Classification of El Niño and La Niña Winters


A number of WRCC ENSO tables and graphs are stratified by June-November Southern Oscillation Index (SOI), following the logic of Redmond & Koch (1991, 2381-2399, Water Resources Research). Slightly better relations are found between western climate and SOI than with SST (Sea Surface Temperatures). Also, better relations are often found when SOI leads winter climate (by about 4 months) rather than being concurrent with climate. And, whether better, similar, or even slightly worse, the predictive utility is retained with the earlier SOI period. Because the SOI is averaged over 6 months, the lead and lag periods are not completely separated (2 month overlap for Oct-Mar climate elements), but by early winter (Sept or Oct) the "ball park" value of the SOI is usually apparent.

Strictly speaking, with this approach ENSO phase is determined by atmospheric quantities (SOI). "El Nino" and "La Nina" are usually defined solely by ocean temperatures (SST). The relationship between the two is strong enough that most moderately positive SOI years are also "La Nina" and most moderately negative SOI years are also "El Nino", and the terms are used interchangeably, at some risk of misinterpretation, to denote ENSO phase.

*** Note ***

(Many El Nino, La Nina, Neither Nino categorization approaches use ocean criteria, such as Sea Surface Temperatures (SST), rather than atmospheric criteria, such as the Southern Oscillation Index (SOI). Most categorizations produce the same years; however, typically a few years are included with one approach that are not included in another, and vice versa. )

SE - Strongly Negative SOI (-1.00 or less) - "Stronger El Nino" 
EN - Mildly   Negative SOI (-0.50 or less) - "Moderate El Nino"
 N - Neither  (SOI between -0.50 and +0.50)- "Neither Nino" or "Nada Nino"
LN - Mildly   Positive SOI (+0.50 or more) - "Moderate La Nina"
SL - Strongly Positive SOI (+1.00 or more) - "Stronger La Nina"

Unless stated otherwise, the term "El Nino" based on SOI usually
  includes EN and SE cases.
Unless stated otherwise, the term "La Nina" based on SOI usually 
  includes LN and SL cases.

    Jun-Nov     Following    Label
 Year   Ave SOI   Winter -- -- - -- --

 1933      0.13  1933-34       N
 1934     -0.05  1934-35       N
 1935      0.18  1935-36       N
 1936     -0.42  1936-37       N
 1937     -0.23  1937-38       N
 1938      1.20  1938-39            SL
 1939     -0.58  1939-40    EN
 1940     -1.80  1940-41 SE
 1941     -1.73  1941-42 SE
 1942      0.28  1942-43       N
 1943      0.22  1943-44       N
 1944     -0.52  1944-45    EN
 1945      0.45  1945-46       N
 1946     -1.05  1946-47 SE
 1947      0.52  1947-48         LN
 1948     -0.25  1948-49       N
 1949     -0.43  1949-50       N
 1950      1.55  1950-51            SL
 1951     -1.07  1951-52 SE
 1952      0.05  1952-53       N
 1953     -0.75  1953-54    EN
 1954      0.18  1954-55       N
 1955      1.50  1955-56            SL
 1956      0.88  1956-57         LN
 1957     -0.62  1957-58    EN
 1958     -0.12  1958-59       N
 1959     -0.18  1959-60       N
 1960      0.27  1960-61       N
 1961     -0.12  1961-62       N
 1962      0.38  1962-63       N
 1963     -0.92  1963-64    EN
 1964      0.87  1964-65         LN
 1965     -1.58  1965-66 SE
 1966     -0.08  1966-67       N
 1967      0.10  1967-68       N
 1968      0.15  1968-69       N
 1969     -0.67  1969-70    EN
 1970      0.77  1970-71         LN
 1971      0.92  1971-72         LN
 1972     -1.28  1972-73 SE
 1973      1.28  1973-74            SL
 1974      0.55  1974-75         LN
 1975      1.83  1975-76            SL
 1976     -0.52  1976-77    EN
 1977     -1.52  1977-78 SE
 1978      0.00  1978-79       N
 1979      0.05  1979-80       N
 1980     -0.38  1980-81       N
 1981      0.42  1981-82       N
 1982     -2.42  1982-83 SE
 1983     -0.12  1983-84       N
 1984     -0.22  1984-85       N
 1985     -0.37  1985-86       N
 1986     -0.22  1986-87       N
 1987     -1.35  1987-88 SE
 1988      1.25  1988-89            SL
 1989      0.22  1989-90       N
 1990     -0.23  1990-91       N
 1991     -0.95  1991-92    EN
 1992     -0.80  1992-93    EN
 1993     -1.08  1993-94 SE
 1994     -1.43  1994-95 SE
 1995      0.00  1995-96       N
 1996      0.47  1996-97       N
 1997     -1.67  1997-98 SE
 1998     +1.05  1998-99            SL
 1999     +0.40  1999-00       N       (ocean temperatures definitely LN)
 2000     +0.57  2000-01         LN
 2001     -0.17  2001-02       N       
 2002     -0.83  2002-03    EN          
 2003     -0.33  2003-04       N
 2004     -0.73  2004-05    EN         (ocean temp met EN criteria, too)
 2005      0.08  2005-06       N       (mild La Nina developed in mid-winter)
 2006     -0.90  2006-07    EN         (ocean temp moderately EN, too)
Questions? Dr. Kelly Redmond krwrcc@dri.edu
Western Regional Climate Center, wrcc@dri.edu