A diagnostic study of the southern Africa rainy season during the 1997–98 El Nino

Despite the strength of the 1997-98 El Nino some of the typical ENSO teleconnections failed to materialize. Among these was the generally weak connection with drought development across southern Africa during the summer rainy season (Nov-Apr). A diagnostic analysis was undertaken to determine how the large scale atmospheric circulation affecting southern Africa departed from the cannonical ENSO response during the 1997-98 El Nino and to what extent these departures were potentially predictable. Observational data (NCEP-NCAR Reanalysis) as well as output from three atmospheric general circulation models, all forced with observed sea surface temperatures, were used in the analysis.

The Reanalysis data indicated that an anomalous southerly component of the vertically integrated moisture flux across the southern Africa, typical of El Nino events, failed to develop in 1997-98, allowing tropical moisture to be advected into the region. By contrast the anomalous, near-equatorial easterly moisture flux, also typical of El Nino events, was significantly enhanced and associated with very heavy rainfall across Kenya and eastern Tanzania. A westward shift of the near-equatorial, anomalous divergence field to the western Indian Ocean; deviations from the typical structure of the Southern Hemisphere extratropical anomalous stationary wave pattern; and a southward displacement of the anomalous low-level anticyclone over the central Indian Ocean were all factors unfavorable to the development of drought. Interestingly, the GCMs forced with observed SSTs were unable to reproduce these features, with the mositure flux and stationary wave patterns more remeniscent of the canonical ENSO pattern. Implications for forecasting the regional rainfall response to future El Nino events - even very strong events - will be discussed.