précipitations au cours de la décennie 1990 et la mise en eau du barrage de Nangbéto en 1988 ont modifié le régime du Mono, dont les débits d'étiages ont augmenté. Les débits entrants et sortants du barrage sont équivalents, même lors des crues. Les maximums annuels des débits entrants peuvent être modélisés par une distribution des valeurs extrêmes (GEV) de type Fréchet, tandis que ceux des débits sortants, suivent une loi GEV de type Weibull (bornée). Le mécanisme dominant de génération des crues dans le bassin est la montée progressive des eaux associée à l'augmentation de la saturation des sols. L'analyse des débits de crue entre 1988 et 2010 n'a pas montré d'augmentation de la magnitude ou de la fréquence des crues. Le modèle GR4J reproduit bien les débits de crues en période calage, mais ces critères de performance se dégradent lorsque l'on s'en éloigne. Dans ces conditions, l'utilisation de ce modèle pour évaluer les risques futurs de crues serait hasardeuse et il est nécessaire d'envisager d'autres types d'approches.

Despite earlier studies over various parts of the world including equatorial Eastern Africa (EEA) showing that intraseasonal statistics of wet and dry spells have spatially coherent signals and thus greater predictability potential, no attempts have been made to identify the predictors for these intraseasonal statistics. This study therefore attempts to identify the predictors (with a 1-month lead time) for some of the subregional intraseasonal statistics of wet and dry spells (SRISS) which showed the greatest predictability potential during the short rainfall season over EEA. Correlation analysis between the SRISS and seasonal rainfall totals on one hand and the predefined predictors on the other hand were initially computed and those that were significant at 95% confidence levels retained. To identify additional potential predictors, partial correlation analyses were undertaken between SRISS and large-scale oceanic and atmospheric fields while controlling the effects of the predefined predictors retained earlier. Cross-validated multivariate linear regression (MLR) models were finally developed and their residuals assessed for independence and for normal distribution. Four large-scale oceanic and atmospheric predictors with robust physical/dynamical linkages with SRISS were identified for the first time. The cross-validated MLR models for the SRISS of wet spells and seasonal rainfall totals mainly picked two of these predictors around the Bay of Bengal. The two predictors combined accounted for 39.5% of the magnitude of the SST changes between the July–August and October–November–December periods over the Western Pole of the Indian Ocean Dipole, subsequently impacting EEA rainfall. MLR models were defined yielding cross-validated correlations between observed and predicted values of seasonal totals and number of wet days ranging from 0.60 to 0.75, depending on the subregion. MLR models could not be developed over a few of the subregions suggesting that the local factors could have masked the global and regional signals encompassed in the additional potential predictors.

This descriptive study attempts to document the diversity of intraseasonal oscillations (ISO) propagative and spatial patterns, as inferred from outgoing longwave radiation (OLR) variability. Main ISO events over the Indo-Pacific Warm Pool are first extracted using a local mode analysis (LMA) applied on the 20–120-d filtered OLR on the 1979–2008 period. One hundred and sixty-nine individual ISO are detected. Their propagative patterns are then objectively regrouped into a few types using a hierarchical agglomerative classification. Three alternative partitionings are retained, depending on the level of details expected from the typology. ISO events first regroup naturally into two well-separated classes, identifying boreal summer northward-propagating events and boreal winter eastward-propagating ones. Retaining a larger number of classes shows that the diversity of the ISO patterns is primarily modulated by the seasonal cycle, with the most pronounced convective perturbations roughly located at the latitude of the Inter-tropical Convergence Zone in the tropics. When a sufficient number of types are considered, one can obtain several types of ISO events for a given season. Apart from seasonality, the variability in the ISO appears to be related to surface conditions, such as sea surface temperature. Analysis of the differences between the types suggests that the ‘canonical’ ISO, often considered in the literature, is actually rather rare and moderately representative of the propagative events taken individually.

