PATTERN OF RAINFALL ANOMALIES IN NIGERIA (1901-2000)

Abstract

The pattern of rainfall anomalies have been largely neglected in the literature on climatology in Nigeria in spite of their significance to agriculture. This study examined the pattern of rainfall anomalies in Nigeria with a view to understanding the possible causal factors.

Monthly rainfall, raindays and dew point temperature data for 27 meteorological stations were obtained from the Nigerian Meteorological Agency. Monthly dew point temperature data of five meteorological stations obtained from the National Meteorological Services, Niger Republic were used in determining the northernmost position of the Inter -Tropical Discontinuity (ITD). Monthly Sea Surface Temperatures (SSTs) of the Atlantic and Pacific oceans and Southern Oscillation Index obtained from the Climatic Prediction Center (CPC) were used in examining the relationships among El-Nino/Southern Oscillation (ENSO), sea surface temperatures and Nigerian rainfall anomalies. Standardized Precipitation Index (SPI), Percentage Deviation Below the Mean (PDBM) index, stepwise multiple regression, simple linear regression and Mann-Kendall tau statistics were used for data analyses.

The north, extreme northeast, south-west and south-east recorded 26, 25, 8 and 6 severe droughts (SPI: -1.50 to -1.99) respectively and all the regions recorded mild (SPI: 0 to -0.99) to moderate (SPI: -1.00 to -1.49) droughts. The causal factors of annual rainfall anomalies in the north and middle belt were South Atlantic SSTs (r2 =0.13, p<0.001) and North Atlantic SSTs (r2 =0.12, p<0.001) respectively, while seasonal rainfall anomalies in the north, middle belt and south-west were caused by South Atlantic SSTs (r2 =0.13, p<0.001), North Atlantic SSTs (r2 =0.10, p<0.001) and South Atlantic SSTs (r2 =0.08, p<0.001) respectively. Annual rainfall anomalies at five soudano-sahelian stations including Katsina, Nguru, Maiduguri, Kano and Sokoto were related to East-Central Pacific SSTs (r2 =0.44, p<0.001), Global SSTs (r2 =0.37, p<0.001), North Atlantic SSTs (r2 =0.29, p<0.001), North Atlantic SSTs (r2 =0.21, p<0.001) and South Atlantic SSTs (r2 =0.16, p<0.05) respectively. Inter-tropical discontinuity accounted for most of the spatial variations in rainfall (r2 =0.96, p<0.001). Annual rainfall anomalies showed a downward trend in the north (β = -0.17, p<0.001), middle belt (β = -0.27, p<0.001), south-west (β = -0.001, p<0.97) and south-east (β = -0.37, p<0.001). Annual rain days anomalies displayed a different pattern with a declining trend in the north (β = -0.22, p<0.001), middle belt (β = -0.04, p<0.001), south-west (β = -0.49, p<0.001) and upward trend in the south-east (β = 0.25, p<0.001).

The pattern of rainfall anomalies in Nigeria were caused by sea surface temperature anomalies over the Atlantic and Pacific oceans as well as seasonal movements of the inter-tropical discontinuity. These factors should therefore be considered for forecasting of rainfall anomalies in the country.

Keywords: Rainfall Anomalies, El-Nino/Southern Oscillation, Inter-tropical discontinuity. Word Count: 431