Recherches sur la pluviométrie de la corne orientale de l'Afrique

The Eastern Horn of Africa (Somalia, Ethiopia, Djibouti and Kenya) exhibits strong rainfall anomalies, considering the latitudinal position of the region. The mean annual rainfall map shows a vigorous meridian contrast between the rainy Western Highlands of Kenya and Ethiopia, and the deficiency of the lowlands of Somalia, the Eastern parts of Kenya and Ethiopia, and the Red Sea coast. This opposition is mainly related to the northern summer season, during which the different caracteristics of the wet "west-african" monsoon and the dry divergent "indian" monsoon are clearly emphasized. They induce single-maximum summer rainfall regimes, and double-peak spring and autumn regimes, respectively. The extension of the summer rainfall maximum as far south as Western Kenya constitutes another anomaly. A contrast also exists between the (unexpected) winter rainfall regimes of the Red Sea Coast, and the summer rainfall regimes of the Ethiopian Plateau. The local and regional influences of relief and sea proximity (the latter generally up to 30 or 50 kms from the coast), and their associated wind circulations, play an important part in the rainfall patterns. That is the case (at least partially) for the coastal winter rains of the Red Sea and the Gulf of Aden, the summer rains along the coasts of Kenya and Southern Somalia, or shelter effects in the Rift Valley. The inter-annual variability is generally high, particularly in the eastern half of the region, where the variations are especially related to the autumn rains. In many of the multiple-peak rainfall areas, there is also a large inter-annual variability in the seasonal repartition of rains, compared to mean regimes. However, year to year persistence of rainfall anomalies is shown to be higher in some of the western continental regions, in particular the Ethiopian Highlands, although there are seldom clear trends. Periodic droughts have affected the region, the worst hit area being Ethiopia, where severe famines have been reported in 1888-92, 1972-74 or 1982-85, among others. According to the few long-term records available, rainfall variations differ from place to place, but one can discern general patterns, such as low rainfall in the 40's, very high rainfall in the 60's, and two dry periods in the early 70's and 80's, separated by above-normal rainfall years. Although comprehensive study of spatial variations is still to be carried out, it seems that Ethiopian rainfall patterns (especially in Eritrea) are often quite different from those of the other regions. This is also shown in the analysis of rainfall periodicities, which indicate 4 peak frequencies: - 2 to 2.5 years, mainly in the Indian Ocean coastal areas; - 3 to 4 years, weak and rare; - 5 to 6 years, common throughout the Horn of Africa (except in Eritrea); - 10 to 12 years, essentially significant in Addis-Ababa

Importance of recent extreme weather variation in Djibouti and need for impact quantification

This analysis shows that the current rainfall deficit is exceptional and historically unique. The significant population migration induced by the drought to Djibouti city must be supervised, especially during their spontaneous settling. This presented example confirms that current rainfall shortages and increasing temperature extremes are impactinglocal people who urgently need adaptation and DRR strategies. It is necessary to reduce exposure to hydrological risks of these affected populations, in order that victims of the drought are not carried away by a rainfall excess.Peer reviewe

Ethnographic context and spatial coherence of climate indicators for farming communities - a multi-regional comparative assessment