This paper documents the accuracy of a post-correction method applied to precipitation regionalized by the Weather Research and Forecasting (WRF) Regional Climate Model (RCM) for improving simulated rainfall and feeding impact studies. The WRF simulation covers Burgundy (northeastern France) at a 8-km resolution and over a 20-year long period (1989–2008). Previous results show a strong deficiency of the WRF model for simulating precipitation, especially when convective processes are involved. In order to reduce such biases, a Quantile Mapping (QM) method is applied to WRF-simulated precipitation using the mesoscale atmospheric analyses system SAFRAN («Système d'Analyse Fournissant des Renseignements Adaptés à la Nivologie») that provides precipitation data at an 8 km resolution. Raw and post-corrected model outputs are next used to compute the soil water balance of 30 Douglas-fir and 57 common Beech stands across Burgundy, for which radial growth data are available. Results show that the QM method succeeds at reducing the model's wet biases in spring and summer. Significant improvements are also noted for rainfall seasonality and interannual variability, as well as its spatial distribution. Based on both raw and post-corrected rainfall time series, a Soil Water Deficit Index (SWDI) is next computed as the sum of the daily deviations between the relative extractible water and a critical value of 40% below which the low soil water content induce stomatal regulation. Post-correcting WRF precipitation does not significantly improve the simulation of the SWDI upon the raw (uncorrected) model outputs. Two characteristic years were diagnosed to explain this unexpected lack of improvement. Although the QM method allows producing realistic precipitation amounts, it does not correct the timing errors produced by the climate model, which is yet a major issue to obtain reliable estimators of local-scale bioclimatic conditions for impact studies. A realistic temporality of simulated precipitation is thus required before using any systematic post-correction method for appropriate climate impact assessment over temperate forests. Fil: Boulard, Damien. Universite de Bourgogne; Francia Fil: Castel, Thierry. Universite de Bourgogne; Francia Fil: Camberlin, Pierre. Universite de Bourgogne; Francia Fil: Sergent, Anne Sophie. Institut National de la Recherche Agronomique; Francia. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; Argentina Fil: Asse, Daphné. Crea Centre de Recherches Sur Les Ecosystèmes Daltitud; Francia Fil: Bréda, Nathalie. Institut National de la Recherche Agronomique; Francia Fil: Badeau, Vincent. Institut National de la Recherche Agronomique; Francia Fil: Rossi, Aurélien. Universite de Bourgogne; Francia Fil: Pohl, Benjamin. Universite de Bourgogne; Francia

22 pages; International audience; Relationships between daily precipitation and daily maximum and minimum temperature (Tx and Tn, respectively) are analyzed at station level over the Greater Horn of Africa (GHA). Rainfall occurrence is associated with either above normal Tn (mostly in cool highland areas) or below normal Tn (especially lowland, hot environments and early parts of the rainy season). Tx generally displays a more consistent response to rainfall occurrence, with cooling peaking 1 day after the rainfall event. However there is often a persistence of this cooling several days after the rainfall event, and the amplitude of the cooling is also greater for heavy rainfall events. These temperature anomalies are thought to be a response to cloudiness (concurrent reduced Tx and concurrent enhanced Tn) and soil moisture (reduced Tx and Tn, suggested to reflect evaporative cooling). These relationships are of relevance to the interpretation of temperature trends. From 1973 to 2013, the GHA shows a clear warming signal, for both Tn (+0.20 to +0.25 °C/decade depending on seasons) and Tx (+0.17 to +0.22 °C/decade). Rainfall shows both negative (mostly between February and July) and positive trends (mostly in October–December). Given the superimposition of temperature and rainfall trends in parts of the GHA and the covariations between daily rainfall and both Tx and Tn, regression models are used to extract the rainfall influence on temperature, accounting for lag effects up to 4 days. The daily residuals from these models are used to depict temperature variations independent from precipitation variations. At some stations, trends computed on these residuals noticeably differ from the raw Tx trends. When averaged across the GHA, these effects do not exceed −0.06 to +0.03 °C/decade (depending on the month) for Tx, and are marginal for Tn, thus do not strongly modify the magnitude of the warming in the last 40 years. Nevertheless, these results show that precipitation-temperature relationships must be addressed when analyzing temperature changes.

