Spatial patterns of rainfall variability over western equatorial Africa

Finding the spatial patterns of rainfall variability is a fundamental practice in climatology and will enhance our understanding of regional climate processes. This study aims to reveal homogenous sub-regions over the poorly studied area of western equatorial Africa (10S-7N and 7E-30E). The analysis is based on interannual rainfall variability. We use monthly totals of 141 stations over the period 1955-1984. In addition to annual totals, four different seasons are examined separately, an approach that has been lacked in previous studies. The four 3-month seasons are defined as follows: January–February–March (JFM), April–May–June (AMJ), July–August–September (JAS) and October–November–December (OND). The most successful techniques recommended in the literature, are examined through two approaches: first, the rotated principal component analysis (RPCA) in conjunction with Ward's method, and second the RPCA in conjunction with k-means method. The principal components that explain about 65% of total variance are retained and then VARIMAX rotated. The corresponding scores are utilized as input for cluster analysis.

Using Ward's method, 5 sub-regions are recognized for AMJ, JAS and OND, and 4 sub-regions for JFM and annual data. The regions are well geographically distributed over the area and consist of roughly the same number of stations. The F-test is used to evaluate the homogeneity of each sub-region. The results show that, all sub-regions are significantly homogenous at the 1% probability level. Assuming the same number of clusters, the k-means method provides comparable spatial patterns with those of Ward's method. However, there are some differences, which are more evident in JAS and OND. Like Ward's method, the values of F-ratio for the k-means algorithm also confirm the homogeneity of all sub-regions for each season. These results show promising improvement in providing homogenous regions, a problem that has been mentioned in some previous studies.