Sea-surface temperature co-variability in the Southern Atlantic and Indian Oceans and its connections with the atmospheric circulation in the Southern Hemisphere

The relationship between sea-surface temperature (SST) inter-annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950–1999 period. Exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer. Interestingly, these areas are also significantly associated with the far southwestern Pacific (Tasman Sea area). The patterns and time series of co-…

Extreme dry spell detection and climatology over the Mediterranean Basin during the wet season

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, an…

June-september rainfall in north-eastern Africa and atmospheric signals over the tropics: A zonal perspective

The connection between rainfall in northeast Africa (Djibouti, Eritrea, Ethiopia, Kenya, Somalia, Sudan, and Uganda) and various atmospheric indicators from the tropics have been investigated for the northern summer season. Variables used include zonal wind at 700 hPa and 200 hPa, sea-level pressure, and rainfall over other tropical areas, for the period 1951–1988. Strong significant correlations are shown with the Southern Oscillation and the components of the Walker cell in the Pacific Ocean. Droughts in the Ethiopia-Uganda area are associated with El Nino events and, moreover, with above normal pressure and droughts over India. Other significant relationships exist with the African monso…

The Stationarity of Lead-Lag Teleconnections with East Africa Rainfall and its Incidence on Seasonal Predictability

East Africa experiences large interannual rainfall variations, which can lead to severe droughts, as in 1984 in Ethiopia, or extensive flooding, as in 1997 in Somalia and Kenya. In the last 15 years, significant advances have been made in relating these variations to large-scale ocean-atmosphere anomalies, of which those associated to ENSO (El Nino Southern Oscillation) come first. The additional importance of sea-surface variations in the Tropical Atlantic and Indian Oceans is becoming established. Although 20th century East African precipitation series did not experience decadal-scale trends as significant as those found in the sahelian belt, the correlation with ENSO, the Indian monsoon …

Timing and patterns of the ENSO signal in Africa over the last 30 years: insights from normalized difference vegetation index data.

Abstract A more complete picture of the timing and patterns of the ENSO signal during the seasonal cycle for the whole of Africa over the three last decades is provided using the normalized difference vegetation index (NDVI). Indeed, NDVI has a higher spatial resolution and is more frequently updated than in situ climate databases, and highlights the impact of ENSO on vegetation dynamics as a combined result of ENSO on rainfall, solar radiation, and temperature. The month-by-month NDVI–Niño-3.4 correlation patterns evolve as follows. From July to September, negative correlations are observed over the Sahel, the Gulf of Guinea coast, and regions from the northern Democratic Republic of Congo…

Effets des émissions anthropiques de gaz à effet de serre sur le climat du bassin de l'océan Indien

La simulation numérique du climat par le modèle couplé de circulation générale ARPEGE-OPA est utilisée ici pour prédire les effets d'un doublement du CO2 atmosphérique en l'an 2100 sur le climat du bassin de l'Océan Indien. Les températures de surface de la mer, les précipitations et les champs moyens de vents à 850 et 200 hPa sont étudiés au pas de temps trimestriel, sur les deux normales climatiques 1968-1997 et 2070-2099. Pour la fin du XX° siècle, le modèle a tendance à surestimer légèrement les TSM et plus fortement les précipitations, résultat d'un cycle hydrologique simulé trop développé. L'extraction d'indices pluviométriques calculés sur des régions homogènes montre que les télécon…

The East African March–May Rainy Season: Associated Atmospheric Dynamics and Predictability over the 1968–97 Period

Abstract This paper focuses on the East African March–May “long rains.” Particularly, it investigates the atmospheric patterns associated to the March–May rainfall anomalies, then proposes a seasonal prediction model. In a preliminary step, in order to define a regional rainfall index, a new form of extended principal component analysis is performed, aimed at capturing both the spatial and intraseasonal rainfall coherence. What emerges is that although the long rains exhibit a low temporal coherence, calling for a separation between the months of March–April and May in teleconnection studies, they show a relatively strong spatial consistency over the Kenya–Uganda inland region. From composi…

Variabilité de la température entre 1951 et 2014 en Allemagne associée à l’évolution de la floraison des pommiers.

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 r…

Oceanic and atmospheric linkages with short rainfall season intraseasonal statistics over Equatorial Eastern Africa and their predictive potential

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 th…

Cycles diurnes de rayonnement solaire et forets en Afrique Centrale.

