Some ENSO impact analyses, particularly those in COAPS, categorize application data as belonging to one of three phases of ENSO: El Niño (warm SST anomalies in the Pacific) El Viejo (cool SST anomalies), or neutral based on an index derived from observed SST anomalies. The particular index chosen is that from the Japan Meteorological Agency (JMA) because it selects well the known ENSO events. The index is a 5-month running mean of spatially averaged SST anomalies over the tropical Pacific: 4°S-4°N, 150°W-90°W. If index values are 0.5°C or greater for 6 consecutive months (including OND), the ENSO year of October through the following September is categorized as El Niño, El Viejo (index values equal or exceed -0.5°C), or neutral (all other values). A plot of these data show their relative values since 1970.
For the period 1949 to present, the JMA-Index is based on observed data. However, for the years 1868-1948 the index is based on reconstructed monthly mean SST fields. The reconstructed SST fields were computed using an orthogonal projection technique (Meyers et al., 1997). The JMA index for 1868-1949 was calculated just as for the 1949-present, meaning the JMA climatological values were used to calculate the JMA SST anomalies. A 5-month running mean was then applied resulting in the JMA index.The JMA Index values for each month of the 1868-1949 period are available via ftp. (ftp://ftp.coaps.fsu.edu/pub/JMA_SST_Index/).
Correction - (17 July 2003): The ENSO years of 1870 and 1974 were reclassified from neutral to cold phases. The original classification of these years was the result of inconsistent application of the -0.5C threshold for cold phases. In these two cases, several 5-month running mean values in the six month period that included OND were equal to -0.5C, but the years were incorrectly classified as neutral. This correction only affects the list in Table 1, not any files available from the ftp site.
Table 1: The years in each category correspond to the first three months of the ENSO year namely October, November, and December. For example, the ENSO year 1970 starts October 1970 and ends September 1971.
Special thanks to David M. Legler for starting this index.
To cite this index, please reference Bove, M. C., J. B. Elsner, C. W. Landsea, X. Niu, and J. J. O’Brien, 1998: Effect of El Niño on U.S. landfalling hurricanes, revisited. Bull. Amer. Meteor. Soc., 79, 2477–2482.