Nadège Martiny

Compared regimes of NDVI and Rainfall in semi-arid regions of Africa

Bi‐monthly normalized difference vegetation index (NDVI) at an 8 km spatial resolution from the advanced very high resolution radiometers (AVHRR) was used from 1981 to 1995 to analyse the vegetation response to rainfall supply in semi‐arid regions of Africa. Within the 200–600 mm annual rainfall belt, for which the apparent NDVI response to rainfall was the strongest, three regions were selected which exhibited different patterns in their NDVI regimes and/or relationships with rainfall. The regions, located in western, southern and eastern Africa, were split into coherent sub‐regions in terms of mean regime of photosynthetic activity through a cluster analysis. Overall, intra‐regional differences were found to be of relatively minor importance compared with inter‐regional differences. The mean annual rain‐use efficiency (RUEa) was the highest in southern Africa, followed by that in eastern Africa, and it was the lowest in western Africa. In eastern Africa, a distinctive feature was found which consisted of the dissimilar vegetation response to the two rainy seasons. For a given rainfall amount, the NDVI peak was comparatively higher for the short versus the long rains. Then, in all three regions, the correlation between rainfall and NDVI regimes was found to be significantly high. Moreover, the lag between the rainfall and NDVI peaks, which was found to be the smallest in western Africa (1 month), and the highest in southern Africa (over 1.5 month), was determined to be related to the increased rainfall rate before the peak. In view of these results, it is shown that the dominant cause for the inter‐regional discrepancies, at the spatial and temporal scales considered, is the structure of the rainy season (distribution, concentration) more than the potential evapotranspiration, vegetation cover or soil type.

Characterization of the interannual and intraseasonal variability of west African vegetation between 1982 and 2002 by means of NOAA AVHRR NDVI data

Abstract The interannual and intraseasonal variability of West African vegetation over the period 1982–2002 is studied using the normalized difference vegetation index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR). The novel independent component analysis (ICA) technique is applied to extract the main modes of the interannual variability of the vegetation, among which two modes are worth describing. The first component (IC1) describes NDVI variability over the Sahel from August to October. A strong photosynthetic activity over the Sahel is related to above-normal convection and rainfall within the intertropical convergence zone (ITCZ) in summertime and is partly associated with colder (warmer) SST in the eastern tropical Pacific (the Mediterranean). The second component (IC2) depicts a dipole pattern between the Sahelian and Guinean regions during the northern summer followed by a southward-propagating signal from October to December. It is associated with a north–south dipole in convec...

Seasonality of meningitis in Africa and climate forcing: aerosols stand out

Bacterial meningitis is an ongoing threat for the population of the African Meningitis Belt, a region characterized by the highest incidence rates worldwide. The determinants of the disease dynamics are still poorly understood; nevertheless, it is often advocated that climate and mineral dust have a large impact. Over the last decade, several studies have investigated this relationship at a large scale. In this analysis, we scaled down to the district-level weekly scale (which is used for in-year response to emerging epidemics), and used wavelet and phase analysis methods to define and compare the time-varying periodicities of meningitis, climate and dust in Niger. We mostly focused on detecting time-lags between the signals that were consistent across districts. Results highlighted the special case of dust in comparison to wind, humidity or temperature: a strong similarity between districts is noticed in the evolution of the time-lags between the seasonal component of dust and meningitis. This result, together with the assumption of dust damaging the pharyngeal mucosa and easing bacterial invasion, reinforces our confidence in dust forcing on meningitis seasonality. Dust data should now be integrated in epidemiological and forecasting models to make them more realistic and usable in a public health perspective.

Interannual persistence effects in vegetation dynamics of semi‐arid Africa

[1]xa0Over 15 years of Normalized Difference Vegetation Index (NDVI) data from the Advanced Very High Resolution Radiometers (AVHRR) are used to study the response of vegetation activity to rainfall in three semi-arid regions of Africa. The relationships between annual NDVI and annual precipitation (PPT) time series are examined using statistical approaches (simple and partial correlations, linear multiple regressions). It appears that annual NDVI highly depends on PPT of the concurrent year and the previous year. An analysis of particularly dry and wet years enables to better diagnose two distinct responses of vegetation activity to rainfall. The “recovery” effect represents the difficulty of vegetation to recover from previous two-year drought conditions. The “memory” effect represents the capacity of semi-arid ecosystems to benefit from a water surplus at a one-year time-lag.

