Anomalous ENSO Occurrences: An Alternate View*

There has been an apparent increase in the frequency and duration of El Niño–Southern Oscillation events in the last two decades relative to the prior period of record. Furthermore, 1990–95 was the longest period of sustained high Darwin sea level pressure in the instrumental record. Variations in the frequency and duration of such events are of considerable interest because of their implications for understanding global climatic variability and also the possibility that the climate system may be changing due to external factors such as the increased concentration of greenhouse gases in the atmosphere. Nonparametric statistical methods for time series analysis are applied to a 1882 to 1995 seasonal Darwin sea level pressure (DSLP) anomaly time series to explore the variations in El Niño–like anomaly occurrence and persistence over the period of record. Return periods for the duration of the 1990–95 event are estimated to be considerably smaller than those recently obtained by Trenberth and Hoar using a linear ARMA model with the same time series. The likelihood of a positive anomaly of the DSLP, as well as its persistence, is found to exhibit decadalto centennial-scale variability and was nearly as high at the end of the last century as it has been recently. The 1990–95 event has a much lower return period if the analysis is based on the 1882–1921 DSLP data. The authors suggest that conclusions that the 1990–95 event may be an effect of greenhouse gas–induced warming be tempered by a recognition of the natural variability