Why Eastern Africa was not dry during the 2020 short rainy season despite La Niña and a negative Indian Ocean Dipole: Interplay between the Madden-Julian Oscillation and La Niña in modulating short rain

La Niña was forecasted to be one of the predominant climatic modes in the 2020 boreal autumn, taking into consideration its impact on the short rain (October to November rainfall) in eastern Africa. Parallel to La Niña and just 2 months before the start of the 2020 short rain, the Greater Horn of Africa Regional Outlook Forum published extremely strong projections of below-normal rainfall in Burundi, Rwanda, most parts of Uganda, Kenya’s southern region, and Tanzania. On the other hand, several regions of East Africa (EA) saw above average rainfall, especially in November. The current study examines the core and potential drivers causing the 2020 November rainfall to deviate from its historical response to the La Niña event. The driving force responsible for the wetter-than-anticipated 2020 November rainy conditions was identified. Although the ocean surface favors a negative phase of the Indian Ocean Dipole mode, it normally acts to suppress the short rain in EA, the mode decayed sooner than its life cycle in October. Lastly, the Madden–Julian Oscillation (MJO) in the Indian Ocean acted to reverse or weaken the Walker circulation (WC), the surface westerly (northeasterly) wind flow from the Congo Basin (central Indian Ocean) which conveys moist (warm) unstable air to EA, thereby promoting convective activity and rainfall. The study further identifies the reasons for dry conditions in October and December. In October, the MJO strength in the Maritime Continent and Western Pacific was associated with Somali low-level jet-like and WC across the Indian Ocean. The latter caused subsidence in EA, whereas the former triggered a strong friction-induced sinking motion along the eastern coast of Africa. To create a dry condition, they both collaborated. In December, La Niña strengthened WC across the Indian Ocean which induced subsidence in EA and resulted in poor rainfall. The diversity of mechanisms of MJO influence is relevant to problems of sub-seasonal weather forecasts for EA.