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 …

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 Highland…

Influence of the Madden-Julian Oscillation on East African rainfall. Part I: Intraseasonal variability and regional dependency

The influence of the Madden–Julian Oscillation (MJO) on rainfall amounts over Equatorial East Africa (Kenya and northern Tanzania) is analysed for the period 1979–95 at the intraseasonal (pentad) time-scale. The two rainy seasons (March to May and October to December) are considered. Intraseasonal wet events in East Africa are embedded in large-scale zonal circulation anomaly patterns along the equator, showing distinct eastward propagation. It is further found that these ‘wet’ events display a clear phasing with respect to the MJO cycle. This phasing is expressed as out-of-phase variations between the Highland and the coastal areas. Such a pattern is suggested to reflect different rain-cau…

Intraseasonal oscillations of the East African long rains and their connection with MJO activity over the Indian Ocean

Our understanding of the East African long rains (March-May) variability remains relatively poor. Interannual variations are quite small compared to intraseasonal vari- ations. An analysis of pentad rainfall and OLR data shows organised variations in the range of 20-75 days, though quite irregular from year to year. However, rainfall and OLR variations are strongly consistent over the highland region only. For this region, NCEP-DOE II reanalysis data are used to detect atmospheric patterns associated to wet events. Significant zonal wind anomalies, of opposite sign at 850 and 200 hPa, are found locally over East Africa. Anomalous low-level westerlies (upper-level easter- lies) are observed d…

Évolution récente des précipitations de mars-mai en Afrique de l'est : configurations spatiales et évolution subsaisonnière.

6 pages; International audience; S'appuyant sur des données stationnelles de précipitations sur la période 1961-2012, l'objectif de cette étude est 1/ deconfirmer la baisse des pluies au cours des Long Rains sur la décennie 2000 détectée par Lyon et DeWitt (2012) à partir dedonnées en points de grille ; 2/ comprendre sa déclinaison à l'échelle intraannuelle sur la base des scénarios pluviométriquessubsaisonniers développés par Moron et al. (2013) ; et 3/ évaluer son impact sur les écoagrosystèmes. Cette baisse se confirmedans les données stationnelles, mais de façon moins marquée que dans les données grillées. Elle affecte plus fortement les stationsles plus sèches et le mois d'avril, pic c…

Impact de la variabilité climatique et du barrage de Nangbéto sur l'hydrologie du système Mono-Couffo (Afrique de l'Ouest)

Resume En Afrique tropicale, de nombreuses etudes sur la variabilite climatique montrent qu'une tendance a la secheresse s'est manifestee a partir de la fin des annees 1960. Cette secheresse a affecte aussi l'hydrosysteme Mono-Couffo, en Afrique de l'Ouest, avec une persistance remarquable qui s'est repercutee sur les ecoulements. Depuis 1988, avec la mise en eau du barrage de Nangbeto, qui coincide avec une legere reprise pluviometrique dans les annees 1990, on assiste a une modification du regime hydrologique du Mono a Athieme. Les methodes statistiques appliquees aux donnees mensuelles de pluie et debit de la periode 1961–2000 ont permis de constater que les deficits pluviometriques des …

Classification of intense rainfall days in southern West Africa and associated atmospheric circulation.

Daily rainfall in southern West Africa (4&ndash

Impact de la variabilité climatique sur les apports liquides dans la basse vallée du Mono (Bénin, afrique de l'ouest)

International audience; Les effets de la variabilité climatique et leur conséquence sur les ressources en eau devient une question de plus en plus préoccupante pour les gestionnaires dans le contexte de la Gestion Intégrée des Ressources en Eau (GIRE) dans de nombreuses régions de l'Afrique de l'Ouest en général et du Bénin en particulier. Les données utilisées sont : précipitations, évapotranspiration et débit du bassin-versant du Mono à Athiémé sur la période 1970-2005. La détermination des précipitations de l'ensemble du bassin est faite en fonction des coordonnées géographiques et de l'altitude. L'analyse statistique de ces données a permis de faire ressortir d'une part la tendance du r…

Dynamique et modélisation des crues dans le bassin du Mono à Nangbéto (Togo/Bénin)

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 Fr…

Climatic gradients along the windward slopes of Mount Kenya and their implication for crop risks. Part 1: climate variability

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 wit…

Spatial coherence and potential predictability assessment of intraseasonal statistics of wet and dry spells over Equatorial Eastern Africa

