influence of madden-julian oscillation (mjo) on rainfall variability over west africa at intraseasonal timescale

Abstract

intraseasonal variability of rainfall over West Africa plays a significant role in the economy of the region that are highly linked to agriculture and water resources. This study therefore has two aims. The study first evaluates the ability of the Atmospheric Model Intercomparison Project (AMIP) simulations performed by Atmosphere General Circulation Models (GCMs) forced with prescribed Sea Surface Temperature (SST) in producing the mean state of West African Monsoon (WAM). This is achieved by analysing the performance of models in reproducing the summer rainfall and temperature climatology, the moving rainbelt and the main dynamical features of WAM such as the strength and position of the African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ). Secondly, this research study investigated the relationship between the Madden Julian Oscillation (MJO) and rainfall over West Africa during the boreal summer as well as the dynamical processes involved using the AMIP type simulations. The results reveal that most of the models are capable of simulating the main features of the West African monsoon and also produce a realistic summer low-level circulation overWest Africa with more intense westerly anomalies over the maximum rainbelt zone. However, some models simulate an equatorward and earlier maximum of rainfall over Guinean coast from March to the end of May. As for the MJO, the simulations show in general good skill in capturing its main characteristics as well as its influence on rainfall over West Africa. On the global scale, most models simulated an eastward propagation of enhanced and suppressed convection similar to the observed one. Over West Africa the MJO signal is too weak in some models although there is good coherence in the eastward propagation. In addition, the ensemble average of models v gives better performance in reproducing these features. The influence on rainfall is well captured in both Sahel and Guinea regions thereby adequately producing the transition between positive and negative rainfall anomalies through the different phases as in the observation. Futhermore, the results show that a strong active convection phase is clearly associated with the AEJ but the weak convective phase is associated with a much weaker AEJ particularly over coastal Ghana. In assessing the mechanisms which are involved in the above impacts the convectively equatorial coupled waves (CCEW) are analysed separately. The analysis of the longitudinal propagation of zonal wind at 850hPa and outgoing longwave radiation (OLR) shows that the CCEW are very weak and their extention are very limited beyond West African region. It was found that the westward coupled equatorial Rossby waves are needed to bring out the MJO-convection link over the region and this relationship is well reproduced by all the models. However, Kelvin waves do not account for the overall impact of MJO signal on convection over West Africa. Results also confirmed that it may be possible to predict the anomalous convection over West Africa with a lead time of 15-20 day with regard to anomalous convection events over the Indian Ocean and AMIP simulations performed well in this regard.