IntraSeasonal Variability of the Belg Rains in Ethiopia

The February to May rainfall season, locally known as Belg, contributes to up to 40% of the annual rainfall over northeast, central and southwest Ethiopia, whereas its contribution exceeds 50% of the annual total rainfall over South and Southeast Ethiopia. The Belg season is characterized by quite significant interannual and intraseasonal variability. However, there are only a few studies addressing the characteristics of this season. Interactions between mid-latitude and tropical systems across northeastern Africa play a major role in modulating the rainfall pattern during this season. We report that Anomalous features in the sub-tropical region, such as the Northern Atlantic Oscillation (NAO) lead to intraseasonal variability of the Belg. The Belg rains are negatively correlated with the NAO index over much of the region, with relatively higher correlation values over the southern and southeastern areas of Ethiopia. Composites of rainfall anomaly also indicate that the negative (positive) phase of the NAO tends to enhance (suppress) the Belg rains. In addition to the sub-tropical influences, there is an MJO related signals in the intraseasonal variability of the Belg rainfall. MJO phases 1 and 2 tend to enhance the Belg rainfall, whereas the Belg rainfall tends to be suppressed during the MJO phases of 5 and 6. In the absence of an active NAO and MJO, the regional circulation patterns become the dominant features that lead to intra-seasonal variability. A shortwave tripole structure with two anomalous mid-upper level warm anti-cyclones and one cold cyclonic trough in the region between the Northeast Atlantic and the Arabian Peninsula tends to suppress the Belg rainfall by way of weakening the interactions between the mid-latitude and tropical systems. In contrast, a tripole structure with two anomalous mid-upper level cold cyclonic, and one warm anti-cyclonic trough tends to enhance rainfall in the Belg season of Ethiopia.