A Possible Mechanism to Explain the Response of Southern Africa Precipitation to ENSO

Southern Africa often, but not always, experiences a northeastward shift in the diagonal band of high precipitation (the South Indian Convergence Zone, or SICZ) in austral summer during warm events. This shift has been realistically reproduced in a series of ensemble GCM simulations designed to study global responses to ENSO in the absence of both inter-ENSO variability and competing sources of external variability.

The cause in the shift of the SICZ and, therefore, the precipitation perturbation, in the model is anomalously low heights through the full depth of the troposphere off the southeast coast of Africa. These low heights are part of a subtropical and midlatitude Rossby wave response to changes in the upper-level divergence field in the tropics. Warming in the eastern Pacific shifts the equatorial Walker circulation over Africa eastward, and the resulting anomalous divergence over equatorial East Africa acts as a Rossby wave source. When Indian Ocean SSTs increase along with the Pacific ENSO event, the response of the southern Africa precipitation field is delayed by one month in the model, from December to January. Thus, the basic mechanism connecting warming in the eastern Pacific with drying over southern Africa acts primarily through the atmosphere, without mediation through Indian Ocean SSTs.