The mean number of dry days in winter increased in the Mediterranean Basin during the 1957-2013 period,mainly because of dry spells becoming longer. The shortest events (Very Short Dry Spells) were less frequentwhereas longer events (Short Dry Spells, Medium Dry Spells and Long Dry Spells) were more frequent on theMediterranean Basin. 76 very long events (Very Long Dry Spells – VLDS) occurred during the 1957-2013period. A hierarchical clustering analysis of these VLDS events allows to detect 5 mainly Mediterraneanconfigurations: North-West Mediterranean, West Mediterranean, North-East Mediterranean, South-EastMediterranean and localized events., En hiver (septembre à avril), le nombre moyen de jours secs a augmenté sur le bassin méditerranéendurant la période 1957-2013, notamment en raison d’épisodes secs de plus en plus longs. Les épisodes les pluscourts (Très Courts Épisodes Secs) ont été de moins en moins fréquents alors que les épisodes plus longs(Courts Épisodes secs, Épisodes Secs Moyens et Longs Épisodes Secs) ont été plus fréquents sur le bassinméditerranéen. 76 épisodes très longs (Très Longs Épisodes Secs - TLES) se sont produits entre 1957 et 2013.Une classification ascendante hiérarchique de ces évènements, considérés comme extrêmes, permet d’observer 5types de configurations typiques à l’échelle du bassin méditerranéen : Nord-Ouest méditerranéen, Ouestméditerranéen, Nord-Est méditerranéen, Sud-Est méditerranéen et épisodes localisés.

This paper documents the capability of the ARW/WRF regional climate model to regionalize near-surface atmospheric variables at high resolution (8 km) over Burgundy (northeastern France) from daily to interannual timescales. To that purpose, a 20-year continuous simulation (1989–2008) was carried out. The WRF model driven by ERA-Interim reanalyses was compared to in situ observations and a mesoscale atmospheric analyses system (SAFRAN) for five near-surface variables: precipitation, air temperature, wind speed, relative humidity and solar radiation, the last four variables being used for the calculation of potential evapotranspiration (ET0). Results show a significant improvement upon ERA-Interim. This is due to a good skill of the model to reproduce the spatial distribution for all weather variables, in spite of a slight over-estimation of precipitation amounts mostly during the summer convective season, and wind speed during winter. As compared to the Météo-France observations, WRF also improves upon SAFRAN analyses, which partly fail at showing realistic spatial distributions for wind speed, relative humidity and solar radiation—the latter being strongly underestimated. The SAFRAN ET0 is thus highly under-estimated too. WRF ET0 is in better agreement with observations. In order to evaluate WRF’s capability to simulate a reliable ET0, the water balance of thirty Douglas-fir stands was computed using a process-based model. Three soil water deficit indexes corresponding to the sum of the daily deviations between the relative extractible water and a critical value of 40 % below which the low soil water content affects tree growth, were calculated using the nearest weather station, SAFRAN analyses weather data, or by merging observation and WRF weather variables. Correlations between Douglas-fir growth and the three estimated soil water deficit indexes show similar results. These results showed through the ET0 estimation and the relation between mean annual SWDI and Douglas-fir growth index that the main difficulties of the WRF model to simulate soil water deficit is mainly attributable to its precipitation biases. In contrast, the low discrepancies between WRF and observations for air temperature, wind speed, relative humidity and solar radiation make then usable for the water balance and ET0 computation. Fil: Boulard, Damien. Universite de Bourgogne; Francia Fil: Castel, Thierry. Universite de Bourgogne; Francia. Agrosup; Francia Fil: Camberlin, Pierre. Universite de Bourgogne; Francia Fil: Sergent, Anne Sophie. Université de Lorraine; Francia. Institut National de la Recherche Agronomique; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina Fil: Bréda, Nathalie. Institut National de la Recherche Agronomique; Francia Fil: Badeau, Vincent. Institut National de la Recherche Agronomique; Francia Fil: Rossi, Aurélien. Universite de Bourgogne; Francia Fil: Pohl, Benjamin. Universite de Bourgogne; Francia

