Figures, tables, and explanatory text for:

Trenberth, K. E. (1997) The Definition of El Niño

Bull. Amer. Meteor. Soc. , 78, 2771-2777.

All figures and tables have been updated to be current through December 1999.

This work explores the benefits of defining El Niño events in more quantitative terms than has been done in the past and compares the utility of such a definition with existing schemes. The focus is on El Niño and La Niña events from Niño regions 3 and 3.4.

A web-based version of the complete text of this paper is available for download from the author's homepage.

For a discussion of El Niño events from Niño regions 3 and and 3.5 refer to Trenberth, K. E. (1996) El Niño Definition, Exchanges, Newsletter of the Climate Variability and Predictability Programme (CLIVAR), Volume 1, No.3, 6-8.

FIGURE 1

Figure 1 shows time series plots of the Niño 3 (upper) and Niño 3.4 (lower) SST indices as five month running means using data from NOAA and relative to a base period climatology from 1950-1979. Values above thresholds of ±0.5°C for Niño 3 and ±0.4°C for Niño 3.4 are stippled to indicate ENSO events. The Niño 3 Region is bounded by 90°W-150°W and 5°S- 5°N. The Niño 3.4 Region is bounded by 120°W-170°W and 5°S- 5°N.

FIGURE 2

Figure 2 shows histograms of the distributions of SST anomalies for Niño 3 and Niño 3.4 from 1950 to December 1999 relative to the mean for the entire period. The contribution from the post-1979 period is shown by the aquamarine areas. Also given is the corresponding normal distribution with the same variance.

TABLE 1

Month Mean Niño 3 s(Niño 3) Mean Niño 3.4 s(Niño 3.4)
1 25.4 0.93 26.4 1.05
2 26.2 0.68 26.6 0.79
3 26.9 0.44 27.1 0.59
4 27.2 0.53 27.6 0.53
5 26.7 0.63 27.5 0.52
6 26.2 0.67 27.4 0.53
7 25.4 0.75 27.0 0.60
8 24.8 0.85 26.7 0.72
9 24.6 0.87 26.4 0.80
10 24.7 0.89 26.5 0.88
11 24.7 1.03 26.4 1.04
12 24.9 1.08 26.4 1.06
All 25.64 0.79 26.83 0.77

Table 1 shows means and standard deviations (s) in the Niño 3 and Niño 3.4 regions for the base period 1950-79 in °C. In the last row ``All'' refers to the overall means and standard deviation of monthly anomalies.

TABLE 2

Niño 3.4 Region (120°W-170°W, 5°S-5°N)
±0.4°C Threshold
La Niña Events El Niño Events
Begin End Duration Begin End Duration
Mar, 1950 Feb, 1951 12
Aug, 1951 Feb, 1952 7
Mar, 1953 Nov, 1953 9
Jun, 1954 Mar, 1956 22
May, 1956 Nov, 1956 7
Apr, 1957 Jun, 1958 15
Jun, 1963 Feb, 1964 9
May, 1964 Jan, 1965 9
May, 1965 Jun, 1966 14
Sep, 1968 Mar, 1970 19
Jul, 1970 Jan, 1972 19
Apr, 1972 Mar, 1973 12
Jun, 1973 Jun, 1974 13
Sep, 1974 Apr, 1976 20
Aug, 1976 Mar, 1977 8
Jul, 1977 Jan, 1978 7
Oct, 1979 Apr, 1980 7
Apr, 1982 Jul, 1983 16
Sep, 1984 Jun, 1985 10
Aug, 1986 Feb, 1988 19
May, 1988 Jun, 1989 14
Mar, 1991 Jul, 1992 17
Feb, 1993 Sep, 1993 8
Jun, 1994 Mar, 1995 10
Sep, 1995 Mar, 1996 7
Apr, 1997 Apr, 1998 13
Jul, 1998 Presently through Dec 1999 18 months and counting

Table 2 lists La Niña and El Niño events after 1950 as defined by SST anomalies in Niño 3.4 Region and exceeding ±0.4°C threshold. The starting and ending month of each event is given with the duration in months. Coupled events where SST anomalies remained of one sign are indicated in blue for La Niña and red for El Niño.

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