The aim of this study was to derive components of the intraseasonal rainfall variations from the daily rainfall in the Equatorial Eastern Africa region and assess their spatial coherence, a pointer to their potential predictability. Daily rainfall observations from 36 stations distributed over Equatorial Eastern Africa and extending from 1962 to 2000 were used. The March to May and October to December periods commonly referred to as the long and short rainfall seasons respectively were considered. Seasonal and intraseasonal statistics at the local (station) level were first defined. The stations were also grouped into near-homogeneous (sub-regional) zones based on daily rainfall. Similarly,…

Interactions entre le cycle diurne de la convection atmosphérique et l'Oscillation de Madden-Julian en Afrique de l'Est

Interactions entre le cycle intrasaisonnier (MJO) et le cycle diurne de la convection atmosphérique. Comment ces deux cycles sont-ils imbriqués ? Quels sont les effets de la MJO sur le déroulement du cycle diurne (durée, intensité, extension spatiale des évènements convectifs) ?

Climate and extreme rainfall events in the Mono river basin (West Africa): investigating future changes with Regional Climate Models.

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

Climate variability and change : hydrological impacts

Utilisation d'ensembles de simulations climatiques sur Modèles de Circulation Générale de l'Atmosphère: concepts, méthodes et applications à la climatologie de l'espace Sud-Africain,

International audience

Cropping system dynamics, climate variability, and seed losses among East African smallholder farmers: a retrospective survey.

Abstract Climate variability directly affects traditional low input and rain-fed farming systems, but few studies have paid attention retrospectively to the cropping system’s ability to mitigate climate risk. This study analyzes the impacts of rainfall variability on farmers’ seed variety losses over time, considering changes in smallholder farming systems. The cropping system dynamics, in favoring maize at the expense of sorghum and pearl millet, have induced an increasing risk of seed loss during drought. Combining ecological anthropology and climatology, a retrospective survey asking farmers about the period 1961–2006 was carried out at three altitudinal levels (750, 950, and 1100 m) on …

African rainfall variability

Dyvalocca : un projet pour l'étude des nuages bas de saison sèche sur la façade atlantique de l'Afrique centrale et de leur impact bioclimatique sur les forêts sempervirentes.

Low-level clouds are key components of the climate but are not well represented in weather and climate models. Recently it has been shown that an extensive low-level cloud cover develops during the June-September main dry season in western equatorial Africa, from the coastal plains of Gabon and Congo-Brazzaville to the inland plateaus downstream of the low-mountain crests. Such a cloudy main dry season is unique in the moist Tropics and is likely to explain the presence of the densest and evergreen forests of Central Africa as evapotranspiration is low and the quality of light is high. The ANR-DFG funded “DYVALOCCA” project will conduct from 2020 to 2022 a field campaign, raise existing in-…

Coastal precipitation regimes in Kenya.

Kenya is under the influence of the seasonal reversal of the Indian ocean monsoons. However, its coastal belt, up to about 50 km inland, exhibits original climatic features. Hierarchical clustering...

L'étude du climat du siècle à venir : Des enjeux, des données et des techniques parfois spécifiques. Application au phénomène ENSO et aux précipitations en Afrique australe

Régionaliser le climat pour en évaluer les impacts : l'apport des modèles numériques

National audience; La simulation de l'évolution future du climat repose sur l'utilisation de modèles numériques complexes. Ces modèles doivent prendre en compte les diverses composantes du système climatique (atmosphère, biosphère, cryosphère et lithosphère) et leurs temps de réponse respectifs pour simuler les réactions de ce système à différents scénarios de perturbations d'origine naturelle (activités solaire et volcanique...) ou humaine (émissions de gaz à effet de serre et d'aérosols). Afin d'étudier l'impact du climat sur une région donnée, il faut disposer d'une résolution spatiale suffisamment fine (quelques kilomètres) dont ne disposent pas les modèles de circulation générale plané…

Climate Adjustments over Africa Accompanying the Indian Monsoon Onset

Abstract 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 …

Application of daily rainfall principal component analysis to the assessment of the rainy season characteristics in Senegal

The interannual variability of the onset and cessation dates of the rainy season (RS) in Senegal is analyzed over the 43 yr period 1950-1992, using daily rainfall data for 34 stations. The use of principal component analysis, based on rainfall only, is explored to identify aggregate, regional indexes for the onset and cessation of the rains. The minimum and maximum values of the cumula- tive scores of principal component 1, for each year, are used to locate the onset and cessation dates, respectively. Very distinct spatial rainfall patterns are found before and after the onset/cessation dates. Mean dates compare favorably with those based on other definitions, though our method is not meant…

Impact des précipitations sur l'activité photosynthétique de la végétation en Afrique semi-aride sub-saharienne

National audience

État actuel des réseaux de mesures éco-climatiques en Afrique centrale : les ambitions du projet de recherche international FORGREENE.

