THE IMPACT OF EL NINO/SOUTHERN OSCILLATION ON RAINFALL CHARACTERISTICS IN FIVE CLIMATIC ZONES OF NIGERIA

Abstract

This research work is aimed at investigating the impact of El Nino/Southern Oscillation (ENSO) on Rainfall Characteristics at five climatic zones of Nigeria. The study specifically investigated the precipitation index for ENSO and Non ENSO years at each station; estimated the onset and cessation dates of rainfall for each of the stations; and compared the impact of ENSO and La Nina on rainfall characteristics which include Rainfall amount, Onset date, Cessation date, Rainy days, Dry days, August rainy days, August dry days and August amount across the stations. The Precipitation index (PI) was calculated for all the years and the Nature of Dry and Wet years were identified during El Niño, La Niña and normal years. Percentage cumulative mean rainfall method was used to estimate the onset and cessation date of rainfall at each of the stations. The long-term mean for each of the characteristics was calculated and compared with the yearly data for each of the selected stations. Cramer’s test was used to determine if there is any difference between the means of ENSO, La Nina and normal years. Results were compared between the ENSO years, La Nina years and normal years over a period of 30 years (1982 - 2011). Generally for all the stations, ENSO years annual rainfall amount is lower than the long term mean, onset is later and cessation of the rainy season is earlier than the normal years. Duration is shorter during ENSO years than other years and rainfall amount in August is lower in ENSO years. Precipitation index for all the stations shows that there are more droughts during ENSO years when compared with Normal years. It was shown that the effects of ENSO was felt more in Nguru (Sahel), Yola (Sudan-Sahel), and Minna (Guinea-Sudan), while, lesser effect of ENSO on rainfall characteristics was felt in Ibadan (Wooded savannah) and Calabar (Forest belt). For La Nina years which are direct opposite of ENSO years, across all the stations, annual rainfall amount is higher than the long term mean, onset is earlier and cessation of the rainy season is later than the normal years. Duration of the raining season is longer and rainfall amount in August is higher than the long term mean. Precipitation index for all the stations shows that there are no droughts during La Nina years. Since there are some teleconnections between ENSO and rainfall at the five climatic zones of Nigeria, it would be possible to predict extreme rainfall events like drought if the periodicity of ENSO can be understood.