Global equatorial variability of 850 and 200 hPa zonal winds from rawinsondes between 1963 and 1989

The longitude-height-time variability of 3-month averaged zonal wind anomalies at 850 and 200 hPa over the equatorial area (5{degrees}N-5{degrees}S) is analyzed using a three-dimensional dataset constructed from rawinsonde data (1963-1989). The first mode, closely related to the Southern Oscillation Index, suggests a strong vertical coupling associated with a horizontal out-of-phase pattern between the central/western Pacific and the remainder of the equatorial belt. The vertical coupling appears to be phase-locked to the annual cycle with strongest intensities found over South America and near the maritime continent early in the calendar year and over the Pacific basin and Africa during th…

Guinean and sahelian rainfall anomaly indices at annual and monthly scales (1933-1990)

The annual and monthly rainfall variability in north tropical Africa are analysed by principal component analysis in order to detect the main coherent modes. The Sahel constitutes the first mode, although it is divided close to 1O"W into an Atlantic section and a Continental section. The Guinean zone is less coherent, and is divided into distinct modes. The subequatorial area never formed a distinct mode. Rainfall variability for the coherent areas is constructed from rainfall anomaly indices (RAI), based on the coherent areas, and a rainfall typology taking into account the spatial pattern of the rainfall anomaly fields and the intensity of the anomaly. Their evolution (mainly since 1950) …

SURCOTES MARINES DANS LE GOLFE DU LION ET FORCAGES ATMOSPHERIQUES : VARIABILITE CONTEMPORAINE ET FUTURE (1950-2100)

Sea surges in the Gulf of Lions are mainly forced by southerly and south-easterly winds. This regional-scale atmospheric circulation is leading by a strong zonal gradient between low pressure system over the Bay of Biscay and high pressure over central Europe. This synoptic-scale circulation mostly happens during “Greenland Above” weather regime. In the second half of the 20th century, a slow increase of the sea-level pressure over central Europe increased the probability of having “Greenland Above” weather types associated with a southerly atmospheric circulation in the Gulf of Lions, thus leading to strong surges. A linear regression is used to simulate the interannual variability of high…

The onset of the rainy season and farmers’ sowing strategy for pearl millet cultivation in Southwest Niger

A multi-year (2004-2009) field survey of on-farm sowing practices in 10 villages located in south-west Niger close to Niamey, is analysed to investigate the relationships (i) between rainfall and the sowing date of pearl millet and the risk of sowing failure, (ii) between sowing and meteorological/agro-climatic onset dates, (iii) between sowing/onset dates, and simulated and observed yield/biomass at the end of the season. Even if some villages sow without any synchronous or anterior rainfall, most parcels (73% out of the 1551 available cases) are sown during and just after a 2-day wet spell receiving at least 10 mm. In fact, there is a strong correlation (r = 0.82-0.95 depending on onset d…

North-Atlantic Oscillation and regional-scale sea-surge variability in Gulf of Lions during the 20th century

Article soumis; International audience; Sea-surge variations recorded at three tide-gauge stations (Grau-de-la-Dent, Sète, and Port-Vendres) around the Gulf of Lions (Northwest Mediterranean Sea) are mostly locally forced by onshore winds blowing from 90° to 180° related to an atmospheric depression usually centered between the Bay of Biscay and the British Island, which is more prevalent during the negative phase of the North-Atlantic Oscillation (NAO). During the second half of the 20th century, the long-term increase of sea-surge height at Grau-de-la-Dent finds no counterpart in the positive deviation of the NAO. The relationship between the monthly frequency of sea surges > 20 cm at Gra…

