Analysis of climate variability and of its influences on seasonal rainfall regimes in West Africa: case of the N'zi (Bandama) watershed in Ivory Coast

The climate variability was generally studied in West Africa and particularly in Ivory Coast. The rainfall variability was especially analysed. Therefore, rainfall variables such as rainy days frequencies and rainfall seasonal duration are little studied. The main purpose of the following work is to show the climate influence on seasonal rainfall regimes on the N'zi (Bandama) watershed. We have documented two main aspects of climate variability. In the first part, the climate variability was characterised by the analysis of air temperature, air humidity, rainfall index, rainfall frequencies days and seasonal rainfall duration. In the second step, we have compared the standard monthly rainfall for 1951-2000 period so as to analyse the behaviour of seasonal rainfall regimes in the climate variability context. An analysis in normalized principal components (ANPC) allowed the analysis of the impact of the climatic factors on the seasonal rainfall regimes. The climate variability was manifested by a decline of the spatial and temporal annual rainfall, a decrease of rainfall frequencies days particularly those which are superiors to 10 mm and a reduction of rainfall seasonal duration (30 to 35 days) since 1970. In this study, the most important result of our investigations is the lack of change in the monthly rainfall regimes on the N'zi (Bandama) watershed. The analysis in normalized principal components (ANPC) based on fourteen (14) variables, made it possible to highlight the impact of climate variability on the seasonal rainfall regimes. The first principal component (F-1) is associated with the factors defining the atmospheric conditions. The principal second and the third components (F-2 and F-3) are associated with the variables related to precipitations. As for the factors which influence the temporal variability of the seasonal pluviometric regimes, it appeared that the six rainfall variables are influenced by air temperature as well as the relative humidity of the air. It is thus concluded that air temperature and the relative humidity of the air are factors of the temporal variability of the seasonal rainfall regimes on the N'zi (Bandama) watershed.