Typology of pentad circulation anomalies over the Eastern Africa - Western Indian Ocean region, and their relationship with rainfall

International audience; The aim of this study was to classify the most frequently observed atmospheric circula- tion anomaly patterns in eastern Africa and the adjacent Indian Ocean. As an example of the useful- ness of such a classification, the second objective was to test whether these patterns account for intraseasonal rainfall anomalies in the region. A partitioning algorithm, known as dynamical cluster analysis, was therefore applied to the zonal (U)and meridional (V)components of the wind anom- alies, obtained from the NCEP-NCAR Reanalysis R-2 at the pentad (5 d) timescale. The 3 geopoten- tial levels 850, 700 and 200 hPa were combined. Focus is on the transition seasons (March to Ma…

Weather Regimes in the Euro-Atlantic and Mediterranean sector and relationship with West African rainfall over the period 1989-2008 from a Self-Organizing Maps Approach

International audience; Weather Regimes (WR) have been defined over the Euro-Mediterranean region [60°W-60°E; 15°N-70°N] from May to October season using the daily Sea Level Pressure, 700 hPa geopotential height and specific humidity from ERA-interim over 1989-2008 period. Computations are based on a neural network lassification technique referred to as Self Organizing Maps and the WR produced can be used by the community for comparison with other periods, projection onto model outputs, seasonal prediction or teleconnection studies. The article particularly examines the relationship between WR and West African (WA) rainfall and our results suggest that changes in particular WR frequencies c…

Active phases and pauses of the West African monsoon and associated atmospheric dynamics

International audience

Time of emergence of Mediterranean summer maximum temperature.

The time at which the signalof climate change emerges from the noise of natural climate variability is called “time of emergence”. This timeis analyzed here based on summertime mean maximum temperature in the Mediterranean basin for RCP8.5 andRCP4.5 scenarios. This time would be around 2040-50 (2060-70) in the eastern (western) basin. Differencesbetween the optimistic and pessimistic scenarios give a mean shift of around 15 years with the time ofemergence occurring earlier for RCP8.5 scenario.

Climate change impact on the West African monsoon from June to October.

Sixteen CMIP5 models are used toanalyse how climate changes over the West African Monsoon region between a “future period”, defined from 2031 to 2070under the rcp4.5 emission scenario, and the “current period”, defined from 1960 to 1999 under the historical emissionscenario. The results show an increase of the September to October rainfall amounts. Circulation anomalies include of anincreases of sea-level pressure over the Mediterranean Sea and Europe that allow a strengthening of the moisture flux fromEuro-Mediterranean regions into the Sahel. These changes are stronger in autumn.

Changes in the frequency of the Weather Regimes over the Euro-Atlantic and Mediterranean sector and their relation to the anomalous temperatures over the Mediterranean Sea.

19 pages; International audience; An exercise has been carried out to assess to what extent the Euro-Atlantic Weather Regimes (WR),described from the ERA-interim Reanalysis in the summer season, projects onto a pool of AGCMAMIPsimulations in which sea surface temperatures (SST) are prescribed from observations.Although the model simulations present some biases in the spatial structure and seasonality of WRs,exhibiting also less variability, they are able to capture main WR over the region in summer season:+Middle East –Middle East, +NAO, -NAO. WR paradigm is used to quantify changes in theatmosphere under warmer/colder than normal conditions over the Mediterranean Sea. To addressthis proble…

Modulations intra-saisonnières de la pluviométrie ouest africaine: pauses et phases actives du système de mousson

National audience; En Afrique de l'Ouest, le déclenchement des différentes saisons des pluies ainsi que les rythmes pluviométriques sont principalement liés à la phase d'installation du système de mousson. Cette étude montre que cette phase peut se décomposer en quatre sous-périodes distinctes, assez fixes dans le temps, appelées « pauses » pendant lesquelles les pluies stagnent, voire décroissent. Ces pauses sont suivies par de fortes croissances des précipitations appelées « phases actives ». Dans le cadre d'études épidémiologiques portant sur des maladies à vecteur très dépendant des précipitations, type paludisme, ces résultats offrent des perspectives de recherches intéressantes.

Regional scale model simulation for West Africa using a Mesoscale Model MM5

Dynamical downscaling is considered as a good tool for deriving regional climate in formation based on large scale climate conditions because they can represent surface conditions like topography and vegetation more realistic than GCM. The fifth generation Pennsylvania State University (PSU)-National Centre for Atmospheric Research (NCAR) Mesoscale Model MM5 is used to study the regional scale process that control West African rainfall, its associated circulations and other surface features. The simulations are carried out for seven months from March to September using 1.125X1.125 degree lat/lon version of ECMWF reanalysis data for defining the initial and laternal boundary conditions. The …

Agronomy and climatology of a 100 000 km² watershed in West Africa.

