Daily Rainfall Modulation of the Long and Short Rains in East Africa By the Madden Julian Oscillation and Convectively Coupled Equatorial Waves

Rainfall variability and especially related extreme events have a large socio-economic relevance for East African countries. Recent years have seen both extreme droughts, but also floods and an as of yet not well understood decline in the long rains during March-April-May (MAM). For the mostly rain-fed agriculture, the onset, withdrawal, and breaks in the two major rainy seasons, the long and short (October-November-December OND) are a pivotal importance. It has been shown in previous studies that the Madden-Julian Oscillation (MJO) modifies intraseasonal variability including onset dates in East Africa. However, a comprehensive study using (a) an extensive network of rain gauges for various decades and climatological regions, (b) investigating the sole and combined influence of the MJO and Convectively Coupled Equatorial Waves (CCEWs), and (c) studying the processes related to convection that are ultimately changing convective activity is currently lacking.

The present study presents results from such a comprehensive approach. It investigated the modulation of East African rainfall by the MJO and four types of CCEWs between 1983 and 2013 for the long and short rains and daily, respectively, by using daily rainfall observations from 91 weather stations and CHIRPS (Climate Hazards InfraRed Precipitation with Stations) satellite-based rainfall estimates. A dynamic and thermodynamic analysis included wind anomalies and perturbations of moisture fluxes, Convective Available Potential Energy (CAPE) and Moist Static Energy (MSE).

It is found that the MJO reveals the strongest rainfall modulation partly exceeding 6 mm day-1 between dry and wet MJO phases. Kelvin and Equatorial Rossby waves demonstrate a lower impact on rainfall, whereas Mixed-Rossby-Gravity and Eastward Inertio Gravity waves show the weakest influence on rainfall. The wave phases generate heterogeneous impacts on different regions, e.g., for coastal and highland areas. Easterly wind anomalies generate positive rainfall perturbations, in particular at the East African coast. Stronger wind and tropospheric moisture fluxes lead to a considerable stronger impact of the waves on rainfall during MAM than during OND. The MJO interacts with Kelvin and ER waves leading to changed and overrode wet and dry phase signals. Weather forecasts and climate projections can now be tested with regard to their ability to modulate rainfall by the MJO and CCEWs. This can result in more robust regionalized rainfall forecasts and projections for East Africa.