This work presents an overview of spatio-temporal quality andspatial density of rainfall networks of Cameroon, Gabon, Congo, CAP and DRC, particularly in the recentperiod (1973-2014) through the GSOD international database. Faced with enormous gaps observed, theseresults will help to select metrological and analytical orientations in FORGREENE project (Tropical forestsgreeness and cloudiness in Central Africa: present conditions and future evolution). A comparison with rainfallsatellite estimates (RFE type) also indicates their great potential, provided they are based on in situ validations.

Montagnes d'Afrique Tropicale : marqueurs de la variabilité climatique ou isolats climatiques ?

International audience; Tropical mountains are often seen as beacons of past and present-day climate change. However, do they actually reflect interannual and decadal-scale climatic variability of the surrounding areas ? At the scale of Africa South of the Sahara, an analysis of the 1941-2000 rainfall variability shows that spatial coherence is seldom modified by relief. Examples are presented in which mountain climates are even indicators of large-scale atmospheric dynamics and climate variability.

Climate change of Benin for the period 1950-2010 and its impact on surface water.

6 pages; International audience; In West Africaand Benin in particular, variability / climate change affects the availability of surface water resources. Thus,this study aims to analyze the rainfall field of Benin from the fifty three (53) meteorological stations from 1950to 2010 and to detect the rainfall amounts and the average temperature of six synoptic stations and thehydrometric stations of Niger river in, Benin, Pendjari, Ouémé, Mono and Couffo. From the analysis a trend ofdeclining precipitation and of rising temperatures have been highlighted. This decrease is more pronounced inthe Sudanese area (north of 8 ° N) and in the sub-equatorial area (south of 8 ° N). This decrease of prec…

Typology of intraseasonal oscillations based on a Local Mode Analysis

The Intraseasonal Oscillation (ISO; 20-120 day) is an important component of the variability of the Tropical convection that strongly perturbs the Asian and the Aus- tralian monsoons. Our knowledge of the physical origin of the ISO however remains largely incomplete, partially because of the large variability of its characteristics from one event to another. The aim is to determine how the patterns of the different ISO events may be objectively regrouped in a few types, related for example to season, ENSO or other large scale forcing. Patterns of the different ISO events are extracted using a Local Mode Analysis (LMA, Goulet and Duvel 2000). The LMA is based on complex empirical orthogonal …

Analysis of cloudiness and solar radiation diurnal cycles for a better understanding of the forests greenness mean annual cycle in Central Africa.

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 star…

Spatial and temporal variability of rainfall over the Republic of Djibouti from 1946 to 2017.

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 contras…

Spatial Coherence of Tropical Rainfall at the Regional Scale

AbstractThis study examines the spatial coherence characteristics of daily station observations of rainfall in five tropical regions during the principal rainfall season(s): the Brazilian Nordeste, Senegal, Kenya, northwestern India, and northern Queensland. The rainfall networks include between 9 and 81 stations, and 29–70 seasons of observations. Seasonal-mean rainfall totals are decomposed in terms of daily rainfall frequency (i.e., the number of wet days) and mean intensity (i.e., the mean rainfall amount on wet days).Despite the diverse spatiotemporal sampling, orography, and land cover between regions, three general results emerge. 1) Interannual anomalies of rainfall frequency are us…

An improvement of June-September rainfall forecasting in the Sahel based upon region April-May moist static energy content (1968-1997)

This study provides statistical evidence that June–September Sahelian rainfall hindcasts currently based on oceanic thermal predictors apprehend more the negative trend than the interannual rainfall variations. Four physically meaningful predictors of June–September Sahel rainfall are first selected through the near-surface April–May information and several experimental hindcasts provided. We then discuss the skills achieved using regression techniques and cross-validated discriminant functions. In that context, 8/11 of the driest seasons and 8/10 of the wettest are correctly predicted. Finally using completely independent training and working periods we show that better and significant hin…

Evolution of dry season low cloud cover over the Atlantic coast of Central Africa from diurnal to interannual scale

Despite its bioclimatic importance, few studies have focused on the stratiform low-level cloud cover ofthe dry season (June-September) of the Atlantic coast of Central Africa. Using in-situ data documenting theperiod 1971-2019, we show that this cloud cover is underestimated by SAFNWC satellite observations andCMIP6 models. We identify three types of days: clear, cloudy at night and totally cloudy, the latterbeing more frequent on the coast and inland, to the windward of the Cristal and Chaillu Mountains. Onan interannual scale, temperature anomalies in the South Tropical Atlantic, the equatorial Pacific and theCongo Basin, by modulating the meridional and zonal circulation cells, influence…

Caractérisation de la variabilité inter et intra-annuelle du NDVI en Afrique de l'ouest sur la période 1982-2002 à l'aide des données NOAA-AVHRR.

