Trenberth and Hoar report on a study they have made on the long El Niño-Southern Oscillation (ENSO) event of 1990-1995. Sea- surface temperatures in the central Pacific Ocean have remained high over the 1990-1995 period, which is highly unusual over the 113-year record for which data are available. This has led to a persistence in the Southern Oscillation, even though temperatures in the traditional El Niño region have fluctuated. Both the recent trend toward more ENSO occurrences since 1976 and the extended ENSO event of 1990-1995 are unexpected from the previous record. Statistical analysis shows that such events would occur only once in 2,000 years, based on natural variability of the last 113 years. This raises the possibility that changes in the ENSO pattern may be in part due to observed increases in greenhouse gases.
Trenberth, K.E. and T.J. Hoar, 1996. Geophys. Res. Lett., 23, 57-60.