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L'objectif de cette étude est d'analyser les liens entre les indices climatiques et la variabilité des séries de précipitations et de débits. Afin de mieux comprendre la non stationnarité des différentes stations, les données de débits et de pluviométries utilisées concernent la station de Tortiya (1960–1996). Les indices climatiques couplés à ces séries sont l'indice NAO (North Atlantic Oscillation) et l'ENSO (El Niño – Southern Oscillation) sur la même période d'étude. La méthodologie a consisté à appliquer l'analyse et la cohérence en ondelettes sur les séries temporelles. Ces méthodes ont mis en évidence les modes de variabilité dans les séries chronologiques : le mode infra annuel, le mode annuel et le mode interannuel (1–2 ans; 2–4 ans; 4–8 ans). D'une part, les résultats montrent que la variabilité du signal est expliquée dans les hautes fréquences (6 mois à 1 an) dans les différentes séries chronologiques. À cette fréquence, c'est le mode annuel (1 an) qui enregistre toute la variabilité du signal comprise entre 30 % et 70 %. D'autre part, ce travail présente aussi des signaux sur d'autres fréquences et périodes assez faibles. L'analyse en ondelette a révélé que le signal dominant est très largement significatif au niveau du cycle annuel. Par ailleurs, l'usage de la cohérence en ondelettes entre les indices climatiques (ENSO, NAO) et les précipitations, indiquent une forte influence du NAO sur les séries pluviométriques et de débits.

The proposed study is based on an analysis of rainfall variability and flood over more than half a century (1947-2014) in two countries in Sahel (Mauritania and Senegal). The rain analysis is based on the use of the chronological graphic method of information processing (MGCTI) of „Matrice Bertin” type. Applied to rainfall-relayed data, this method allows, in a first stage, the analysis of the spatiotemporal distribution of the meteorological parameter, then, in a second stage, the determination of data on cycle changes, if there are manifest cyclic, due to the regional analysis. The results demonstrate that at the end of the Sahelian climatic drought and a return of rains confirmed over the last decade of the studied period (1947-2014).

The type of cold anti-cyclonic winter weather represents a climatic situation that reinforces the stability of the low atmospheric layers. It creates ideal conditions for the occurrence of temperature inversions (the air is cooler in the lower layers than in the upper layers). These phenomena favor the blockage of pollutant particles and accentuate their concentration in atmosphere. In recent years, the alerts to pollution by particles have multiplied in France. This situation is new, because of the changed measurement protocol in January 2007 and by lowering the alert and information thresholds since January 2012. The pollution alerts which multiplied during the month of December 2016 generated a great political debate on this subject. The alternating traffic has been adopted twice in Paris. A "Crit'Air" vignette issued to the clean cars is now mandatory from 15 January 2017 to circulate in Paris. The present paper aims to study this new situation from the December 2016 pollution episode.

The results of different studies referring to evolution of precipitations show that, on a worldwide scale, the climatic change appears as an intensification of precipitations and a recurrence of extreme events (more noticeable during the last decennia). These studies also show an increase of precipitations in equatorial areas and at Poles, at the expense of Mediterranean and of dry and temperate tropical areas. Nowadays, Mediterranean area is considered as a hot spot area of climate change. The impact of this change and the high human pressure on the environment would result in a shortage of water for about 290 million people. The aim of this paper is to analyze rainfall variability in a wide area of North Africa (from the Morocco in the west up to Tunisia in the east). This study is undertaken for a period of nearly half a century. The rainfall data were collected from 35 meteorological stations. To analyze the time series signal in time and space, graphical method of information processing (MGCTI) was used.

In order to characterize the climatic changes observed on a regional and local scale, an analysis of the pluviometric parameters has been proposed. It relies on the calculation of the centered index decreased and the quintile method to characterize their long-term evolution. This investigation allows for an explanation for the extreme rain variability. It also shows the severe and durable character of the climatic drought noticed in this region beginning with the 1970s and the shy return towards a possible humid phase observed beginning with 2008. At the same time, the analysis of the evolution of the extreme rains has highlighted a relevant relation with the North Atlantic Oscillation (NAO). This relation is in reversed phase with the humid periods (negative NAO index and high frequency of the extreme rains from the humid categories) and in phase with the dry periods (positive NAO index and high frequencies of the extreme rains from the dry categories).