28 novembre-2 décembre 2005; International audience

Définition d’un événement de poussières désertiques au Sahel : apport de nouvelles mesures de PM10 au Burkina Faso.

This study isdedicated to the desert dust of the dry season in an inhabited Sahelian rural area whose populations are widely exposed tosanitary risks such as meningitis outbreaks (Martiny & Chiapello, 2013). Here, the objective is to give a definition of a dust“event”, a concept describing the dustiest moments in a dusty environment, but that remains rather unclear, notably becauseof the lack of ground-truth measurements. The analysis of new PM10 measurements in Burkina Faso has enabled us to definethe dust events as episodes which experience concentrations greater than 189μg/m3 for at least 10h. Five types of dust eventshave been defined depending on the concentrations measured. On the fir…

Comparison of rainfall structures between NCEP/NCAR reanalyses and observed data over tropical Africa

A comparison is made between modelled (NCEP/NCAR reanalysis) and observed (CRC and CRU dataset) annual and monthly precipitation over tropical Africa during the period 1958–1997. The split moving-windows dissimilarity analysis (SMWDA) is used to locate abrupt changes in rainfall time series. In the NCEP reanalysis data, we identify a main abrupt shift, which occurs in 1967 and concerns more than 50% of grid points. In the observation, this shift is only found over parts of tropical North Africa. Three other NCEP abrupt shifts (1975, 1983 and 1987) in the reanalysis shown by the SMWDA, each concerning about 20% of tropical Africa, are not identified in the observation. One hypothesis concern…

Identification of processes that control the stable isotope composition of rainwater in the humid tropical West-Central Africa.

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 …

The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought

Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought…

Intraseasonal variations of June?September rainfall and upper-air circulation over Kenya

In the Northern Summer, Kenya is located under the influence of the divergent Indian monsoon flow, and therefore is dry except for two separate areas: the coastal strip and the western regions. Analysis of daily rainfall data for June–September 1982 to 1988 has revealed that, although there are many distinct rainfall events between the two regions, an out-of-phase relationship is also evident, rain on the Coast being frequently accompanied by a drop in the precipitation over the Rift Valley area. It is shown that two types of wind forcing accompany these patterns. Alternating westerly and easterly anomalies at the 700 hPa level are associated with persistent wet and dry conditions (respecti…

Précipitations en Afrique du Sud: changements climatiques simulés par le modèle de circulation générale ARPEGE/OPA et par la désagrégation régionale

International audience; En Afrique du Sud, l'accès à la ressource en eau est source de nombreux enjeux et les besoins sont croissants. Les conséquences du réchauffement global sur la disponibilité en eau peuvent être importantes. Pour étudier le climat du XXIe siècle, les modèles de circulation générale (MCG) intégrant la modification prévisible de la composition chimique de l'atmosphère (gaz à effet de serre et aérosols) sont des outils incontournables. Outre les incertitudes avec lesquelles les MCG doivent composer (scénarios d'émissions), leur aptitude à simuler le climat à l'échelle régionale est variable. Pour l'été austral (janvier-mars), sur la période 1970-1999, le modèle couplé ARP…

Climatic gradients along the windward slopes of Mount Kenya and their implication for crop risks. Part 2 : crop sensitivity.

16 pages; International audience; 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 t…

Determinants of the interannual relationships between remote sensed photosynthetic activity and rainfall in tropical Africa

International audience; The response of photosynthetic activity to interannual rainfall variations in Africa South of the Sahara is examined using 20 years (1981-2000) of Normalised Difference Vegetation Index (NDVI) AVHRR data. Linear correlations and regressions were computed between annual NDVI and annual rainfall at a 0.5° latitude/longitude resolution, based on two gridded precipitation datasets (Climate Prediction Center Merged Analysis of Precipitation [CMAP] and Climatic Research Unit [CRU]). The spatial patterns were then examined to detect how they relate to the mean annual rainfall amounts, land-cover types as from the Global Land Cover 2000 data set, soil properties and soil typ…

Influence de l'Oscillation de Madden-Julian sur la variabilité intrasaisonnière des pluies en Afrique de l'Est (Kenya-Tanzanie)

Les incidences de l'Oscillation de Madden-Julian, mode de variabilité atmosphérique intrasaisonnier dominant dans la bande tropicale, sur la répartition des pluies d'Afrique de l'Est durant le trimestre Mars-Avril-Mai, sont étudiées sur la période 1979-2002. L'analyse révèle une modulation significative des précipitations. Cependant, de fortes hétérogénéités existent du point de vue des pluies entre les Hautes terres et la côte kenyane, couplées à des mécanismes pluviogènes de nature différente.

