Summary of current conditions in the Tropical Pacific

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Early 1997 witnessed the rapid development in the tropical Pacific of a significant El Niño/Southern Oscillation (ENSO) warm event. Warm sea surface temperature (SST) anomalies erupted during April - June in the tropical eastern Pacific, and by July SST anomalies were the warmest observed in the past hundred years in that region. SST anomalies in the eastern Pacific subsequently exceeded 5°C*. The Southern Oscillation Index, the difference between surface air pressure at Tahiti minus that at Darwin, Australia, dropped precipitously to negative values March 1997, and remained negative throughout 1997. Wind and SST conditions in the equatorial Pacific at the time of TIP-6 are illustrated in Figure 1.

Surface winds along the equator were punctuated by a series of westerly events of increasing intensity and eastward fetch beginning in early 1997. These westerly episodes excited downwelling equatorial Kelvin waves that propagated eastward, eventually depressing the thermocline by over 90 m in the eastern Pacific. At the same time, the thermocline in the western Pacific shoaled by 20 - 40 m in response to anomalous upwelling Rossby wave generation. Changes in SST due to ocean dynamical effects and anomalous air-sea heat exchanges lead to an eastward and equatorward shift in atmospheric deep convection and precipitation in the tropical Pacific. Resultant heating of the atmosphere altered the position of the sub-tropical jet streams and storm tracks, affecting weather patterns worldwide. The coastal ocean regions off both western North and South America were also affected by remote oceanic and atmospheric forcing from the tropics during this event.

The intensity of this El Niño event caught the scientific community by surprise. Though several state-of-the-art prediction models indicated 1997 would witness the development of an El Niño, none of them accurately forecast its magnitude, or the timing of its onset which occurred much earlier in the calendar year than for previous events. The Lamont-Doherty coupled ocean-atmosphere forecast model, which uses no ocean data for initialization, failed to predict the event at all.

*Editors note: Monthly mean SST anomalies in the Niño3 (5°N - 5°S, 150°W - 90°W) region of the eastern Pacific in November and December 1997 exceeded those observed at anytime during the 1982 - 83 El Niño, which up to that time was the strongest of the century.

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