[A] Nonetheless, the 1997 - 1998 El Nino Was an unusual one. It developed so rapidly that every month between June and December 1997 set a new monthly record high for sea surface temperatures in the eastern equatorial Pacific. Anomalies (that is, deviations from normal in December 1997 were the highest ever recorded along the Equator in the eastern Pacific. Moreover, before 1997 - 1998, the previous record setting El Nino occurred in 1982 - 1983. These two "super El Ninos" were separated by only 15 years, compared with a typical 30 -40 year gap between such events earlier in the 20th century.
[B] Identifying why it was so strong challenges our understanding of the physical mechanisms responsible for El Nino. This is more than simply an academic question: the 1997 - 1998 El Nino severely disrupted global weather patterns and Pacific marine ecosystems, and by one estimate caused $ 33 billion in damage and cost 23,000 lives worldwide.
[C] There were warning
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