Trenberth, K., and T. J. Hoar, 1995: The 1990-1995 El
Nino-Southern Oscillation Event: Longest on Record.
Geophysical Research Letters, 23, 57-60.
The tendency for more frequent El Nino events and fewer La Nina events
since the late 1970's has been linked to decadal changes in climate throughout
the Pacific basin. Aspects of the most recent warming in the tropical Pacific
from 1990 to 1995, which are connected to but not synonymous with El Nino,
are unprecedented in the climate record of the past 113 years. There is a
distinction between El Nino (EN), the Southern Oscillation (SO) in the
atmosphere, and ENSO, where the two are strongly linked, that emerges more
clearly on decadal time scales. In the traditional El Nino region, sea
surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the
central equatorial Pacific are better linked to the SO and have remained
positive from 1990 to 1995. We carry out several statistical tests to assess
the likelihood that the recent behavior of the SO is part of a natural
decadal-timescale variation. One test fits an autoregressive-moving average
(ARMA) model to a measure of the SO given by the first hundred years of the
pressures at Darwin, Australia, beginning in 1882. Both the recent trend for
more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are
unexpected given the previous record, with a probability of natural occurrence
about once in 2,000 years. This opens up the possibility that the ENSO changes
may be caused by the observed increases in greenhouse gases.
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