Quasi-periodic patterns in the East African rainfall records

Abstract

In this study East African rainfall records within the period 1922-80 were subjected to spectral analysis in order to examine whether the fluctuations in rainfall during 1922-80 exhibited any periodic or quasi-periodic patterns. The characteristics of the rainfall auto-correlations at the individual stations were also analyzed. The data used here included monthly and annual rainfall records of about 100 stations distributed all over East Africa. Fluctuations in the regionally averaged rainfall records were also examined. Results from spectral analysis indicated that a family of four spectral peaks were common in many rainfall records. The period of these peaks was centred generally around 2.2 to 2.8 years, 3.0 to 3.7 years, 4.8 to 6.0 years, and 10.0 to 12.5 years. Although all these spectral peaks appeared in many rainfall series, the proportion of the total rainfall variance explained by each varied significantly from region to region. Results- from auto-correlation analysis indicated that lag one auto-correlations were significantly different from zero at 16 stations for the annual records. The nature of persistence at these 16 stations was observed to be close to the linear markov type. "Red noise'" hypothesis was used in testing the statistical significance of the spectral peaks observed at these stations while the "white noise" hypothesis was employed in the other cases.