A model perspective on the dynamics of the shadow zone of the eastern tropical North Atlantic – Part 1: the poleward slope currents along West Africa

International audienceThe West African seaboard is one of the upwelling sectors that has received the least attention, and in situ observations relevant to its dynamics are particularly scarce. The current system in this sector is not well known and understood, e.g., in terms of seasonal variability, across-shore structure, and forcing processes. This knowledge gap is addressed in two studies that analyze the mean seasonal cycle of an eddy-permitting numerical simulation of the tropical Atlantic. Part 1 is concerned with the circulation over the West African continental slope at the southernmost reach of the Canary Current system, between  ∼ 8 and 20°N. The focus is on the depth range most directly implicated in the wind-driven circulation (offshore and coastal upwellings and Sverdrup transport) located above the potential density σt = 26.7 kg m−3 in the model (approx. above 250m of depth). In this sector and for this depth range, the flow is predominantly poleward as a direct consequence of positive wind stress curl forcing, but the degree to which the magnitude of the upper ocean poleward transport reflects Sverdrup theory varies with latitude. The model poleward flow also exhibits a marked semiannual cycle with transport maxima in spring and fall. Dynamical rationalizations of these characteristics are offered in terms of wind forcing of coastal trapped waves and Rossby wave dynamics. Remote forcing by seasonal fluctuations of coastal winds in the Gulf of Guinea plays an instrumental role in the fall intensification of the poleward flow. The spring intensification appears to be related to wind fluctuations taking place at shorter distances north of the Gulf of Guinea entrance and also locally. Rossby wave activity accompanying the semiannual fluctuations of the poleward flow in the coastal waveguide varies greatly with latitude, which in turn exerts a major influence on the vertical structure of the poleward flow. Although the realism of the model West African boundary currents is difficult to determine precisely, the present in-depth investigation provides a renewed framework for future observational programs in the region

Etude des connaissances, des attitudes et des pratiques des jeunes lyceens en matière des IST du VIH et du SIDA dans la Commune urbaine de Sikasso, Mali

Nous avons mené en mai 2012 une étude transversale, descriptive à passage unique dans 3 établissements scolaires de la commune urbaine de Sikasso. Cette étude avait pour objectif général d’étudier les connaissances, les attitudes et les pratiques des jeunes lycéens en matière des IST du VIH et du SIDA. Les élèves de sexe féminin étaient prédominants (60,2 %). La moyenne d’âge était de 19,4 ans avec des âges extrêmes de 15 ans et 24 ans. Plus de la moitié (69,2 %) des élèves avaient déjà entendu parler des IST et la prévalence déclarée d’IST et/ou des symptômes associés à la présence d’IST était estimée à 27,0 %. La majorité des élèves, 97,9 %, avait déjà entendu parler du VIH et du Sida, les principales sources d’information de ces élèves étaient : la télévision (93,3 %), la radio (87,3 %) et l’école (84,8 %) et leurs connaissances sur le Sida étaient bonnes. Selon leurs attitudes, 39,1 % des élèves ont déclaré avoir fait le test de dépistage parmi lesquels 97,8 % des garçons et 95,6 % des filles ont retiré leur résultat. La majorité des élèves, 97,1 % des filles et 94,7 % des garçons savaient que le préservatif protège du VIH et du SIDA dont 59,5 % ont déclaré l’avoir utilisé lors des derniers rapports sexuels. Ils étaient 156 soit 54 %, à avoir déjà eu des rapports sexuels dont 22,9 % l’ont eu avec deux partenaires sexuels au cours des 12 derniers mois. Une forte proportion des élèves (64,4 %) a assisté au moins une fois à une séance de sensibilisation sur les IST, le VIH et le SIDA

A numerical modeling study of the Southern Senegal upwelling shelf: Circulation and upwelling source waters

The regional ocean circulation along the Southern Senegal shelf is studied using a numerical model for varying forcing fields during the upwelling season (November-May). The main objective is to study the shelf circulation of the marine environment in the Southern Senegal Upwelling Shelf (SSUS), the Lagrangian pathways by which waters are transported to the surface in the upwelling tongue and the fate of these waters. Model results are compared with hydrographic measurements during UPSEN2-ECOAO survey and satellite images datasets. The upwelling source waters and also the warm nearshore waters are analyzed, from a Lagrangian point of view, by computing the paths of particles that are passively advected by the regional ocean model velocity field (ROMS). It was found that the model is capable of representing the main characteristics of the SSUS whose Sea Surface Temperature (SST) patterns and their variability have an important similarities but the assessment of the current simulated by the model is less obvious because of the few available observations. The present results suggest that high frequency of wind has a strong influence over the mesoscale patterns of the cross-shore circulation in association with local topography and the presence of the Cape Verde peninsula. These cross-shore circulations are dominated by an onshore flow around 14.5°N and an offshore flow around 14.1°N. The onshore flow is in agreement with the previous eulerian studies regarding the coastal divergence of the surface flow (upwelling) that was strongly localized within a few tens of kilometers south of the Cape Verde in the Hann Bight. Lagrangian experiments show also very clearly that coastal warm waters are upwelling waters that were advected from Hann Bight and underwent a strong warming by the heat flux from the atmosphere to the ocean in a shallow area (∼10 m); therefore, easy to warm up. All numerical solutions have shown a tendency of upwelling particles remaining on the shelf for more than a month. These results may explain the enrichment of water in the SSUS and suggest the need to consider the whole shelf as a retention area