The pattern of rainfall over East Africa is both intricate and fundamentally not well understood. This gives rise to low skill in short-term and seasonal predictability of precipitation for the region and a lowered confidence in future projections for the region provided by climate models. The focus of this work is to develop a better understanding of how the atmospheric circulation features of East Africa, e.g., Somali jet, are responsible for the complex rainfall signal across the region and the interannual variability thereof. From ERA-Interim, daily values of zonal and meridional winds and specific humidity were used to quantify the seasonal and interannual variability of the dominate circulation and atmospheric moisture transport features in the region. These features were then described as dynamically responsible for (e.g., by way of convergence), and/or as a source of moisture to drive particular rainfall features in this region. From this developed a fundamental understanding of how interseasonal variations in precipitation, derived from available datasets such as TRMM, are tied to variability in specific atmospheric circulation mechanisms. These results will provide a better foundation for further studies into understanding how extreme drought and flooding events like those of 2005 and 2006, respectively, are forced by atmospheric conditions, and they will offer an improved means by which to measure the ability of a climate model to reproduce the climate of East African rainfall.