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

SummaryThe 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.ZusammenfassungDie zeitlichen Charakteristika der Niederschlagsvariabilität in Afrika werden mit Hilfe von Abweichungsserien von 84 Gebieten des Kontinents und 5 großräumigeren Zonen untersucht. Die untersuchten nicht zufälligen Aspekte sind lineare Trends, Persistenz und quasiperiodische Fluktuationen. Es lassen sich keine langfristigen Trends im Niederschlag Afrikas beweisen. In einigen Regionen, am deutlichsten an der bengalischen Küste und im äquatorialen Afrika, neigen abnormale Niederschlagsverhältnisse dazu, über mehrere Monate anzudauern und die Korrelationen zwischen den Jahreszeiten sind hoch. Spektralanalysen zeigten signifikante Periodizitäten, die in 4 Periodenbändern gehäuft waren: 2.2–2.4, 2.6-2.8, 3.3–3.8 und 5.0–6.3 Jahre. Das kann in ganz Äquatorial- und Südafrika nachgewiesen werden, kaum dagegen in Nordafrika. Eine Kreuzspektralanalyse mit der „Southern Oscillation” legt einen starken Einfluß auf die Niederschlagsvariabilität für Süd- und Teile Äquatorialafrikas nahe und nahezu keinen für Nordafrika. Die Koherenz mit der „Southern Oscülation” ist am stärksten im Bereich der „Quasi Biannual Oscillation” von 2.2–2.4 Jahren. Allgemein legt das Koherenzspektrum einen inversen Zusammenhang zwischen „Southem Oscillation Index” und Niederschlag in den Äquatorialregionen (das bedeutet: mehr Niederschlag in Jahren mit niedrigerem Index) und einen direkten Zusammenhang in den meisten anderen Gebieten, in denen der Einfluß nachgewiesen werden kann, nahe.

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