Interannual memory effects for spring NDVI in semi‐arid South Africa

[1]xa0Almost 20 years of Normalized Difference Vegetative Index (NDVI) and precipitation (PPT) data are analysed to better understand the interannual memory effects on vegetation dynamics observed at regional scales in Southern Africa (SA). The study focuses on a semi-arid region (25°S–31°S; 21°E–26°E) during the austral early summer (September–December). The memory effects are examined using simple statistical approaches (linear correlations and regressions) which require the definition of an early summer vegetation predictand (December NDVI minus September NDVI) and a consistent set of potential predictors (rainfall amount, number of rainy days, rainfall intensity, NDVI and Rain-Use-Efficiency) considered with 4 to 15-month time-lag. An analysis over six SA sub-regions, corresponding to the six major land-cover types of the area reveals two distinct memory effects. A “negative” memory effect (with both rainfall and vegetation) is detected at 7 to 10-month time-lag while a “positive” memory effect (with vegetation only) is observed at 12 to 14-month time-lag. These results suggest that interannual memory effects in early summer vegetation dynamics of semi-arid South Africa may preferably be driven by biological rather than hydrological mechanisms.

The association between respiratory tract infection incidence and localised meningitis epidemics: an analysis of high-resolution surveillance data from Burkina Faso

Meningococcal meningitis epidemics in the African meningitis belt consist of localised meningitis epidemics (LME) that reach attack proportions of 1% within a few weeks. A meningococcal serogroup A conjugate vaccine was introduced in meningitis belt countries from 2010 on, but LME due to other serogroups continue to occur. The mechanisms underlying LME are poorly understood, but an association with respiratory pathogens has been hypothesised. We analysed national routine surveillance data in high spatial resolution (health centre level) from 13 districts in Burkina Faso, 2004–2014. We defined LME as a weekly incidence rate of suspected meningitis ≥75 per 100,000 during ≥2 weeks; and high incidence episodes of respiratory tract infections (RTI) as the 5th quintile of monthly incidences. We included 10,334 health centre month observations during the meningitis season (January-May), including 85 with LME, and 1891 (1820) high-incidence episodes of upper (lower) RTI. In mixed effects logistic regression accounting for spatial structure, and controlling for dust conditions, relative air humidity and month, the occurrence of LME was strongly associated with high incidence episodes of upper (odds ratio 23.9, 95%-confidence interval 3.1–185.3), but not lower RTI. In the African meningitis belt, meningitis epidemics may be triggered by outbreaks of upper RTI.

Timing and patterns of the ENSO signal in Africa over the last 30 years: insights from normalized difference vegetation index data.

A more complete picture of the timing and patterns of the ENSO signal during the seasonal cycle for the whole of Africa over the three last decades is provided using the normalized difference vegetation index (NDVI). Indeed, NDVI has a higher spatial resolution and is more frequently updated than in situ climate databases, and highlights the impact of ENSO on vegetation dynamics as a combined result of ENSO on rainfall, solar radiation, and temperature. The month-by-month NDVI‐Ni~ correlation patterns evolve as follows. From July to September, negative correlations are observed over the Sahel, the Gulf of Guinea coast, and regions from the northern Democratic RepublicofCongotoEthiopia.However,theyarenotuniforminspaceandaremoderate(;0.3). Conversely, positive correlations are recorded over the winter rainfall region of South Africa. In October‐ November, negative correlations over Ethiopia, Sudan, and Uganda strengthen while positive correlations emerge in the Horn of Africa and in the southeast coast of South Africa. By December with the settlement of the ITCZ south of the equator, positive correlations over the Horn of Africa spread southward and westward while negative correlations appear over Mozambique, Zimbabwe, and South Africa. This pattern strengthens and a dipole at 188S is well established in February‐March with reduced (enhanced) greenness during ENSO years south (north) of 188S. At the same time, at ;28N negative correlations spread northward. Last, from April to June negative correlations south of 188S spread to the north (to 108S) and to the east (to the south of Tanzania).

