Sub-seasonal variability of the Belg rains in Ethiopia

The February-May rainfall season, locally known as Belg, contributes up to 40% of the annual rainfall over northeastern, central and southwestern Ethiopia. Its contribution exceeds 50% over southern and southeastern Ethiopia. The Belg season is characterized by significant inter-annual and intra-seasonal variability. However, there are only a few studies addressing the characteristics of this season. Interactions between extratropical and tropical systems across the Red Sea region play a major role in modulating the rainfall pattern during this season. It is shown in this article that the North Atlantic Oscillation (NAO) seems to play a major role in the variability of the Belg rains on the sub-seasonal time scale. The Belg rains are negatively correlated with the NAO index over much of the region, with southern and southeastern Ethiopia exhibiting relatively highest correlation values. NAO rainfall anomaly composites also indicate that the negative (positive) phase of the NAO tends to enhance (suppress) the Belg rains. Two modes of regional circulation patterns that modulate Belg rainfall variability have also been identified in this study. A ridge/trough pattern, featuring two anomalous mid- to upper-level warm anticyclones and one cold cyclonic trough in the region between the northeast Atlantic and the Arabian Peninsula, tends to suppress the Belg rainfall due to reduced tropical-extratropical interactions. In contrast, a tripole structure with two anomalous mid- to upper-level cold cyclonic troughs and one warm anticyclone tends to enhance rainfall during the Belg season of Ethiopia. It is further shown in this article that the Madden-Julian Oscillation (MJO) tends to modulate rainfall during the Belg season.