Following the onset of the strong El Nino of 1997-98 historical rainfall teleconnection patterns and dynamical model predictions both suggested an enhanced likelihood of drought for southern Africa, but widespread dry conditions failed to materialize. Results from a diagnostic study of NCEP-NCAR reanalysis data are reported here demonstrating how the large- and regional-scale atmospheric circulations during the 1997-98 El Nino differed from previous events. Emphasis is placed on the January-March 1998 season and comparisons with the strong 1982-83 El Nino, although composites of eight events occurring between 1950 and 2000 are also considered. In a companion paper, simulation runs from three atmospheric general circulation models ( AGCMs), and forecasts from three fully coupled models are employed to investigate the extent to which the anomalous atmospheric circulation patterns during the 1997-98 El Nino may have been anticipated.Observational results indicate that the 1997-98 El Nino displayed significant differences from both the 1982-83 episode and the composite event. An unusually strong Angola low, exceptionally high sea surface temperatures ( SSTs) in the western Indian and eastern tropical South Atlantic Oceans, and an enhanced northerly moisture flux from the continental interior and the western tropical Indian Ocean all appear to have contributed to more seasonal rainfall in 1997-98 over much of the southern Africa subcontinent than in past El Nino events.
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