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GEOPHYSICAL RESEARCH LETTERS,
VOL. 24, NO. 23,
PAGES 3057–3060,
1997
El Niño and Climate Change
Kevin E. Trenberth
National Center for Atmospheric Research, Boulder, Colorado
Timothy J. Hoar
National Center for Atmospheric Research, Boulder, Colorado
Abstract
A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar [1996], with a focus on the unusual nature of the 1990-1995 El Niño-Southern Oscillation (ENSO) warm event in the context
of an observed trend for more El Niño and fewer La Niña events after the late 1970s. The conclusions of that study have been
challenged by two studies which deal with only the part of our results pertaining to the length of runs of anomalies of one
sign in the Southern Oscillation Index. They therefore neglect the essence of Trenberth and Hoar, which focussed on the magnitude
of anomalies for certain periods and showed that anomalies during both the post-1976 and 1990-mid-1995 periods were highly
unlikely given the previous record. With updated data through mid 1997, we have performed additional tests using a regression
model with autoregressive-moving average (ARMA) errors that simultaneously estimates the appropriate ARMA model to fit the
data and assesses the statistical significance of how unusual the two periods of interest are. The mean SOI for the post-1976
period is statistically different from the overall mean at < 0.05% and so is the 1990-mid-1995 period. The recent evolution
of ENSO, with a major new El Niño event underway in 1997, reinforces the evidence that the tendency for more El Niño and fewer
La Niña events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.
Received 26
August
1997;
accepted 22
October
1997.
Subscriber Access to Full Article (Nonsubscribers may purchase for $9.00, Includes print PDF)
Citation: Trenberth, K. E., and T. J. Hoar
(1997),
El Niño and Climate Change,
Geophys. Res. Lett.,
24(23),
3057–3060.
Copyright 1997 by the American Geophysical Union.
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