Influence of Madden Julian Oscillation on Precipitation Extremes and its State-of-Art Climate CMIP6 Historical Simulations over West Africa

West Africa regions has been witnessing increasingly severe extreme weather events, particularly floods. Extreme events have important implications for rain-fed agriculture therefore the West Africa are particularly vulnerable to those events because of low adaptive capacity. However, the processes linked to the significant changes in climate over the regions in response to future warming is not well understood. This study aims to investigate the impact of Madden Julian Oscillation on the occurrence of extreme rainfall events and its state-of-art climate CMIP6 (Coupled Model Intercomparison Project Phase 6) historical simulations over West Africa. The results show there is clear evidence that MJO influences rainfall pattern by favoring strong rainfall during the enhanced phase while the opposite is observed during the suppressed phase. In addition, the analysis shows that MJO exerts a large and significant impact on the probability of observing extreme precipitation. The MJO skill scores in GCMs based on pattern correlations in simulating the eastward propagation allow to categorize the model into “Good”, “Intermediate” and “Poor” MJO models. Looking at how CMIP6 models simulate the likelihood of extreme precipitation events using the different categories identified previously, the analysis shows that the MJO continues to represent a great challenge for these latest generation GCMs. During enhanced MJO phases, we observed that the likelihood of extreme precipitation events is well simulated in “Good” MJO models but we noticed less coherency in “Intermediate” and “Poor” ones.

The MJO impact is more pronounced over Gulf of Guinea and West Africa regions are dominated with strong trough during strong MJO phases. The increase moisture supply over the region lead to an increase precipitation intensity and frequency. In addition, the analysis of observations shows the existence of a contrast between the Indian Ocean and West Africa in terms of anomalous convection which is very important and might be used as a potential predictor. This relationship could lead to a better forecast of extreme events. That mechanism is very well represented in “Good” models while “Intermediate” and “Poor” ones don’t show a very clear signal on how the moisture transport give rise to MJO-related precipitation extremes.