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 …

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…

Uses and limits of thermal indices: the case of Sahel

International audience; Our main goal here is to analyse extreme heat waves (HWs) in the Sahel (13°N-18°N; 16°W-30°E), using different thermal indices. In the ACASIS project funded bythe French “Agence Nationale de la Recherche”, HWs analyses are characterized forthe first time during the hottest season in the Sahel, using the Global Summary of theDay (GSOD) synoptic observations during April-May-June 1973-2013. Such extremehigh temperatures are usually defined by 3 criteria: 1/ Their low probability ofoccurrence: less than the 10% of the days. The use of absolute thresholds,associated with heat budget and physiological impacts, could be an alternative(Seneviratne et al., 2012). 2/ Their in…

Rainfall variability in subequatorial America and Africa and relationships with the main sea-surface temperature modes (1951–1990)

The rainfall variability of subequatorial South America and Africa is poorly documented owing to the scarcity of data. We present a new land-only data set of monthly precipitation from 1951 to 1990, focusing on subequatorial South America and Africa, which improves the knowledge of rainfall variability and allows comparisons with GCM outputs. The results of multivariate analyses are compared with those performed on the best actual global rainfall data set developed by Mike Hulme. The main modes of bimonthly rainfall variability are not located in the major rain-forest basins of Za'ke and Amazonia, but rather on the tropical margins, such as Venezuela or Sudan, and near-coastal equatorial ar…

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 …

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-…

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…

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…

Spatial coherence of monsoon onset over Western and Central Sahel (1950-2000)

Abstract The spatial coherence of boreal monsoon onset over the western and central Sahel (Senegal, Mali, Burkina Faso) is studied through the analysis of daily rainfall data for 103 stations from 1950 to 2000. Onset date is defined using a local agronomic definition, that is, the first wet day (>1 mm) of 1 or 2 consecutive days receiving at least 20 mm without a 7-day dry spell receiving less than 5 mm in the following 20 days. Changing either the length or the amplitude of the initial wet spell, or both, or the length of the following dry spell modifies the long-term mean of local-scale onset date but has only a weak impact either on its interannual variability or its spatial coher…

Spatial coherence of agronomic boreal monsoon onset over Western and Central Sahel (1950-2000).

Poster Presentation

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…

Variability of the African convection centre as viewed by outgoing longwave radiation records and relationships with sea-surface temperature patterns

International audience; The main African convection centre (ACC) is delimited by the 5"-square areas with outgoing longwave radiation less than 235 W m-' in the domain 17.5°N-17.5"S and 18"W-46"E. Four parameters (latitude and longitude of the centre of gravity, mean intensity, and extension) were analysed during the period June 1974 to December 1991. The relationships between the variability of the ACC and the main sea-surface temperature (SST) patterns were then studied. Correlation and composite analyses demonstrate that the first principal component of SST, representing mainly the variability of the eastern and the central tropical Pacific, is associated strongly with the extension and …

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…

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 …

Trends of mean temperatures and warm extremes in northern tropical Africa (1961-2014) from observed and PPCA-reconstructed time series

Trends in daily maximum (TX) and minimum (TN) temperatures and indices of warm extremes are studied in tropical North Africa, west of the eastern African highlands, from 1961 to 2014. The analysis is based on the concatenation and cross-checking of two observed databases. Due to the large number of missing entries (~25%), a statistical infilling using probabilistic principal component analysis was applied. Averaged over 90 stations, the linear trends of annual mean TX and TN equal respectively +0.021 °C/yr and +0.028 °C/yr. The frequency of very hot days (TX > 35°C) and tropical nights (TN > 20°C), as well as the frequency of daily TX and TN above the 90th percentile (p90) (“warm days” and …

Durée et fréquence des vagues de chaleur en Afrique tropicale septentrionale selon 5 indices de chaleur

Thermal extremes generate health risks that are increasingly taken into account, including in tropical Africa. This work compares the duration and the frequency of spring heat waves (HWs) in West Africa according to 5 indices: Warm Spell (WS), Heat Index (HI) with and without the relative humidity, Apparent Temperature (AT), and Excess Heat Factor (EHF). The daily relative humidity and temperature data (minimum, maximum, dew point are provided by the GSOD database for the period 1973-2014 and cover 145 stations spread over a large African area (3-24°N; 24°W-36°E). The results show that the duration of the HWs is short on the coast (3 days) and they last longer inland the continent and in th…

Durée et fréquence des vagues de chaleur en Afrique tropicale septentrionale selon 5 indices de chaleur.