Insolation cycles as a major control equatorial Indian Ocean primary production

Analysis of a continuous sedimentary record taken in the Maldives indicates that strong primary production fluctuations (70 to 390 grams of carbon per square meter per year) have occurred in the equatorial Indian Ocean during the past 910,000 years. The record of primary production is coherent and in phase with the February equatorial insolation, whereas it shows diverse phase behavior with δ 18 O, depending on the orbital frequency (eccentricity, obliquity, or precession) examined. These observations imply a direct control of productivity in the equatorial oceanic system by insolation. In the equatorial Indian Ocean, productivity is driven by the wind intensity of westerlies, which is rel…

Characterization of the interannual and intraseasonal variability of West African vegetation between 1982 and 2002 by means of NOAA AVHRR NDVI data

AbstractThe interannual and intraseasonal variability of West African vegetation over the period 1982–2002 is studied using the normalized difference vegetation index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR).The novel independent component analysis (ICA) technique is applied to extract the main modes of the interannual variability of the vegetation, among which two modes are worth describing. The first component (IC1) describes NDVI variability over the Sahel from August to October. A strong photosynthetic activity over the Sahel is related to above-normal convection and rainfall within the intertropical convergence zone (ITCZ) in summertime and is partly associated …

Interactions tropicales – tempérées et pluviométrie au Mozambique.

International audience

Climats tropicaux d'aujourd'hui à demain : variabilité et changements

Present and future tropical climates : their climate variability and their climate changes The oriffttality of tropical climates is re-examined under the light of climate variability and climate change, both present and future . The interannual variability of rainfall is on average larger in the tropics than in the extra-tropics, but it is also spatially more contrasted. Human communities have often responded to this constraint through efficient coping strategies. The regions showing the most variable climate do not necessarily have lower population densities, an indication that in the long run variability is not decisive. However, the recent demographic growth of these regions is often low…

Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon

Abstract In light of the droughts and subsequent food crises that have plagued the Ethiopia–Sudan region in the course of its history, and especially during the last 3 decades, the author examines both the interannual and intraseasonal variabilities of the July–September rains and compares them to the Indian summer monsoon. Regional rainfall indexes for the region stretching from Eritrea to Lake Victoria are computed using seasonal totals for the period 1901–88. Daily data for 1982–88 are also considered. Though all these regions are only partly affected by the Indian monsoon cross-equatorial flow and although they are separated from India by an extensive dry belt (Red Sea, Somalia, west Ar…

Compared regimes of NDVI and rainfall in semi‐arid regions of Africa

Bi‐monthly normalized difference vegetation index (NDVI) at an 8 km spatial resolution from the advanced very high resolution radiometers (AVHRR) was used from 1981 to 1995 to analyse the vegetation response to rainfall supply in semi‐arid regions of Africa. Within the 200–600 mm annual rainfall belt, for which the apparent NDVI response to rainfall was the strongest, three regions were selected which exhibited different patterns in their NDVI regimes and/or relationships with rainfall. The regions, located in western, southern and eastern Africa, were split into coherent sub‐regions in terms of mean regime of photosynthetic activity through a cluster analysis. Overall, intra‐regional diffe…

Extracting subseasonal scenarios: an alternative method to analyze seasonal predictability of regional-scale tropical rainfall.

Abstract Current seasonal prediction of rainfall typically focuses on 3-month rainfall totals at regional scale. This temporal summation reduces the noise related to smaller-scale weather variability but also implicitly emphasizes the peak of the climatological seasonal cycle of rainfall. This approach may hide potentially predictable signals when rainfall is lower: for example, near the onset or cessation of the rainy season. The authors illustrate such a case for the East African long rains (March–May) on a network of 36 stations in Kenya and north Tanzania from 1961 to 2001. Spatial coherence and potential predictability of seasonal rainfall anomalies associated with tropical sea surface…

Vegetation structure and greenness in Central Africa from Modis multi-temporal data.

