INFLUENCE OF THE QUASI-BIENNIAL OSCILLATION ON RAINFALL PRODUCING SYSTEMS OVER WEST AFRICA

Abstract

Rainfall is a major ingredient for economic development for most West African countries. It has serious implications on the socio-economic activities because more than 95% of its agricultural activities are rainfed and rain-fed agriculture serves as the main source of income for most of the economies. Therefore, rainfall variability in terms of amount and time can lead to good or poor agricultural production as either too much rainfall causes floods and water logging or very little rainfall results in inappropriate agricultural planning and activities. This study used ERA-Interim reanalysis and observational GPCP v2.2 to investigate the seasonal rainfall variability and relationship with the tropospheric jets. This work focuses on the Influence of the Quasi-biennial Oscillation on Rain Producing Systems over West Africa also the relationship of the sub-tropical westerly jet (STJ) with the West African monsoon and its associated tropospheric jets. The association and relative influence of the low-level West African Westerly Jet (WAWJ), the AEJ and TEJ during wet and dry situation were also examined. Results showed that the sudden appearance of the TEJ in June and the intensification of the AEJ are linked to the weakening and disappearance of the stratospheric easterly flow (QBO) and the fast poleward retreat of the STJ. A close association between the northward movements of the AEJ core and rainfall belt was found. However, no clear relationship was seen between the northward advance of the rainfall distribution and the TEJ slow movement, as this jet core always lags the rainfall maximum. By considering rainfall variability causes, results also showed that the Subtropical jet (STJ) retreated poleward faster while the TEJ, WAWJ and ascending motion were all stronger, resulting in higher rainfall during wet years than dry. Furthermore, it vii was found that the so-called monsoon jump and the reversal of the shape of the monsoon layer both occur in May/June and that this may be linked to the weakening of the QBO and the sudden poleward retreat of the sub-tropical jet in the same months. The influence of stratospheric quasi-biennial oscillation (QBO) on the African Easterly Jet, Tropical Easterly Jet and West African precipitation was investigated through simulations using the Global Climate Model (GCM) using the Coupled Models Intercomparison Project Phase 5 (CMIP5). The performance of the CMIP5 models in reproducing the quasi-biennial oscillation was evaluated while the influence of the QBO on West African rainfall precipitation was studied using wavelet analysis over each zone of the regions (Sahel, Savannah, Guinea). Also, the composite of the effect during QBO and non-QBO years as well as of the QBO phases on rainfall variability were studied. The results show that all the models capture the general structure of the QBO but with some biases while HadGEM2-CC produced results closer to observation (ERAINT). It was also found that there is good coupling between QBO and precipitation over all zones of West Africa. The wavelet coherence analysis gives confirmation of the results. From this study it is concluded that the quasi-biennial oscillation has an important influence on West Africa precipitation that could lead to improved rainfall prediction over West Africa.