Abstract:
Monthly mean of rainfall data (mm) between January to December for the period 1901 to 2006 for 23 stations (grid points) over Nigeria were analysed in this study. The trends of rainfall over the 23 stations were examined statistically based on the time series, mean, standard deviation of the rainfall data and sub-dividing the study period into 30-year Climatic means and comparing with World Meteorological Organization (WMO) Climatic Normal Period of 1961 to 1990. Results obtained showed that the annual long-term mean precipitation in all the stations exhibit a spatial variation of values ranging between less than 3000mm in the Southern region and greater than 300mm in the Northern region and long-term annual rainfall also decreases from North to South in response to the Annual migration cycle of ITD from south to north. Time series and change detection analysis performed revealed an interannual variation of rainfall trends in the stations considered while annual rainfall gradually decreases in the 1960s and 1970s, a period of change which lasted till 1990s and subsided at the start of 21st century resulting in wetter interannual variability. Spatial variability of rainfall over Nigeria was also investigated by Principal Component Analysis and Surfer plots. First three of explained variance with cumulative 72% were considered and associated with prominent factors influencing rainfall in Nigeria. The first, which account for about 52% explained variance, can be attributed to the effect of ITD which traverses South-North during the course of annual rainfall cycle in Nigeria. Second, which account for about 14.5% can be attributed to isolated topographical features such as mountains, large body of water, topography of the stations, altitude (high or low) and so on. The third, which is about 5.5%, can be attributed to local effects e.g. diurnal heating cycle.