In tropical mountains, the way topographical gradients translate in terms of intra-seasonal and interannual climate variability (especially rainfall and evapotranspiration) is relatively unknown, yet it is an important issue for agriculture and food security. The eastern slopes of Mount Kenya, in East Africa, with their wide range of agro-ecological conditions, are appropriate for the study of these aspects. Daily (monthly) rainfall data from 11 (24) stations at different elevations and exposures are collected for the period 1961-2006. For the elevation belts suitable for farming and agro-pastoral activities, mostly below 2000 m, it is found that not all rainfall characteristics co-vary with altitude. Four distinct behaviours are revealed: (1) Parameters which show a relatively regular increase/decrease with elevation. This is the case for the duration of the two rainy seasons found across the region, and the mean onset and cessation dates, (2) Parameters which show a relatively uniform pattern throughout the region, with little differences both horizontally and vertically. This applies to the interannual variability of the October-December seasonal rains, which are quite homogeneously modulated by Indian Ocean sea-surface temperatures (SSTs), (3) Parameters markedly influenced by exposure. This applies to daily rainfall intensities, which peak along the southeast-facing slopes, whatever the altitude. The frequency of rainy days shows both a vertical gradient and some influence of exposure, (4) Parameters which show very weak spatial coherence, or pronounced vertical discontinuities. This is obvious for the cessation date of the March-May rains, whose poorly organized interannual variability contrasts with that of the onset. Variables involved in potential evapotranspiration (ET0) computation display either a strong (maximum temperature) or a weak (global radiation) vertical coherence, reflecting the existence/absence of an elevation control on the mean distribution of these variables. However, the interannual variations of global radiation have the strongest influence on those of ET0. (Resume d'auteur)

Uncertainties in simulating the seasonal mean atmospheric water cycle in Equatorial East Africa are quantified using 58 one-year-long experiments performed with the Weather Research and Forecasting model (WRF). Tested parameters include physical parameterizations of atmospheric convection, cloud microphysics, planetary boundary layer, land-surface model and radiation schemes, as well as land-use categories (USGS vs. MODIS), lateral forcings (ERA-Interim and ERA40 reanalyses), and domain geometry (size and vertical resolution). Results show that (1) uncertainties, defined as the differences between the experiments, are larger than the biases; (2) the parameters exerting the largest influence on simulated rainfall are, in order of decreasing importance, the shortwave radiation scheme, the land-surface model, the domain size, followed by convective schemes and land-use categories; (3) cloud microphysics, lateral forcing reanalysis, the number of vertical levels and planetary boundary layer schemes appear to be of lesser importance at the seasonal scale. Though persisting biases (consisting of conditions that are too wet over the Indian Ocean and the Congo Basin and too dry over eastern Kenya) prevail in most experiments, several configurations simulate the regional climate with reasonable accuracy.

International audience; El Niño Southern Oscillation (ENSO) and given phases of the Madden-Julian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (i) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (ii) the intraseasonal (30-60 days) and interannual (4-5 years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (iii) extreme values of the Indian Ocean wind shear result from the combination of El Niño and the MJO phase enhancing atmospheric convection over Africa, or La Niña and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.

Rainfall and circulation changes accompanying the Indian monsoon onset are examined, focusing on the African continent and neighboring areas. The Indian Meteorological Department official monsoon onset dates over Kerala (MOK; on average on 1 June) are used. Composites are formed at a pentad (5 days) time scale to compare pre- and postonset conditions. Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data for 1979–2007 indicate that a substantial rainfall decrease over several parts of Africa is associated with MOK. Significant rainfall anomalies, after removal of the mean seasonal cycle, are found in eastern Africa and the nearby western Indian Ocean. Indian monsoon onset dates over the period 1958–2001 are correlated at 0.55 with the cessation dates of the March–May rainy season (the long rains) averaged over Kenya and northern Tanzania. The rainy season cessation leads the onset by 12 days, on average. An early cessation of the March–May rains tends to precede an early onset over India. Over East Africa, the rainfall decrease is associated with a strengthening of the southerly winds (Somali jet) shortly before MOK, enhancing wind divergence and wind shear. A weaker rainfall signal is noted over western Africa and the Gulf of Guinea. MOK coincides with a pause in the progression of the West African monsoon. The pause is associated with anomalous descent over the Sahel, culminating two to three pentads after MOK. Composite maps further indicate that MOK is followed over much of the African continent by a sea level pressure rise and, between India and North Africa, a westward propagation of easterly wind and positive geopotential height anomalies. Many of these signals are associated with Madden–Julian oscillations, but independent features, suggesting Rossby wave propagation over North Africa, are also found to connect MOK and West Africa. Overall, these results are indicative of a large-scale adjustment of the atmospheric dynamics across both eastern and western Africa in conjunction with the monsoon onset over India.

