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Study region : The study considers six precipitation stations located in Senegal, West Africa. Senegal is located in the Sahel, an area that is threatened by climate variability and change. Both droughts and extreme rainfall have been an issue in recent years. Study focus : Two different statistical downscaling techniques were applied to the outputs of four regional climate models at six selected precipitation stations in Senegal. First, the delta-change method was applied to the mean annual precipitation as well as the 5, 10, 20, 50 and 100-year return period daily precipitation events. Second, a quantile–quantile transformation (QQ) was used to downscale the monthly distributions of precipitation simulated by regional climate models (RCMs). The 5, 10, 20, 50 and 100-year daily precipitation events were afterward calculated. All extreme events were calculated assuming that maximum annual daily precipitations follow the generalized extreme value (GEV) distribution. The two-sided Kolmogorov–Smirnov (KS) test was finally used to assess the performance of the quantile–quantile transformation as well as the GEV distribution fit for the annual maximum daily precipitation. New hydrological insights for the region : Results show that the two downscaling techniques generally agree on the direction of the change when applied to the outputs of same RCM, but some cases lead to very different projections of the direction and magnitude of the change. Projected changes indicate adecline in mean precipitation except for one RCM over one region in Senegal. Projected changes in extreme precipitations are not consistent across stations and return periods. The choice of the downscaling technique has more effect on the estimation of extreme daily precipitations of return period equal or greater than ten years than the choice of the climate models.

Alors que l'ensemble du territoire mexicain appartient au domaine tropical, caractérisé par l'alternance d'une saison des pluies correspondant aux mois les plus chauds et d'une saison sèche ou relativement sèche, marquée par les mois les plus froids, la majeure partie de la moitié septentrionale, plus précisément 60 % du pays, correspond à des zones arides ou semi-arides, qui reçoivent, en moyenne, moins de 500 mm de pluie par an. Cette vaste région, située entre le tropique du Cancer et le 3...

In Cameroon, a 3-year randomized, double-blind controlled trial was conducted to determine if ivermectin, given at 3-monthly intervals and/or at high doses (800 microg/kg), had a greater effect on adult Onchocerca volvulus than standard annual doses of 150 microg/kg. Adverse reactions were recorded and analysed in a logistic regression model with random effects to assess the influence of the dose and rhythm of treatment on their occurrence. After the first dose, 3-monthly treatment was associated with a clearly reduced risk of reactions, especially oedematous swellings, pruritus and back-pain. Oedematous swellings and subjective ocular troubles were found to be associated with high doses of ivermectin. These results reinforce former parasitological conclusions that it would be desirable to evaluate the feasibility and effects on transmission of large-scale 3-monthly treatments with standard doses of ivermectin for onchocerciasis control. Owing to the unexpected ocular reactions, the use of high doses to counteract any future resistance of O. volvulus to ivermectin should be considered with caution.

Heat fluxes are estimated across transatlantic sections made at 4°30′S and 7°30′N in January–March 1993, following Hall and Bryden (1982. Deep-Sea Research 29, 339–359). Particular care is given to the computation of Ekman volume and heat fluxes, which are assessed both (a) from the windstress data for the period of the cruise and (b) from the comparison between geostrophic and Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP) velocities. In contrast with previous studies, the two estimates for Ekman fluxes do not converge for either section: (a) (11.5±0.5 Sv; 1.01±0.05 PW) across 7°30′N and (−9.3±1.2 Sv; −0.85±0.12 PW) across 4°30′S when windstress data at the date of the hydrographic stations are used; (b) (6.3±1.1 Sv; 0.56±0.09 PW) across 7°30′N and (−3.4±3.0 Sv; −0.35±0.24 PW) across 4°30′N when the ageostrophic transport above the thermocline is used. The divergence would have been even greater at 4°30′S if the strong ageostrophic signal beneath the thermocline, which brings a transport of (8.4 Sv; 0.82 PW), had been considered. The corresponding total meridional heat fluxes are: (a) 1.40±0.16 PW and (b) 0.95±0.20 PW across 7°30′N, (a) 1.05±0.12 PW and (b) 1.67±0.14 PW (2.39±0.14 PW when the subthermocline ageostrophic transport is taken into account) across 4°30′S. The estimates based on windstress data are compared with the results from an inverse model (Lux and Mercier, 1999) to show the importance of the heat flux due to the deviation of the local depth-averaged potential temperature from its average over the section, which is neglected in the Hall and Bryden (1982. Deep-Sea Research 29, 339–359) method but is not negligible in our computation in which we do not isolate the transport of the western boundary current east of the 200 m isobath; this corrective flux amounts here to −0.19 PW across 7°30′N and 0.33 PW across 4°30′S. The seasonal variability of the meridional heat flux across 7°30′N is studied through the hydrographic data collected during the ETAMBOT 1–2 cruises, which repeated the 7°30′N section west of 35°W in September 1995 and April 1996. When the section is completed east of 35°W with CITHER 1 data and when windstress data are used for the computation of the Ekman transport, the estimates for the meridional heat fluxes are 0.20±0.14 PW in September 1995 and 1.69±0.27 PW in April 1996. The estimates fit well with results from numerical models.