African forests within the Congo Basin are generally mapped at regional scale as broad-leaved evergreen forests, with a main distinction between terra-firme and swamp forests types. At the same time, commercial forest inventories, as well as national maps, have highlighted a strong spatial heterogeneity of forest types. A detailed vegetation map generated using consistent methods is needed to inform decision makers about spatial forest organisation and theirs relationships with environmental drivers in the context of global change. We propose a multi-temporal remotely sensed data approach to characterize vegetation types using vegetation index annual profiles. The classifications identified…

Exploring the predictability of the‘Short Rains’ at the coast of East Africa

The boreal autumn ‘Short Rains’ at the coast of East Africa are deficient when there is weak development of a zonal circulation cell along the Indian Ocean equator, an anomalously low sea-surface temperature in the western portion of the basin, and in the high phase of the southern oscillation. Such large-scale circulation departures and their precursors are described by compact indices. September values of these indices for the period 1958–96 are used to explore the predictability of an index (RON) of October–November rainfall at the coast of East Africa. Regressions with cross-validation over the entire 1958–96 period are evaluated for the early (1958–77) and late (1978–96) halves of the …

A typology for intraseasonal oscillations

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-sepa…

Temporal evolution of heavy rainfall in the southern West Africa coastline belt.

The Southern Coastalbelt of West Africa (SCWA) is exposed to recurrent flooding. The two objectives of the study are to show thespecificity of SCWA to heavy rainfall occurrence and to determine the trends of the latter. Daily rainfall data of41 stations in the southern parts of Côte d’Ivoire, Ghana, Togo and Benin for the period 1951–2015 and ClimateHazards Group InfraRed Precipitation data were used. The approach is based on the determination (for eachstation and grid point) of the 95th centile (P95) of precipitations greater than or equal to 1 mm. Results indicatethat rainfall is heavier on the coastal belt than inland (average P95 is 65,4 and 45,5 m/day for coastal and inlandstations res…

Interannual persistence effects in vegetation dynamics of semi-arid Africa

[1] Over 15 years of Normalized Difference Vegetation Index (NDVI) data from the Advanced Very High Resolution Radiometers (AVHRR) are used to study the response of vegetation activity to rainfall in three semi-arid regions of Africa. The relationships between annual NDVI and annual precipitation (PPT) time series are examined using statistical approaches (simple and partial correlations, linear multiple regressions). It appears that annual NDVI highly depends on PPT of the concurrent year and the previous year. An analysis of particularly dry and wet years enables to better diagnose two distinct responses of vegetation activity to rainfall. The “recovery” effect represents the difficulty o…

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 t…

Inter-Relationships Between Groundnut Yield in Senegal, Interannual Rainfall Variability and Sea-Surface Temperatures

Groundnut production strongly contributes to Senegal’s economy. Interannual variations of groundnut yield for the country as a whole, and their relationship with rainfall amounts, are examined for the 31-yr period 1960–1990. It is shown that on that scale, and after removing decadal trends, almost half of the variance is explained by rainfall variability, especially that of the early part of the rainy season (July–August). Given the high spatial coherence of seasonal rainfall in the region, teleconnections with global- and regional-scale climate dynamics, including sea-surface temperatures, are assessed. Though some features are similar to the rest of the Sahel, others are more specific, su…

Can we expect some predictability of intra-seasonal rainfall patterns over East Africa ?

Précipitations tropicales : quelle prévisibilité potentielle à l'échelle intrasaisonnière et locale ?

6 pages; International audience; Les précipitations tropicales résultent de phénomènes imbriqués. Les cumuls saisonniers à l'échelle régionale permettent de filtrer une partie des variations spatiales liées notamment aux échelles les plus fines et ainsi de magnifier l'action des forçages plus vastes. La variabilité interannuelle des totaux saisonniers est partiellement prévisible à partir de l'état antérieur des températures de surface océanique. Cependant, ce total saisonnier ne constitue pas toujours l'élément le plus prévisible, notamment dans le cas où les pluies les plus abondantes en moyenne sont fortement incohérentes. La saison février-juin au Kenya et au nord de la Tanzanie montre …

Interannual variability of rainfall in the eastern horn of Africa and indicators of atmospheric circulation

Relationships between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932–83. Rainfall data consist of stations and regional indices calculated for three selected areas experiencing quite different rainfall patterns. The results indicate significant negative correlations between northern autumn rains in Somalia and the Southern Oscillation during the same season. These rains are also negatively correlated with pressure in the Western Indian Ocean, and positively correlated in the Eastern Indian Ocean. The reverse pattern is shown with sea-surface temperature. This agrees ver…

Extreme dry spells over the Mediterranean Basin during the wet season: assessment of HyMeX/Med-CORDEX regional climate simulations (1979-2009).