Towards understanding the epidemiology of Neisseria meningitidis in the African meningitis belt: a multi-disciplinary overview.

OBJECTIVESnNeisseria meningitidis is the major cause of seasonal meningitis epidemics in the African meningitis belt. In the changing context of a reduction in incidence of serogroup A and an increase in incidence of serogroups W and C and of Streptococcus pneumoniae, a better understanding of the determinants driving the disease transmission dynamics remains crucial to improving bacterial meningitis control.nnnMETHODSnThe literature was searched to provide a multi-disciplinary overview of the determinants of meningitis transmission dynamics in the African meningitis belt.nnnRESULTSnSeasonal hyperendemicity is likely predominantly caused by increased invasion rates, sporadic localized epidemics by increased transmission rates, and larger pluri-annual epidemic waves by changing population immunity. Carriage likely involves competition for colonization and cross-immunity. The duration of immunity likely depends on the acquisition type. Major risk factors include dust and low humidity, and presumably human contact rates and co-infections; social studies highlighted environmental and dietary factors, with supernatural explanations.nnnCONCLUSIONSnEfforts should focus on implementing multi-country, longitudinal seroprevalence and epidemiological studies, validating immune markers of protection, and improving surveillance, including more systematic molecular characterizations of the bacteria. Integrating climate and social factors into disease control strategies represents a high priority for optimizing the public health response and anticipating the geographic evolution of the African meningitis belt.

Atmospheric corrections over coastal waters for SeaWiFS: validation using ground-based measurements

We use ground-based measurements in order to validate the SeaWiFS calibration as well as the SeaWiFS atmospheric correction scheme over coastal waters. Ground-based extinction measurements provide the required information to compare to the SeaWiFS aerosol products. For most of the cases, the aerosol optical thicknesses at 865 nm agree while large discrepancies occur in the blue. The SeaWiFS algorithm quite systematically selects the maritime model while the actual aerosol models correspond to a broader variety offered by the complexity of the sources near by the European coastal areas. The radiometer also measures at several wavelengths the atmospheric downwelling radiances in the almucantar and in the principal plane (PPL). Knowing by measurements the aerosol optical thickness (tau) a, a simple scheme was developed in order to derive the aerosol phase function Pa from the PPL radiances. We then have the required inputs to a radiative transfer code to properly account for the aerosols. In a forward mode, it is then applied to simulate the signal at SeaWiFS time of overpass in order to express the TOA radiances for comparison to the SeaWiFS measurements. In a backward mode, it can be used to convert the top of atmosphere radiance into a water leaving radiance. We present here some preliminary results for 3 days over the Venice site in Italy.

Le climat, un facteur de risque pour la santé en Afrique de l'Ouest.

Cet article fait le point sur certaines connaissances acquises dans le cadre du programme AMMA (phase 1) au sujet des impacts de la variabilite climatique sur la sante en Afrique de lOuest. Les auteurs ont travaille dune part sur les maladies sevissant majoritairement en saison des pluies, a vecteur tel que le paludisme a Plasmodium falciparum, ou non comme le cholera, et dautre part sur les maladies respiratoires et non vectorielles, telles que les meningites bacteriennes a meningocoques a Neisseira Meningitidis, qui sevissent en plein coeur de la saison seche. Le paludisme est connu pour sa relation a leau et a la temperature, indispensable pour le developpement de son vecteur et son parasite. Le cholera, bien que lie a leau, peut aussi etre observe en saison seche : le lien entre cholera et environnement merite donc un etat des lieux eclaire par les nouveaux parametres et outils disponibles aujourdhui du cote du climat. Lintensite de la saison de la meningite est tres liee aux conditions de vent et dhumidite predominants en octobre-novembre de lannee precedente. Son demarrage est en phase avec le maximum de lhiver et larrivee des grands evenements de poussieres dans les basses couches atmospheriques. Enfin, son arret est tres lie a larrivee de la mousson. Ces resultats marquent une etape importante dans la mise en place de systemes dalerte precoce.