Does the South American Monsoon Influence African Rainfall?

Abstract

Teleconnections between the South American monsoon and southern African rainfall are investigated for years with Benguela Niño or Niña events in the South Atlantic. During these events, it is found that substantial rainfall anomalies also occur over South America in addition to those previously known for southern Africa. The appearance of large rainfall anomalies in the South American monsoon region prior to the onset of the Benguela Niño proper suggests that anomalous convection over South America may influence the evolution of both the SST anomalies and the African rainfall anomalies associated with Benguela Niño events. This teleconnection between South America and southern African rainfall may occur directly, via atmospheric circulation anomalies induced by convection over South America, or indirectly, via the effect of induced circulation anomalies on regional SST. To investigate these teleconnections, a vorticity equation model, which is linearized about a realistic basic state and which includes the divergence in this state and the advection of vorticity by the divergent wind, is applied to the events. The model is forced with anomalous divergence patterns observed during the events, and the steady-state solutions show that anomalies of convection during the South American monsoon produce the main circulation anomalies observed during the Benguela Niño events and hence influence rainfall and circulation patterns over Angola and other southern African countries. An influence function analysis confirms this result, indicating that South America is the most efficient source region to produce the observed anomalies, and also shows that there is no influence of convection over Africa on the South American monsoon. Based on these linear model and observational results, it is concluded that the South American monsoon can influence the evolution of Benguela Niños and associated rainfall anomalies in southern Africa.