Thermal extremes generate health risks that are increasingly taken into account, including in tropical Africa. This work compares the duration and the frequency of spring heat waves (HWs) in West Africa according to 5 indices: Warm Spell (WS), Heat Index (HI) with and without the relative humidity, Apparent Temperature (AT), and Excess Heat Factor (EHF). The daily relative humidity and temperature data (minimum, maximum, dew point are provided by the GSOD database for the period 1973-2014 and cover 145 stations spread over a large African area (3-24°N; 24°W-36°E). The results show that the duration of the HWs is short on the coast (3 days) and they last longer inland the continent and in th…

Cohérence spatiale et prévisibilité potentielle du démarrage de la saison des pluies en Afrique soudano-sahélienne

Sahel droughts and Enso dynamics

Correlations between summer Sahel rainfall and Southern Oscillation Index has increased during the last thirty years. At high frequency time scale (periods lower than 8 years), an intertropical Atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature (SST) anomalies between the eastern equatorial parts of Pacific and Atlantic. This zonal connection worked well during most of the E1 Nino/Southern Oscillation (ENSO) events occurring after 1970; positive/negative SST anomalies in the eastern Pacific/Atlantic led to rainfall deficits over the whole West Africa. At low frequency time scale (periods greater than 8 years), positive SST anomalies in the In…

Les vagues de chaleur au Sahel : caractérisation, mécanismes, prévisibilité.

The mechanisms controlling Sahelian heat wave (HW) variability are examined on the period 1979-2014 using the GSOD observational database and ERA-Interim reanalyses. HW events are analyzed through all terms of the atmospheric energy balance, showing a predominant role of incoming shortwave radiation on daily maximum temperature (Tx) and atmospheric water vapor on minimum temperature (Tn). The low-frequency warming trend, not explained by the previous terms, is thought to relate to the increase of greenhouse gases concentrations, due to anthropogenic emissions. The predictability of Sahelian HW events is assessed for lead times reaching up to 15 days. The model's skill, biases and uncertaint…

Variabilité intra-saisonnière et multi-décennale de la téléconnexion entre les pressions de surface (100°W–50°E ; 30°–70°N) et les ENSO/LNSO (1873–1996)

Abstract The relationships between an index of the NINO3.4 region and the wintertime sea level pressure (hereafter SLP) anomalies on the extratropical North Atlantic and the bordering areas (100°W–50°E; 30°–70°N) are studied for 1873–1996. This study emphasizes the need of a careful pooling of months and the multi-decadal variability of the ENSO/LNSO influence on the extratropical North Atlantic. We calculate the mean monthly climate anomalies for the 20 warmest and the 20 coldest NINO3.4 (170°–120°W; 5°N–5°S) years from October to March. The composite of SLP anomalies for the 20 warmest NINO3.4 years shows an anomalous trough centered near 30°W in November–December, and positive (respectiv…

Global atmospheric response to specific linear combinations of the main SST modes. Part I: numerical experiments and preliminary results

Abstract. This article investigates through numerical experiments the controversial question of the impact of El Niño-Southern Oscillation (ENSO) phenomena on climate according to large-scale and regional-scale interhemispheric thermal contrast. Eight experiments (two considering only inversed Atlantic thermal anomalies and six combining ENSO warm phase with large-scale interhemispheric contrast and Atlantic anomaly patterns) were performed with the Météo-France atmospheric general circulation model. The definition of boundary conditions from observed composites and principal components is presented and preliminary results concerning the month of August, especially over West Africa and the …