Accurate seasonal predictions of rainfall may reduce climatic risks that farmers are usually faced with across the tropical and subtropical zones. However, although regional-scale seasonal amounts have regularly been forecasted since 1997/98, the practical use of these seasonal predictions is still limited by myriad factors. This paper synthesizes the main resultsof a multi-disciplinary ethnographic and climatic project (PICREVAT). Its main objective was to seek the climatic information ? beyond the seasonal amounts ? critical for crops, both as an actual constraint to crop yields and as identified by the current and past practices and perceptions of farmers. A second goal was to confront the relevance and signifiCance of this climatic information with its spatial coherence, which gives an upper bound of its potential predictability. The ethnographic and climatic analyses were carried out on three very different fields: North Cameroon (mixed food crops associated with a cash crop ? cotton ? integrated into a national program); Eastern slopes of Mt Kenya (mixed food crops, with a recent development of maize at the expense of sorghum and pearl millet);and Central Argentina (mixed crops and livestock recently converting to monoculture of transgenic soybean, referred to as soybeanization).The ethnographic surveys, as well as yield?climate functions, emphasized the role played by various intra-seasonal characteristics of the rainy seasons beyond the seasonal rainfall amounts, in both actual yields and people?s representations and/or crop management strategies. For instance, the onset of the rainy season in East Africa and North Cameroon, the season duration in the driest district of the eastern slopes of Mount Kenya, or rains at the core (August) and at the end of the rainy season in North Cameroon have been high lighted. The dynamics of farming systems (i.e. soybeanization in Central Argentina, increas-ing popularity of maize in East Africa, recent decline of cotton in North Cameroon) were also emphasized as active drivers; these slow changes could increase climatic vulnerability (i.e. soybean is far more sensitive to rainfall variations than wheat, maize is less droughtresistant than sorghum or millet), at least for the least flexible actors (such as the non-capitalized farmers in Central Argentina). The cross between ethnographic surveys and climatic analyses enabled us to identify climate variables that are both useful to farmers and potentially predictable. These variables do not appear to be common across the surveyedfields. The best example is the rainy season onset date whose variations, depending on regions, crop species and farming practices may either have a major/minor role in crop performance and/or crop management, or may have a high/low potential predictability.Fil: Moron Vincent. Columbia University; Estados Unidos. Aix-Marseille University; FranciaFil: Boyard-Micheau Joseph. Universite de Bourgogne; FranciaFil: Camberlin Pierre. Universite de Bourgogne; FranciaFil: Hernandez, Valeria Alicia. Universidad Nacional de San Martín. Instituto de Altos Estudios Sociales; Argentina. Université Paris Diderot - Paris 7; FranciaFil: Leclerc, Christian. No especifíca;Fil: Mwongera, Caroline. No especifíca;Fil: Philippon, Nathalie. Universite de Bourgogne; FranciaFil: Fossa Riglos, María Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín; ArgentinaFil: Sultan, Benjamin. Sorbonne University; Franci

The Precipitation Inferred from Soil Moisture (PrISM) Near Real-Time Rainfall Product: Evaluation and Comparison

Near real-time precipitation is essential to many applications. In Africa, the lack of dense rain-gauge networks and ground weather radars makes the use of satellite precipitation products unavoidable. Despite major progresses in estimating precipitation rate from remote sensing measurements over the past decades, satellite precipitation products still suffer from quantitative uncertainties and biases compared to ground data. Consequently, almost all precipitation products are provided in two modes: a real-time mode (also called early-run or raw product) and a corrected mode (also called final-run, adjusted or post-processed product) in which ground precipitation measurements are integrated in algorithms to correct for bias, generally at a monthly timescale. This paper describes a new methodology to provide a near-real-time precipitation product based on satellite precipitation and soil moisture measurements. Recent studies have shown that soil moisture intrinsically contains information on past precipitation and can be used to correct precipitation uncertainties. The PrISM (Precipitation inferred from Soil Moisture) methodology is presented and its performance is assessed for five in situ rainfall measurement networks located in Africa in semi-arid to wet areas: Niger, Benin, Burkina Faso, Central Africa, and East Africa. Results show that the use of SMOS (Soil Moisture and Ocean Salinity) satellite soil moisture measurements in the PrISM algorithm most often improves the real-time satellite precipitation products, and provides results comparable to existing adjusted products, such as TRMM (Tropical Rainfall Measuring Mission), GPCC (Global Precipitation Climatology Centre) and IMERG (Integrated Multi-satellitE Retrievals for GPM), which are available a few weeks or months after their detection

Nile Basin Climates

The climate of the Nile Basin is characterised by a strong latitudinal wetness gradient. Whereas the areas north of 18°N remain dry most of the year, to the south there is a gradual increase of monsoon precipitation amounts. Rainfall regimes can be divided into 9 types, among which summer peak regimes dominate. In the southern half of the basin, mesoscale circulation features and associated contrasts in local precipitation patterns develop as a result of a complex interplay involving topography, lakes and swamps. Precipitation changes and variability show up as 3 distinct modes of variability. Drying trends since the 1950s are found in central Sudan and to some extent the Ethiopian Highlands. The equatorial lakes region is characterised by occasional very wet years (e.g. 1961, 1997). The interannual variations are strongly, but indirectly influenced by El-Nino / Southern Oscillation. Sea surface temperature variations over other ocean basins, especially the Indian and South Atlantic Oceans, also play a significant role. Projections for the late twenty-first century show a 2-4°C temperature increase over the basin, depending on the scenario, but rainfall projections are more uncertain. Most models tend to predict a rainfall increase in the equatorial regions, but there is little consistency between models over the tropical regions

More variable tropical climates have a slower demographic growth

International audienceA 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

Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

Daily rainfall in southern West Africa (4–8° N, 7° W–3° 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 1998–2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8° 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° 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