Influence of the Madden-Julian Oscillation on East African rainfall. Part I: Intraseasonal variability and regional dependency

The Madden–Julian Oscillation (MJO) was shown in Part I to have a significant impact on both East African rainy seasons at pentad time-scale. The case of the ‘long rains ’ (March–May) is further examined by considering both shorter (daily) and longer (interannual) time-scales. Based on composite analyses, extreme daily rainfall events in the Highland (west) and in the coastal (east) regions of Equatorial East Africa (Kenya and Tanzania) are extracted. Low-level westerly wind anomalies are seen to accompany wet events in the west and easterly ones in the east. These opposite circulation anomalies preferentially occur at distinct phases of the MJO, which indicates that the latter has a major influence on rainfall at the daily time-scale. However, this influence undergoes significant year-to-year variations. It is found that the common variance between smoothed rainfall time series (5-day low-pass filter) and MJO indices varies from 5 % to 53 % in the Highland region. Significantly lower air temperatures in the upper troposphere are recorded during the MJO cycles that present the highest common variance with East African rainfall. Such a cooling is seen to be related to the Kelvin wave propagation in the upper layers which favours upward atmospheric motion over the region. At the interannual time-scale, fluctuations in MJO amplitude contribute to the March–May rainfall variability, and 44 % of the March–May seasonal rainfall variance in the 1979–95 period in East Africa is explained by this parameter. Years of high MJO amplitude are characterized by earlier onset of the rains, and higher seasonal amounts