A study of climatic variability in Nigeria based on the onset, retreat, and length of the rainy season

The long‐term variability of rainfall conditions in Nigeria in terms of the onset, retreat, and length of the rainy season has been analysed, using pentad data for the period 1919–1985. Data were grouped into four areas, arranged in a south‐north transect; the Coastal, Guinea‐Savanna, Midland and Sahelian Zones. The series for retreat of rainfall showed evidence for quasi‐triennial and quasi‐6‐year oscillations, while that for rainy season length displayed quasi‐biennial and quasi‐triennial oscillations. No consistent spectral peaks emerged for changes in the date of onset of the rainy season. There is spatial coherence in variation in the date of the retreat of rainfall over the whole country, whilst for the date of onset of the season spatial coherence is limited to southern Nigeria (Coastal and Guinea‐Savanna Zones). Northern Nigeria (Midland and Sahelian Zones) and southern Nigeria (Coastal and Guinea‐Savanna) emerge as distinct areas in terms of spatial coherence in the variation of the length of the rainy season. There is also evidence for a secular change in the date of the retreat of rainfall for the whole country during the period 1939–1985, and in the date of onset of rainfall for southern Nigeria for 1968–1985.