Modulation of East African Boreal Fall Rainfall: Combined Effects of the Madden–Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO)

Abstract

Abstract Climate variabilities can have significant impacts on rainfall in East Africa, leading to disruption in natural and human systems and affecting the lives of tens of millions of people. Subseasonal and interannual variabilities are critical components of total rainfall variability in the region. The goal of this study is to examine the combined effects of the Madden–Julian oscillation (MJO), operating at a subseasonal time scale, and El Niño–Southern Oscillation (ENSO), operating at an interannual scale, on the modulation of East African boreal fall [October–December (OND)] rainfall, commonly called the short rains. Composite analysis shows that daily rainfall responses depend on MJO phase and its interaction with ENSO state. In particular, MJO modulation of rainfall is generally stronger under El Niño conditions relative to ENSO neutral and La Niña conditions, leading to increased potential for daily precipitation excesses during wet MJO phases under El Niño. There is evidence for both dynamic and thermodynamic mechanisms associated with these impacts, including an increase in westerly moisture transport and easterly advection of temperature and moist static energy. Seasonal analysis shows that the frequency and intensity of wet MJO phases during an El Niño contribute notably to the seasonal total precipitation anomaly. This suggests that the MJO can mediate El Niño’s impact on OND rainfall in East Africa.