До 85-річчя Ісаака Бенціоновича Клеймана

У 2006 р. виповнилося 85 років одному з найшановніших археологів Одеси, відомому досліднику стародавньої Тіри Ісааку Бенціоновичу Клейману

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

International audienceThere 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-August-September (JAS) rainfall shows significant relationships with the eastern Mediterranean basin. Let us recall also that in JAS 2003, warmer SST over the whole basin has been shown to be associated with rainfall excess (Jung et al., 2006). Using the Weather Regional Forecast model (WRF) developed by the NCAR, we defined 2 sensitivity experiments by adding/subtracting anomalies in the eastern basin for the year 2003 in order to design warm/cold SST forcing fields. Our results exhibit significant positive rainfall anomalies over the eastern Sahel when the basin is warmer than normal. The response to the warming can be resumed as follows. The moisture is brought in the low levels from the Mediterranean sea to the Sahara and the eastern Sahel. The additional moisture fuels the convection inside the ITCZ that leads to increased moisture convergence sustained by the moisture flux from the Atlantic, consistent with the positive feedback mechanism proposed by Rowell (2003). The main vertical motions within the ITCZ are located east of Greenwich where the rainfall surplus occurs. To the West of the primary meridian, the vertical ascent in the heat low is significantly enhanced and the associated mid levels high pressure reinforced. This is accompanied by a northward migration of the ITCZ and of the heat low. A subseasonal analysis shows that the precipitation positive anomalies then developed are found to prevail until 15 days after the onset of the rainy season at the end of June

Le concept de « time of emergence » appliqué aux températures maximales d’été sur le bassin méditerranéen.

6 pagesInternational audienceThe 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.Dans le contexte du changement climatique se pose la question du moment à partir duquel latendance au réchauffement deviendra significative par rapport à la variabilité naturelle. C’est le concept de« time of emergence ». Ce moment est défini ici à partir des températures maximales estivales moyennes sur lebassin méditerranéen selon les trajectoires RCP4.5 et RCP8.5. La cartographie obtenue montre que la périodese situe autour de 2040-50 (2060-70) sur le bassin oriental (occidental). Ce changement définitif d’état moyendevrait être en moyenne plus précoce d’environ 15 ans pour la trajectoire RCP8.5 par rapport à la trajectoireRCP4.5

Impact du changement climatique sur les pluies de mousson d'Afrique de l'Ouest entre juin et octobre.

International audienceSixteen 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.Seize modèles CMIP5 sont utilisés pour analyser l’impact du changement climatique sur la fin de saison des pluiessahélienne (Septembre-Octobre). Les différences sont faites entre les périodes futurs (2031-2070 sous scénario rcp4.5) etactuelles (1960-1999 sous scénario historique). Les précipitations augmentent dans la simulation future en lien avec unrenforcement des flux d’humidité en provenance de l’Europe et de la partie Est de la mer Méditerranée. Un renforcement dela pression avec une circulation anormalement anticyclonique au-dessus de l’Europe permettent ces changements : l’impactdu changement climatique en région de mousson d’Afrique de l’Ouest en automne se fait principalement par le biais deslatitudes moyennes

Monsoon onset over Sudan-Sahel: Simulation by the regional scale model MM5.

International audienc

Euro-Atlantic weather regimes and Mediterranean rainfall patterns: present-day variability and expected changes under CMIP5 projections.

17 pagesInternational audienceFour monthly rainfall patterns (RPs) describing four monthly typical recurrent rainfall spatial features over the Mediterranean area were computed with agglomerative hierarchical clustering: north-western Mediterranean (NWM), western and south-western Mediterranean (W&SM), Mediterranean generalized deficit (MGD) and north-eastern Mediterranean (NEM). The relationship between the monthly frequency of daily weather regimes (WRs) computed for the Euro-Atlantic region and monthly Mediterranean RPs is analysed over the 1979-2005 period. Overall, the NAO− WR is clearly associated with NWM, the NAO+ with MGD, the Atlantic Ridge (AR) with NAM and the East-Atlantic with W&SM. Over the 21st century, we find that the winter occurrence of WRs are not significantly modified under future RCP4.5 and RCP8.5 projection. Nevertheless, an eastward shift of the low pressure system during NAO− regimes, an eastward extension of Azores high pressure towards the Mediterranean during NAO+ regimes and an increase in high pressure over the Azores during AR regimes could strengthen the mean relationship between WRs and RPs. These within-class barometric changes in WRs are mainly observed under the RCP8.5 scenario and in the late 21st century, highlighting their strong dependence on radiative forcing over time

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

26 pagesInternational audienceDaily 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

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

International audienc

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

National audienceEn 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 objective is to study the monsoon structure from the first rainy season in Guinea (5N) to the sahelian rainfall peak in August. The year 2000 is taken as a reference year in order to validate the model simulation for further study. The results are compared with reanalysis and observed data, then statistical methods are used to study temporal and spatial relationships among various field parameters with 60 km resolution. Preliminary results show that the model is able to reproduce a more realistic rainfall field than the reanalysis