Thanks to the agreement found between in situ measurements and TOPEX/POSEIDON data in the western tropical Atlantic Ocean, a realistic picture of the spatial and temporal variability over the 1992–1997 period is obtained. The sea level variability clearly emphasizes three ranges of variability. The intraseasonal variability is associated with propagating features north of the equator, consistent with a first baroclinic Rossby wave characteristic. The annual variability, which represents the largest part of the variability, describes the seasonal cycle of the North Equatorial Countercurrent; but, more interesting, is a clear year-to-year variability in the northernmost part of the area. The surface currents also reveal an intraseasonal tendency with a peak of energy at 62 days at 5°N. Upper layer volume transport across 38°W and between 3°N and 9°N shows a regular seasonal contrast mostly due to the 3°N–6°N region. Year-to-year variations are also clearly evidenced. The eastward transport loses about 35% of its strength in the second half of 1995 compared with 1994. This event is followed by a stronger than usual westward transport in early 1996, and in early 1997, the transport seems abnormally eastward. However, contrary to the seasonal cycle, this interannual variability occurs mainly in the 6°N–9°N band. Interhemispheric transport computed across 7°30′N, between 50°W and 35°W, is northward during the whole period 1992–1995. A seasonal cycle can be detected with maximum transport during boreal winter and minimum in spring. An interesting result is the increasing tendency of the northward transport from April 1993 until boreal fall 1995, when the maximum value for the period is reached.

Expendable bathythermograph (XBT) data and the climatological temperature/salinity relationship were used to calculate the mean annual cycle of dynamic height and geostrophic transport of major currents relative to 400 db along five shipping tracks covering a large part of the tropical Pacific Ocean. The data were selected in bands centered on the most frequently repeated XBT tracklines for the period 1967-1988. Long-term bimonthly mean temperature was calculated in 1° latitude bins along the tracks. The transport function (vertically integrated dynamic height) was then calculated using the mean temperature/salinity relationship. The mean annual cycle of transport of the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC) (south of 2.5°S) were determined between the ridges and troughs of the transport function. The stochastic errors in bimonthly mean transports were 1-2 Sverdrups on the most sampled tracks. Mean transports of the NEC and NECC increase regularly with longitude from east to west. The NECC has a large annual cycle with a transport-maximum during northern fall and winter. Seasonal variations of the NEC are small. Seasonal variations of the SEC are slightly larger, and they have considerably different phase from track to track. The variation of thermal structure associated with the currents is described. The results of this study are compared in detail to the results of earlier studies of the transports. The differences between the studies are larger than the expected stochastic errors in the mean transports due to differences in the definition of boundaries of the currents and to differences in the procedure for calculating the mean annual variation. The results of all the studies are summarised to facilitate future comparisons to ocean general circulation models and other applications. (Résumé d'auteur)

The production of aboveground soft tissue represents an important share of total net primary production in tropical rain forests. Here we draw from a large number of published and unpublished datasets (n=81 sites) to assess the determinants of litterfall variation across South American tropical forests. We show that across old-growth tropical rainforests, litterfall averages 8.61±1.91 Mg ha−1 yr−1 (mean ± standard deviation, in dry mass units). Secondary forests have a lower annual litterfall than old-growth tropical forests with a mean of 8.01±3.41 Mg ha−1 yr−1. Annual litterfall shows no significant variation with total annual rainfall, either globally or within forest types. It does not vary consistently with soil type, except in the poorest soils (white sand soils), where litterfall is significantly lower than in other soil types (5.42±1.91 Mg ha−1 yr−1). We also study the determinants of litterfall seasonality, and find that it does not depend on annual rainfall or on soil type. However, litterfall seasonality is significantly positively correlated with rainfall seasonality. Finally, we assess how much carbon is stored in reproductive organs relative to photosynthetic organs. Mean leaf fall is 5.74±1.83 Mg ha−1 yr−1 (71% of total litterfall). Mean allocation into reproductive organs is 0.69±0.40 Mg ha−1 yr−1 (9% of total litterfall). The investment into reproductive organs divided by leaf litterfall increases with soil fertility, suggesting that on poor soils, the allocation to photosynthetic organs is prioritized over that to reproduction. Finally, we discuss the ecological and biogeochemical implications of these results.

