Extreme rainfall and floods in southern Africa in January 2013 and associated circulation patterns

During January 2013, very heavy rainfall over central and southern Mozambique led to severe flooding more than 100 deaths, and the displacement of about 200, 000 people. The atmospheric and oceanic patterns associated with this devastating event are analysed. An active South Indian Convergence Zone (SICZ) in January 2013 was associated with the heavy rainfall event. The SICZ was sustained by a low-level trough, linked to a Southern Hemisphere planetary wave (wavenumber-4) pattern and an upper-level ridge over south-eastern Africa. The low-level trough and upper-level ridge contributed to the convergence of moisture over south-eastern Africa, particularly from the tropical South East Atlantic (specifically offshore of Angola in the so-called Benguela Nio region), which in turn contributed to the prolonged life span of the event. Positive SST anomalies (1-2 A degrees C) in the Benguela Nio region were favourable for the substantial contribution of moisture fluxes to the event from the South Atlantic Ocean. This contribution is of particular interest since previous work has tended to ignore this basin and regard the Indian Ocean as the most important moisture source for rainfall over south-eastern Africa. The guidance forecast issued by the Regional Specialised Meteorological Centre, Pretoria for the period indicated its likely occurrence with a lead time of 4 days; however, the magnitude was underestimated, which may be linked to the threshold system used in the forecast system.