DETERMINATION OF THE ONSET OF RAINFALL IN NIGERIA FROM LOW-LEVEL WIND CIRCULATIONS

Abstract

The commencement of rainy season (onset) across Nigeria is highly variable with respect to stations north and south of the country. Associated with the onset are significant transitions of large scale atmospheric and oceanic circulation patterns which are observed over the Sahara and the Equatorial Atlantic regions. This study, proposes a new method of determining the monsoon onset in Nigeria, based on large scale circulation index over Nigeria. The ESSEMBLE dataset provided by the European project ESSEMBLE-based Predictions of Climate Changes and their Impacts (ESSEMBLES) and the daily observed rainfall dataset from Nigerian Meteorological Agency was used in this work. The Onset Circulation Index (OCI) is determined by averaging the daily zonal (u) wind at 850hPa over each station from the ESSEMBLE dataset for the period 1190 – 2007 (18years). The zonal wind on the mean onset date, Pc, is averaged for the period under study to obtain a single threshold value for each station. The onset of rainfall, OROCI, will be defined as the first day when the OCI exceeds the threshold value, with the provision that it (OCI) also exceeds that value in ensuing 6days and the wind consistently westerly within those days. A prominent onset precursor on the biweekly time scale is the westward extension of the convection center (Sub-Tropical Troughs) from the eastern high lands (Cameroun Mountains, and Adamawa hills) toward the western parts. On the intra-seasonal time scale, the onset is led by northeastward propagation of an intra-seasonal monsoon flow from the Atlantic Ocean. The determination of the onset based on the low-level westerly is a characteristic representation of the complex process of the West African Monsoon onset. The proposed Onset Circulation Index (OCI) onset definition provides a useful metric for verifying numerical model performance in simulating and predicting the onset of rainfall in Nigeria. The results show that the sudden commencement of strong rainfall over Nigeria from February (in Rain forest zone) and April-June (Other zones) to October/November in all climatic zones is closely related to the strengthening of the 850-hPa zonal wind to the west of Nigeria, extending from 200W to 150E along 20N to 150N latitudinal window. This is the basic idea of the Onset Circulation Index (OCI). The monsoon westerly was mainly monitored in ten locations (stations) over Nigeria (30E to 150E and 40N to 140N). The critical value ranges from 1ms-1 (in Southern Nigeria) to 3.8ms-1 (in Northern Nigeria). Stations locations in the Rainforest and Guinea zones show clearly double maxima pattern of annual rainfall. Sahel and some parts of Sudan zones show single peak rainfall pattern. This simple index has an average skill of 76% from the ten stations and 18 years data analyzed. The minimum mean error between the actual and the determined onset dates is zero (0) day for Ikeja; and the maximum of forty-two (42) days earlier for Ibi; implying that the average deviation of the obtained onset dates from the actual is nil and forty-two days for Ikeja and Ibi respectively.