International audience; A classical approach to assess the amplitude of rainfall variations is based on the coefficient of variation. Using worldwide station data for the twentieth century, an alternative method, involving the development of a polynomial fit, was shown to be more relevant to semi-arid climates. The results singularised the tropical and subtropical regions, whose amplitude of interannual rainfall variability was larger than that of the extra-tropical regions, for a given mean rainfall value. However, the tropical belt also showed large contrasts between highly variable climates—corresponding mainly to regions where the sea-surface temperature forcing is strongest—and more steady climates. A separate analysis documented the relationship between the amplitude of rainfall variations and human demography. Population densities did not show any systematic decline with increasing variability. However, in the tropics, there was often a coincidence between reduced demographic growth and high rainfall variability. This smaller demographic growth may result from both reduced natural growth (especially enhanced mortality) and out-migration from regions affected by strong rainfall variations, as evidenced from a number of African cases studies. In contrast, tropical regions with a fast-growing population had on average a lower rainfall variability.

The predictability of the mean August–September photosynthetic activity of vegetation over the Sahel for the period 1982–2002 is explored through a Model Output Statistics approach using ECHAM4.5 retrospective forecasts. Given the poor ability of Atmospheric General Circulation Models (AGCMs) to correctly simulate rainfall over the Sahel, the stress is put on using atmospheric dynamics alone. The mean July–September predicted fields of zonal wind at 600 hPa, and humidity flux at 850 hPa, are selected because of their key role in the West African Monsoon system and their consistency in AGCMs. Coupled modes of NDVI/atmospheric dynamics are extracted using Canonical Correlation Analyses performed in leave-one-out cross-validation. The most relevant modes (using NCEP/DOE 2 or ECHAM4.5 atmospheric dynamics) associate enhanced greenness to a weakened African Easterly Jet displaced northward, and strengthened moisture fluxes from the Guinean Gulf. They are linked with increased rainfall over the Sahel and positive (negative) SST anomalies over the Mediterranean (the eastern equatorial Pacific). Used as predictors in a Multiple Linear Regression (MLR) model, the third cross-validated canonical coefficient (CC) derived from ECHAM4.5 simulations added to the August–September NDVI value of the previous year enable to explain 30% of the variance of a Sahelian regional index with a 2-month lead time. Applied at an 8-km spatial resolution, the statistical model possesses a usable skill (>.5) for 24% of the pixels analysed. The NDVI of pixels covered by open grassland appears as the most predictable. Copyright © 2008 Royal Meteorological Society

L’objectif de cette étude est d’appréhender les caractéristiques spatio-temporelles des phénomènes pluviométriques à l’échelle du bassin méditerranéen de 1950 à 2013 et notamment des évènements intenses. Les résultats montrent une baisse du cumul de pluie, durant la période de septembre à avril, quasi-généralisée à l’ensemble du bassin méditerranéen. De plus, le bassin méditerranéen connait une baisse du nombre de jours de pluie particulièrement marquée dans le nord du Portugal et de l’Algérie, au nord-est de l’Anatolie, au sud-ouest et sud-est des Balkans et sur une majeure partie de l’Italie. Dans le contexte actuel de questionnement face à l’impact du changement climatique sur la fréquence des pluies intenses en méditerranéen, il est impossible de mettre en avant une tendance significative propre à l’ensemble du bassin méditerranéen en terme de fréquence et d’intensité de ces évènements. Seuls certains secteurs tels que le sud-ouest de l’Anatolie, le sud-est et le centre-nord/nord-est des Balkans ainsi que le sud et le nord-ouest de la Péninsule Ibérique montrent une augmentation de la part des précipitations intenses dans le cumul total pour la période 1950-2013. The main objective of this study is to characterize Mediterranean rainfall spatiotemporal features from 1950 to 2013, with a focus on intense events. Results show a general decrease in precipitation amounts during the September to April period. The Mediterranean Basin also shows a strong decrease in the number of rainy days in northern Portugal, Algeria, northeast Anatolia, in southern Balkans and in almost all of Italy. In the current context of global warming, changes in intense Mediterranean precipitation events remain less obvious, with no significant evolutions in the number and intensities of such events. Only few zones such as southwest Anatolia, eastern Balkans and parts of the Iberian Peninsula, show an increase in the contribution of intense rainfall in the total precipitation amount over the 1950-2013 period.

