The role of regional circulation features in regulating El Nino climate impacts over southern Africa: A comparison of the 2015/2016 drought with previous events

Extremely dry conditions were experienced across most of southern Africa during the austral summer (October-March) of 2015/2016, associated with one of the strongest observed El Nino events in the Pacific. Dry conditions peaked in the early austral summer months (October-December) producing the most intense drought in the 116-year historical record, as measured by the intensity of the standardized precipitation index across all spatial scales up to the sub-continental. We estimate the return period of this extreme early summer drought to be greater than 200years. The interior and eastern parts of South Africa were particularly hard-hit with station data showing rainfall totals being at their lowest since at least 1950. The early summer dry conditions make the 2015/2016 event atypical compared to past El Nino events of similar magnitude. We find that key regional circulation patterns, influenced by planetary-scale processes, play an important role in modulating the spatial and temporal evolution of the summer rainfall during these El Nino events. Specifically, (a) the Angola Low and the South Indian Ocean High, two dominant low-level circulation features that drive moisture convergence to support convective precipitation in the region, were anomalously weakened in early austral summer of 2015/2016 resulting in less moisture being transported over the continent, and (b) the mid-level Botswana High was stronger than in previous El Nino years further producing unfavourable conditions for rainfall through stronger subsidence in the mid- to upper levels over southern Africa.