Intraseasonal variability of the impacts of the Madden-Julian Oscillation on East African long and short rains

The livelihood of millions of people in East Africa is intricately and critically linked to the spatiotemporal variability of precipitation in the region, and, from the perspective of floods, flash droughts, and crop production, variability on intraseasonal timescales is a critical component of total variability. The principal objective of the present study is to explore impacts of the Madden-Julian oscillation (MJO) on tropospheric circulations affecting East Africa during the long (March–May) and short (October–December) rains and associated changes in precipitation on intraseasonal time scales. For the long rains, it is found that variability in May precipitation is strongly associated with MJO convection at 70⁰ to 80⁰ E and 120⁰ W. Associations are also found in March, in the middle of the long rains (April), on the other hand, the MJO influence is obscured but is evident in lead-time associations. In the short rains, November and December precipitation is significantly associated with MJO convection at 80⁰ E and 120⁰ W, while October is not significantly associated with any of the MJO indices except in the coastal region, where wet (dry) spells are associated with suppressed MJO convection at 120⁰ E (10⁰ W). Analyses of thermodynamic fluxes, outgoing longwave radiation (OLR), divergence and vertical velocity indicate a mechanism of influence active in March, May, November, and December in which warm air advection and convergence lead to enhanced upward vertical motion in the region when there is enhanced (suppressed) MJO convection at 80⁰ E (120⁰ W). In May, the Somali Low Level Jet, which enhances frictionally induced subsidence, increases when the MJO convection at 120⁰ W (80⁰ E) is enhanced (suppressed). Better understanding of the MJO influence on intraseasonal precipitation variability in both rainy seasons can contribute to improved long-range forecasts and, potentially, projections of MJO influence on East Africa under climate change. Key words: MJO, East Africa, long rains, short rains