Influence of the Madden-Julian Oscillation on East African rainfall. Part II: March-May season extremes and interannual variability

Benjamin Pohl (1) , Pierre Camberlin (1)

1 CRC - Centre de Recherches de Climatologie

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.

Mots clés: Africa; Composite analysis; Tanzania; Kenya; East Africa; Observation data; Madden Julian oscillation; Kelvin wave; atmospheric circulation; Intraseasonal variation; daily variations; Interannual variation; Spatial variability; extreme value; time series analysis; Rainfall rate; rainfall; atmospheric precipitation; Rainy season; Climatology

Domaines: Climatologie; Milieux et Changements globaux

Benjamin Pohl, Pierre Camberlin. Influence of the Madden-Julian Oscillation on East African rainfall. Part II: March-May season extremes and interannual variability. Quarterly Journal of the Royal Meteorological Society, 2006, 132 (621B), pp.2541-2558. ⟨10.1256/qj.05.223⟩. ⟨hal-00076677⟩