The Saloum river (Senegal, West Africa) is an inverse estuary, with salinities of more than 80 g l−1 reached 100 km from the sea. Monthly salinity measurements have been done 120 km inland since 1927. Seasonal salinity increase (during dry season) proceeds at a constant rate (⋍ 0.3 g l−1d−1). This would indicate that evaporating water masses are shallow (average depth ⋍ 0.4 m). Since 1950, annual maximum and minimum salinities have been increasing, with decreasing rains, at a rate of about 1.3 g l−1 per year. Across the 1927–1987 period, both yearly extremes are well correlated with rainfall in the previous years, indicating a “memory” spanning three years or less. We have computed a water budget as a function of rainfall with three different hypotheses about the extent of the evaporating surfaces. Comparison with actual data indicate that about 60% of the lowlands are evaporating as shallow open waters would. We discuss the implications of these results for the possible future of the estuaries in the region, especially in the “green-house effect” hypothesis.

In the more than 80 references related to banded vegetation patterns, five continents and 12 countries are represented. The early research concerned only the vegetation pattern itself (Clos-Arceduc 1956). Hypotheses regarding ecological functioning were then proposed between 1956 and 1970 and clearly synthesized by (1971) for seven countries. He defined the minimum common characteristics necessary for the existence of banded vegetation spatial structure. These characteristics are now well known and include a semiarid climate, high-intensity rainfall, and a gentle slope. These characteristics are cited in almost all the publications on banded vegetation. We will not further review them here but focus instead on what they imply: the importance the spatial redistribution of water in the dynamics and the functioning of tiger bush bands. The areas of sealed, substantially bare soils yield a high proportion of runoff. On encountering the thicket bands immediately downslope, the flow rate diminishes and infiltration occurs, thus providing an additional water supply to the thicket. This process is more active in the upslope part of the thicket and progressively decreases to nil on the downslope edge, where many dead trees are observed. The zone located immediately upslope of the thicket is often called the “pioneer front” (Table 5.1) and is a zone of active plant colonization.

Soil erodibility - its susceptibility to erosion - is usually expressed by the K factor of the universal soil loss equation. There have been relatively few papers in Brazil involving soil erodibility studies, due to the tardiness in obtaining results from natural rainfall experiments. The knowledge of erodibility is important for conservation planning, which contributes to soil sustainability. The present study was developed in the 1989 to 1993 period at the Empresa Brasileira de Pesquisa Agropecuaria - Centro Nacional de Pesquisa de Solos (EMGOPA/EMBRAPA-CNPS) Experimental Station, in Senador Canedo, Goiania county, State of Goias, Brazil. The objectives were to identify correlations between different rainfall parameters and soil losses, and to determine the erodibility for a clayey, cerrado phase, Dark Red Latosol (Anionic Acrustox), from the Goiania region, under natural rainfall. No significative difference was found between the correlation coefficients for the different rainfall parameters. The estimated erodibility (K factor) was 0.009 t h M/j/mm. It was observed that the erosivity (expressed ty the EI30 parameter) values above 500 MJ mm/ha/h/month were enough to cause soil losses beyond the tolerance for the studied soil (1.5 t/ha/month), indicating the need for conservation practices to maintain sustainability. (Résumé d'auteur)

International audience

La connaissance et l'utilisation des ressources en eau est d'une importance vitale dans toute la partie semi-aride du Nordeste brésilien, qui couvre plus de 800.000 km2. Pour cela, depuis près de trente ans, on a entrepris le suivi et l'analyse du comportement hydrologique de plusieurs dizaines de petits bassins versants représentatifs, choisis pour représenter les principaux types de bassins. On constate une grande diversité des régimes hydrologiques, aggravée par une irrégularité climatique extrême. Une modélisation hydrologique systématique a permis d'identifier deux grandes zones climatiques, Sertao et Transition, à l'intérieur desquelles, chaque 100 mm de pluie supplémentaire provoque un accroissement de 40% des écoulements moyens annuels. Une typologie des bassins a été établie en fonction, principalement, des caractéristiques des sols et de la couverture végétale. Nous proposons une méthode de transposition des résultats obtenus vers des bassins dépourvus d'information hydrologique. Cette méthode permet une estimation simple des ressources en eau et des débits de pointe des crues en fonction des sols, de la pluviométrie moyenne du bassin et d'autres facteurs correctifs. (Résumé d'auteur)

The data provided by ships of opportunity operated by SURTROPAC Group (Centre ORSTOM de Noumea) are used to study surface temperature and salinity. In 1979 and 1982, negative anomalies of salinity occurred in the equatorial western Pacific. These are usually induced by El Nino events, although no El Nino was noticed in the eastern Pacific during the beginning of the year. However, along the Tahiti–Panama shipping route, the warm season in 1979 and 1982 lasted longer than usual. Empirical orthogonal analysis applied to the surface temperature along shipping routes in the eastern and central Pacific reveal other events with the same signature as the 1976 El Nino. A weak event occurred in 1979 and we can also see the beginning of the 1982 major event. Eastern, central and western anomalies are significant correlated.

Forme et mesure au microscope optique des graines d'amidon chez Euphorbia alluaudii s.sp. alluaudii, variation selon l'origine, a l'interieur d'un meme pied et au cours de l'annee. Etude chez 18 especes. Le caractere de forme des grains ne peut pas etre pris comme critere taxonomique