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.
Annual series of light rainfall, moderate rainfall and heavy rainfall are computed for 4 zones arranged from south to north in Nigeria: Coastal, GuineaSavanna, Midland and Sahelian zones. Daily rainfall data for the period 1919-85 are utilized. Each series is examined for evidence of change in structure in terms of pattern of decrease and increase in dry and wet years, the overall trend, and the occurrence of runs of dry and wet years. The northern Nigeria (Midland and Sahel) heavy rainfall series and the Sahel moderate rainfall series are found to depict evidence of climatic change as defined by Landsberg (1975) that climatic conditions must change to a new equilibrium position with the values of climatic elements changed significantly. On the other hand Landsberg's definition of climatic fluctuations as involving temporary deflection which can revert to earlier conditions is found to fit the 4 regional light rainfall series and the Midland area moderate rainfall series. The southern Nigeria moderate and heavy rainfall series are found to depict only evidence of high frequency oscillations about a stable long-term mean. The recent drought in Nigeria north of about 9 ~ N is shown to be associated with a large decline in moderate and heavy rainfalls over this part of the country.
Rainfall amount and rainfall frequency for five individual dry and five individual wet years were compared with the 1941-1987 averages for Nigeria. Five spatial-anomaly types were found to emerge for dry years, namely above average rainfall in southern Nigeria but below average rainfall in northern Nigeria (spatial type I), above average rainfall in the centre but below average rainfall in the coastal and extreme northern parts of the country (spatial type 11) and below average rainfall in southern Nigeria but above average rainfall in northern Nigeria (spatial type 111). The other two anomaly types are country-wide occurrence of below average rainfall (spatial type IV) and country-wide occurrence of above average rainfall (spatial type V). These spatial-anomaly types also hold true for wet years, except that the spatial type I1 is replaced by the Occurrence of below average rainfall in the centre but above average rainfall in the coastal and extreme northern parts of the country (spatial type VI).The spatial type I and I1 anomalies support the hypothesis of a restricted northward advance of the Inter Tropical Discontinuity (ITD) in dry years and on a complementary basis the spatial patterns for the rainfall amount anomaly (RAA) and the rainfall frequency anomaly (RFA) depicted these two anomaly types in 40 per cent of the monthly periods for the set of dry years considered. The spatial type IV anomaly, which occurs during the peak of the rainy season, supports the hypothesis of a weakening of the rainy season intensity and on a complementary basis the spatial patterns for the RAA and the RFA depicted this pattern in 30 per cent of the monthly periods for the dry years studied. This shows that both hypotheses are valid for explaining dry years in subtropical West Africa.The spatial type I11 and V anomalies support the hypothesis of a considerable northward incursion of the ITD in wet years. It was found that the spatial patterns of the RAA depicted these anomaly types in 53.3 per cent of the monthly periods whilst the spatial patterns of the RFA depicted the anomaly types in 66.7 per cent of the monthly periods for the set of wet years considered.
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