L'influence des centres d'action atmosphériquesdes açores,de l'île sainte-hélenne et du phénomène el NIÑO sur la dynamique spatio-temporelle de la pluviométrie au Cameroun

Abstract

Cameroon “Africa in miniature” is characterized by a great diversity of its climate. This area is pivotal for Central Africa due to its geographical position and like most African countries, Cameroon experiences the harmful effects of climatic disturbances. The objective of this work is to establish the influence of atmospheric action centers such as Azores, the island of Saint Helena and El Niño episodes on the spatio-temporal dynamics of rainfall in Cameroon. For this, the rainfall data collected daily in the main meteorological stations (pluviometric stations) of Cameroon between 1950 and 2015 were converted to monthly, seasonal and annual scales then, subjected to time series analysis, in order to establish the equations and trends. Through an inferential statistical analysis, the evolution of climatic trends at different scales was developed. The study made it possible to identify and assess the dates of the start and end of the rainy seasons, the intensity and distribution of the amounts of precipitation, the length of the said seasons, to develop the typologies of the amounts of precipitation and the situation of extreme rainfall events at the national level. In order to better understand the spatio-temporal dynamics of rainfall at the national level, an analysis of the interrelationships between the rainfall recorded in the main regional stations of Cameroon and the dynamics of the determining factors of the climate in particular, OST, atmospheric pressure, positions in the center of the Azores, Saint Helena Island highs and the intensities of El Niño episodes in the equatorial Pacific were recorded. These factors are, according to many authors, considered to be the mechanisms of the global climate system, and whose influence on the climate of the Gulf of Guinea area is proven. The analysis of the dynamics of the said factors, combined with the migration of the Intertropical Front, as described by numerous studies, makes it possible to better control the spatio-temporal dynamics of rainfall at the local level, and therefore to propose a seasonal model of climate forecasting. The following results emerge from this study: (1) a high degree of instability in the dates at the start and end at the rainy seasons is observed throughout the national territory. The rains sometimes start early, sometimes late, regardless of locality, with a significant impact on the length of the seasons; (2) a strong disturbance of the spatio-temporal distribution (monthly, seasonal and annual) of rainfall is observed all over the national territory; (3) the quantities of annual precipitation and the number of rainy days have sharply decreased, independent of the localities all over the national territory from 1950 to 2015. The recorded decrease is on average 30mm in the Extreme North and Adamaoua regions, between 50 and 75mm in the Center, South and East regions, between 50 and 200 mm in the South-West and Littoral regions.They remained around average in the Western and Northwestern regions and slightly increasing in the North region; (5) the annual averages recorded over the period are 863.4 mm of rainfall over an average of 72 days in the Far North region, between 1,500 and 1,600 mm over a period of between 125 and 140 days in the Central and Eastern regions, between 1600 and 1800 mm of rain spread over an average of 160 to 175 days in the South region, between 3000 and 3700mm, spread over an average of 188 days in the Southwest region, with records of around 9000mm of rain at Debundscha; (6) a heterogeneity of the typology of precipitation is observed in the different localities. In fact, three types of rainfall regimes (monomodal, bimodal and trimodal) have been identified and their importance varies from one locality to another. The difference between the types of rainfall regimes can be quantified by the length and duration of the seasons; (7) an upsurge in extreme climatic events is observed through the behavior of climate change indicators over the national territory as defined by the IPCC (8) a significant influence of OST, atmospheric pressure, the dynamic positions of the action centers of Azores, Saint Helena Island and El Niño episodes is observed on the dates of the start and end of the seasons, as well as on the spatio-temporal distribution of precipitation amounts at the national level ; (9) it is possible to develop a climate forecasting model at the local level on the basis of the behavior of interactions between the centers of atmospheric action of Azores, the island of Saint Helena and El Niño episodes on the rainfall recorded at the local level.