7 pages; International audience; Sahelian regions are considered as particularly vulnerable to climatic variability and change for two reasons: the dominant role of rainfed agriculture in their economies and their weakness of water resources management. The physiological response of an individual plant to the climatic variations of parameters is well documented, but extrapolation to regional food production still remains very dubious. On the Bani, a tributary of the Niger River in Mali, climatic variables representative of the agroclimatic constraints of the cultures (dynamics of the rainy season and occurrence of extreme events) were evaluated from observed past data and simulated future d…

The role of the eastern Mediterranean sea on the WAM in 2003 using the WRF regional model

International audience; There is empirical evidence that sea surface temperatures (SST) exert a prominent role on the WAM, particularly over the Atlantic ocean where they can act to modify the location of the ITCZ and moisture advection. However, the role of the Mediterranean sea has been studied more recently. Rowell (2003) has first shown that the basin could play a significant role on the monsoon in modifying the moisture transport and convergence of humidity. Using GCM simulations, he demonstrated that warmer (colder) than normal SST on the Mediterranean sea is associated with a more/less intense monsoon in the Sahel region. A simple composite analysis between the SST and the Sahel July…

Active phase and pauses of the West African monsoon and associated atmospheric dynamics

This study comes within the research of the characterization of the intra-seasonal modulations of the West Africa monsoon system during its annual cycle. Indeed, these fluctuations are not yet well understood and documented, particularly with regard to the main periods of installation of the various Western African rainy seasons and the phases of transition separating them. Also very few studies were interested in these onsets. Objective diagnostic analyses based on PCA decomposition, Varimax rotation and low-pass filters (≥ 1 month) are performed on the 5-day CMAP rainfall data over West Africa. The northward excursion of the rainbelt can be divided into 4 basic sub-periods each composed o…

Climate adjustments over the African-Indian monsoon regions accompanying Mediterranean Sea thermal variability

International audience; This study analyses the atmospheric and rainfall anomaly patterns in the African/Indian monsoon region concomitant of warm/cold anomalies confined in the Mediterranean sea in northern summer. It examines first the similarities and contrasts observed in longitudes, then discusses the results obtained regarding the normal in terms of climate impacts in the Sahelian belt. Statistical results show an opposite African/Indian relationship with stronger (weaker) African (Indian) tropical circulations occurring in warmer (colder) Mediterranean situations and favouring significant increases (decreases) in monsoon circulation, atmospheric moisture content and deep convection, …

Which rainfall dataset can be used to study African Monsoon?

Méditerranée et mousson africaine : mécanismes et évolution dans l’actuel et le futur.

6 pages; International audience; À partir d’observations in situ et analysées, de réanalyses atmosphériques, d’expériences numériques forcées parles températures de la Méditerranée, et de sorties CMIP5 issues de seize modèles couplés ayant participé au dernier exercicedu GIEC, nous tentons une synthèse des travaux collaboratifs entrepris au CRC ces cinq dernières années et portant sur lesinteractions tropiques/extra-tropiques au long du fuseau africain et sur leurs liens avec la mousson africaine. Après avoirrappelé le rôle des téléconnexions avec les grands bassins océaniques, de la circulation sur le secteur euro-atlantique et dela mousson indienne via le mécanisme de Rodwell-Hoskins, nou…

Definition and predictability of an ORL monsoon onset in the West Africa monsoons

International audience

Impact du climat sur la santé : modélisation régionale des poussières pour les épidémies de méningites au Sahel.

6 pages; International audience; Les épidémies des méningites bactériennes constituent encore à l’heure actuelle un problème de santé publiquemajeur en Afrique de l’Ouest. Chaque année, au coeur de la saison sèche (de janvier à mars), 25 000 à 250 000 cas sontenregistrés par l’Organisation Mondiale de la Santé (OMS) dans la zone latitudinale 10-15°N, plus communément appelée« ceinture de la méningite ». L’importance des facteurs sociétaux dans l’expansion des épidémies est indéniable. Cependant,une série d’études récentes a permis de mettre en lumière le rôle important du climat. Ainsi, l’intensité des épidémies sembleliée à des conditions de circulation atmosphérique spécifiques quelques m…