Modulation of Daily Precipitation over East Africa by the Madden–Julian Oscillation*

Abstract

Spatiotemporal variability in East African precipitation affects the livelihood of tens of millions of people. From the perspective of floods, flash droughts, and agriculture, variability on intraseasonal time scales is a critical component of total variability. The principal objective of this study is to explore subseasonal impacts of the Madden-Julian oscillation (MJO) on tropospheric circulations affecting East Africa (EA) during the long (March-May) and short (October-December) rains and associated variability in precipitation. Analyses are performed for 1979-2012 for dynamics and 1998-2012 for precipitation. Consistent with previous studies, significant MJO influence is found on wet and dry spells during the long and short rains. This influence, however, is found to vary within each season. Specifically, indices of MJO convection at 70 degrees-80 degrees E and 120 degrees W are strongly associated with precipitation variability across much of EA in the early (March) and late (May) long rainy season and in the middle and late (November-December) short rainy season. In the early short rains (October) a different pattern emerges, in which MJO strength at 120 degrees E (10 degrees W) is associated with dry (wet) spells in coastal EA but not the interior. In April the MJO influence on precipitation is obscured but can be diagnosed in lead time associations. This diversity of influences reflects a diversity of mechanisms of MJO influence, including dynamic and thermodynamic mechanisms tied to large-scale atmospheric circulations and localized dynamics associated with MJO modulation of the Somali low-level jet. These differences are relevant to problems of subseasonal weather forecasts and climate projections for EA.