East African rainfall variability associated with the Madden-Julian Oscillation

Abstract

Rainfall is the most important weather parameter that affects social and economic activities in the tropics. High rainfall variability characterized by anomalous wet events and dry spells occur at intraseasonal time scales in East Africa. These extreme events impact negatively on the economy of the countries that depend especially on rain-fed agriculture and hydroelectric power generation. Studies have been conducted on seasonal (long rage) and Numerical weather (short range) predictions. But little has been done on rainfall variability at intraseasonal time scale. Skillful prediction of the extreme rainfall events at intrasesaonal time scales is therefore critical in planning and reduction of negative impacts associated with it. The main objective of this study is to diagnose the relationship of rainfall over East Africa with the Madden-Julian Oscillation (MJO). The data used include daily rainfall obtained from the National Meteorological Centers (NMC) of Uganda, Tanzania and Kenya, daily Madden-Julian (MJO) indices from the Bureau of Meteorology Research Center (BMRC), daily Outgoing Longwave Radiation (ORL) from Climate Diagnostics Center (CDC) and wind data obtained

from NCEP-NCAR. Correlation and composite analysis is used to establish the association and

the relationship between the MJO and rainfall over East Africa. The skill of forecast is verified using cross validation method. The results reveal strong association between East African rainfall and the MJO to the west of the region especially around the Lake Victoria basin. Strong convective development to the west associated with strong westerly and easterly winds configuration at 700hpa and 200hpa favors the relationship between rainfall and the convectively driven MJO. Based on composite results the extreme rainfall events occur during preferential phases of the MJO. Higher chances of

enhance convection during phase 2 than in phase 7 and 8. Phase 5 and 6 of associated with the depressed phases of the MJO over East Africa. The out of phase relationship between west and the east is an indication of different rain causing mechanisms for the two regions. Stronger winds configuration (westerly and easterly winds at 700hpa and 200hpa) and high negative OLR values also coincide with phases2 of the MJO indicating the influence of the MJO on East African rainfall at intraseasonal time scales. The study also reveals that skillful prediction of rainfall at interaseasonal time scale is possible up to 10 days. The east showed no skill of making prediction at intraseasonal time scales. The study provides evidence of the potential predictability of intarseasonal rainfall over East Africa using the MJO indices up to 10 days. However, only explains 18% of the variance is explained by using the MJO indices. The inclusion of other modes of variability such as Seas surface Temperatures (SSTs) and the Indian Ocean Dipole (IOD), as predictors might be necessary in predicting intraseasonal rainfall.