The Mechanisms Associated with Ethiopian Belg Subseasonal Rainfall Variability

The February to May rainfall season, locally known in Ethiopia 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 interannual and intraseasonal variability and failure to this season’s performance can have a negative impact on Ethiopia food security. However, despite its important in Ethiopia’s economy, the characteristics associated with this season are not widely documented in the literature. This study examines the role of tropical-extratropical interactions in modulating Belg seasonal rainfall. The data include daily rainfall data from 117 stations of the National Meteorological Agency (NMA) of Ethiopia for the 1980 to 2010 period, and February to May 2015 and NCEP reanalysis to construct rainfall and circulation anomaly composites for the dry and wet Belg. CPC daily North Atlantic Oscillation (NAO) index data are used to define NAO events and to construct rainfall anomaly composites for the negative and positive phases of the NAO. It is shown in this paper that the North Atlantic Oscillation (NAO) seems to play a major role in the variability of the Belg rains on the subseasonal 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 been identified in this study. A ridge/trough pattern, featuring two anomalous mid-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 by way of weakening tropical-extratropical interactions. In contrast, a tripole structure with two anomalous mid-upper level cold cyclonic troughs and one warm anticyclone tends to enhance rainfall during the Belg season of Ethiopia. The role of MJO in strengthening the interactions between the tropics and the extratropics in Belg seasonal rainfall modulation is presented.