Meteorological scenario of Ethiopian floods in 2006–2007

Abstract

The meteorological scenario of Ethiopian highlands floods is studied. Daily rainfall in the period 1997–2007 reveals two peaks: 23–28 July 2006 and 26–31 July 2007. National Center for Environmental Prediction (NCEP) composites suggest that anomalous southerly monsoon flow over the West Indian Ocean is re-directed by an anomalous Arabian ridge westward across the Red Sea and Ethiopia. A tongue of moisture stretches from the Congo towards the highlands, but westerly equatorial wind anomalies are absent. Anomalous sinking motions and dry conditions are evident over the West Indian Ocean. Diurnal analysis reveals northwesterly flow over eastern Sudan during afternoon hours, whilst back-trajectory analysis highlights a Red Sea source and lifting over the eastern escarpment of Ethiopia. The upper level tropical easterly jet connects Indian and Ethiopian rainfall at intra-seasonal (~40 days) time scale; whilst low-level meridional flow convergence is evident during flood events. Hovmoller analysis on 10°N reveals cyclonic signals propagating westward from the Arabian Sea at 500 km day−1 that produces a 10-day cycle in Ethiopian rainfall. The floods in 2006–2007 occurred at the peak of the annual cycle, with diurnal controls inducing ¾ of rain in the late evening. Whilst cold surges from southern Africa played a role in the 2006 flood, bursts in the northern Hadley cell are a more general determinant. The convection associated with the 2007 flood went on to become a destructive Atlantic hurricane.