The quasi-periodic behavior of rainfall variability in Africa and its relationship to the southern oscillation

Abstract

The temporal characteristics of rainfall variability in Africa are examined using departure series for 84 regions of the continent and five larger-scale zones. The forms of non-randomness which are investigated include linear trends, persistence and quasi-periodic fluctuations. No long-term trends in African rainfall are evident. In some sectors, most noteably along the Benguela coast and equatorial Africa, rainfall anomalies tend to persist over several months and interseasonal correlations are also high. Spectral analysis revealed significant quasi-periodicities clustered in four bands at 2.2–2.4, 2.6–2.8, 3.3–3.8 and 5.0–6.3 years. These are common throughout equatorial and southern Africa, but only weakly evident in northern Africa. A cross-spectral analysis with the Southern Oscillation suggests a strong influence on rainfall variability in southern Africa and parts of the equatorial region and minimal influence in northern Africa. Coherence with the Southern Oscillation is particularly strong in the QBO range of 2.2–2.4 years. In general, the co-spectra suggest an inverse relationship between the SOI and rainfall in equatorial regions (i.e., higher rainfall during low-index years) and a positive relationship with rainfall in most other regions where an influence can be demonstrated.