14 pages; International audience; Central Africa hosts the world's second largest tropical forest after the Amazonian basin. However, as compared to its Amazonian counterpart, the Central Africa forests receive much less rain (∼1500 mm/year in mean spread over two rainy seasons in March-May and September-November, and two dry seasons). They also experience a slower deforestation rate, so that the main threat for the next decades might come from climate variations. Nonetheless, their response to the annual cycle of solar radiation and rainfall/clouds is still poorly known. Analysing high resolution remote sensing data of Enhanced Vegetation Index, rainfall, cloudiness, and solar radiation for a target region located between 0 and 5°N and 12–19°E, we explore the climatic drivers of the forests greenness mean annual cycle. Three main points emerge; first, the diurnal cycle is a key-scale for understanding the mean annual cycles of rainfall and incoming solar radiation at surface, then how climate shapes the greenness mean annual evolution; second, neither the two dry seasons nor the two rainy seasons resemble each other in terms of cloud cover, solar radiation and rainfall, and their links with greenness levels; third, whereas the first rainy season (March-May) appears optimal for greenness especially because of favorable light conditions, water availability is the main controlling factor during the main dry season and at the start of the first vegetative season (February). Regarding the little dry season (mid-June-mid-August) and the second rainy season (September-October), light availability might be the main limiting factor. These findings pave the way for further studies of the climate interannual variability and change impacts on the Central Africa forests, taking into account time-scale interactions.

To better understand the Central Africa forests sensitivity toclimate variability, we jointly analyse the mean annual cycles of greenness, rainfall, cloudiness and solarradiation for the target region 0-5°N/12-19°E using high resolution satellite data. Our results demonstrate theimportance of the diurnal scale for understanding the mean annual cycles of rainfall, cloudiness and solarradiation and the way they shape those of forest greenness. They also suggest that whereas the March-Mayrainy season appears optimal for greenness especially because of favorable light conditions, water availabilityis the main controlling factor in December-January the main dry season and in February at the start of the firstvegetative season. Regarding the little dry season and the second rainy season (July-October) light availabilitymight be the main limiting factor to forests photosynthetic activity., Afin de mieux comprendre la sensibilité de la forêt d'Afrique Centrale à la variabilité climatiqueactuelle, une analyse détaillée des cycles saisonniers moyens d'activité photosynthétique, de précipitations, decouverture nuageuse et de radiation solaire est menée pour la région 0-5°N/12-19°E, en s'appuyant sur desobservations satellites haute résolution spatiale et temporelle. Nos résultats montrent tout d'abord que l'échellediurne est une échelle clé pour comprendre les cycles saisonniers moyens de précipitations, nébulosité etradiation solaire, et comment ils façonnent celui d'activité photosynthétique des forêts. Ensuite, alors que lasaison mars-mai semble optimale pour l'activité photosynthétique des forêts en raison d'une bonne disponibilitéen lumière, la disponibilité en eau est le facteur de contrôle principal durant la grande saison sèche (décembrejanvier)et au démarrage de la 1ère saison végétative (février). De juillet à octobre, la plus faible disponibilitéen lumière pourrait être le principal facteur explicatif à la baisse de l'activité photosynthétique.