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…

Épisodes secs en hiver dans le bassin méditerranéen : variabilité et forçages atmosphériques (1957-2013).

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.

Zonal circulations over the Indian and Pacific Oceans and the level of lakes Victoria and Tanganyika

Level records of two East African Great Lakes, Lake Victoria and Lake Tanganyika, which are considered as hydroclimatic proxies, are analysed. Comparisons between the two lake signals show synchronisms, which can only be accounted for by large-scale mechanisms. Lake-level variations associated with the short rains season (October–January) appear to have a prominent effect on the annual lake levels. The relations between lake-level variations and atmospheric circulation indexes are then investigated. Over the period 1946–2000, four indexes are selected to characterize the October–December zonal circulation over the Pacific and Indian Oceans. Over the Indian Ocean two surface zonal wind index…

Seasonality and atmospheric dynamics of the teleconnection between African rainfall and tropical sea-surface temperature: Atlantic vs. ENSO

A 47-year record (1951-1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIN O3, South Atlantic and North Atlantic indices. El Nino -Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to d…

Montagnes d’Afrique tropicale : marqueurs de la variabilité climatique ou isolats climatiques ?

6 pages; International audience; Les montagnes tropicales sont souvent perçues comme des sentinelles des changements climatiques. Pour autantles variations climatiques interannuelles à décennales sont-elles le reflet de celles des espaces environnants ? A l’échelle del’Afrique au sud du Sahara, une analyse de la variabilité des pluies entre 1941 et 2000 montre une cohérence spatiale peudiminuée par les reliefs. Des exemples sont présentés de reliefs constituant même des révélateurs de la dynamiqueatmosphérique et de la variabilité climatique d’échelles larges.

Le changement climatique contemporain: développement d'un champ d'étude à l'interface Climat & Societé

Acte des journées de Climatologie de la "Commission Climat et Société" (17-19 mars 2005); National audience

Types de configurations pluviométriques sur le bassin méditerranéen, forçages atmosphériques et évènements intenses (1979-2013).

Sixrecurrent and redundant daily rainfall patterns have been extracted over the Mediterranean area from 1979 to 2013: five areassociated to regionals rainfall excess (South-West Mediterranean, Mid-North Mediterranean, East Mediterranean, WestMediterranean and north-west Mediterranean) and one to generalized deficit. Each rainfall pattern is associated with atypical barometric spatial configuration leading to local wind bringing moisture to the coast leading to specific local rainfallsurplus. Rainfall patterns also explain a large part of the spatial characteristics and the probability of occurrence of intenseevents. For example, more than 20% of days of “West-Mediterranean” rainfall pattern…

Intraseasonal wind anomalies related to wet and dry spells during the ?long? and ?short? rainy seasons in Kenya

The largest part of Kenya exhibits two major rainy seasons, the March–May «long rains» and the October–December «short rains», both related to the passage of the ITCZ, but differing in the amount of rainfall recorded and its interannual variability. In order to investigate whether these differences also apply at intraseasonal time-scales, daily rainfall data for the peak month of each rainy season (April and November) were collected for 7 consecutive years (1982–1988). The network comprises 68 stations, from which a classification of the spatial patterns of daily rainfall anomalies has been performed. Wind anomalies corresponding to the various rainfall types and to specific regional rainfa…

Très longs épisodes secs hivernaux dans le bassin méditerranéen : variabilité spatio-temporelle et impact sur la production céréalière en Espagne.

This study concerns the long winter sequences without rain at the whole Mediterraneanbasin scale. Objectives are (i) to define theses extreme events as geographical and climatological objectscharacterized by a location, a spatial extension and a temporal length and (ii) measured impacts of these eventson the cereal production in Spain. 76 events of very long dry spells (eVLDS) are detected over the 1957-2013period. These eVLDS are grouped into 4 typical recurrent configurations at the Mediterranean basin scale: North-East, West, Scattered & Localised and South-East. The eVLDS have a greater impact on barley, wheat and oatyields (winter crops grown through rainfed agriculture) than rainfall …

Forecasting the vegetation photosynthetic activity over the Sahel: a Model Output Statistics approach

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 perfor…

Predictability of NDVI in semi-arid West, East and South Africa

Bias correction of dynamically downscaled precipitation to compute soil water deficit for explaining year-to-year variation of tree growth over northeastern France.

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 Rense…