The Influence of the Madden-Julian Oscillation on Large Daily Precipitation Events in West Africa

The Madden-Julian oscillation (MJO) is a global-scale disturbance that originates over the Indian Ocean and propagates eastward, occasionally circumnavigating the Equator. Although it is known to modulate weather and extreme events throughout the tropics, relatively little attention has been paid to its impacts in West Africa, perhaps because it is nearing the end of its life cycle by the time it arrives there. This study focuses on the modulation of West African precipitation by the MJO especially near the Guinean coast, where variations in large short-term rainfall events that cause flooding that damages crops and infrastructure are more important than variations in total seasonal precipitation. Using composites based on thresholds of daily precipitation amounts, variations associated with the MJO's phase are presented. Mean precipitation is enhanced during MJO phases that show upward motion over western Africa, and suppressed during phases with downward motion. Similarly, a two-to-one modulation of the likelihood of extremes (e.g., rainfall rates above the 90th percentile) occurs across these phases. Conservative statistical tests of local and field significance indicate unambiguous changes during some phases and marginal significance during others.