This study investigates the seasonal variations of rainfall and its decadal anomaly over Nigeria and the possible role of solar and geomagnetic activities. Sunspot number and geomagnetic aa index were used as solar indices. Monthly mean daily rainfall data from Nigeria Meteorological (NIMET) Agency was used in this study. Descriptive, bivariate, and power spectral density (PSD) analyses, as well as Mann-Kendall trend test were employed in analyzing the data, while the variability of rainfall was presented using ArcGIS software. Rainfall anomaly was calculated using the mean of the base period, 1981 – 2010. Seasonally, rainfall varies from the coastal areas to the hinterland from January to December during rainy and dry seasons. On the decadal rainfall anomaly, positive anomalies increases steadily towards the coastal regions from first decade (1951 – 1960) to sixth decade (2001 – 2010); indicating an increase in rainfall characteristic, whereas negative anomalies increases towards the northern regions; depicting decrease in rainfall in this region. This was confirmed from the Mann-Kendall trend test. These variations are evidence of climate change. Correlation analysis revealed that the correlation of rainfall with sunspot and aa index were not significant at 0.05 level of significant. The spectral analysis revealed that there were indications of Schwabe, Hale and Gleissberg cycles on rainfall spectrum. This depicts signatures of solar and geomagnetic activities. We therefore infer that, in addition to anthropogenic activities, solar and geomagnetic activities might play important role in the observed climate change in Nigeria; since rainfall is used as climate change indicator.
Keywords: Rainfall variability, solar and geomagnetic activities, climate change, Nigeria.