Journal of Climate: Vol. 14, No. 8, pp. 1697–1701.
LETTERS: Indices of El Niño Evolution
Kevin E. Trenberth and and David P. Stepaniak
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
(Manuscript received 14 November 2000, in final form 6 December 2000)
To characterize the nature of El Niño–Southern Oscillation (ENSO), sea
surface temperature (SST) anomalies in different regions of the Pacific have been
used. An optimal characterization of both the distinct character and the evolution
of each El Niño or La Niña event is suggested that requires at least
two indices: (i) SST anomalies in the Niño-3.4 region (referred to as N3.4),
and (ii) a new index termed here the Trans-Niño Index (TNI), which is given
by the difference in normalized anomalies of SST between Niño-1+2 and
Niño-4 regions. The first index can be thought of as the mean SST throughout
the equatorial Pacific east of the date line and the second index is the gradient
in SST across the same region. Consequently, they are approximately orthogonal. TNI
leads N3.4 by 3 to 12 months prior to the climate shift in 1976/77 and also follows
N3.4 but with opposite sign 3 to 12 months later. However, after 1976/77, the sign
of the TNI leads and lags are reversed.
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Hongjun Zhang:
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