Given the tendency for drought to develop across southern Africa during El Niño events the failure of this regional response during the very strong El Niño of 1997-98 was notable. Seasonal climate forecasts made at the time indicated a low probability for near- or above-average rainfall resulting in considerable consternation on the part of early adopters of climate outlooks and forecasters alike. In this paper we report on a study undertaken to investigate how the atmospheric circulation affecting southern Africa departed from the typical El Niño in 1997-98 and to what extent this was at least potentially predictable. Observational data, simulations from 3 atmospheric general circulation models (AGCMS) and integrations from 3 coupled models are analyzed.

Observations indicated some significant circulation departures from other El Niño events on both the regional and large-scale. These included an unusually enhanced Angola Low throughout much of the rainy season, and an anomalous northerly flux of moisture from the western tropical Indian Ocean, which was related at least in part to unusually high sea surface temperatures in the western Indian Ocean. The AGCMS forced with observed sea surface temperatures failed to capture these features, tending to instead show a canonical ENSO response in terms of anomalous circulation and precipitation fields. Evaluation of the coupled model runs suggests they did not provide substantial improvement over the AGCMs. While having implications for seasonal forecasting in southern Africa, some of the results from this study have relevance in addressing the utility of using model simulations versus coupled model runs in diagnostic studies of southern African climate more generally.

Note: This paper is based on the results of the second part of a two-part study, which is in preparation for submission to the Journal of Climate.