ANOMALOUS ENSO OCCURRENCES - AN ALTERNATE VIEW

There has been an apparent increase in the frequency and duration of E l Nino-Southern Oscillation events in the last two decades relative to the prior period of record. Furthermore, 1990-95 was the longest peri od of sustained high Darwin sea level pressure in the instrumental rec ord. Variations in the frequency and duration of such events are of co nsiderable interest because of their implications for understanding gl obal climatic variability and also the possibility that the climate sy stem may be changing due to external factors such as the increased con centration of greenhouse gases in the atmosphere. Nonparametric statis tical methods for time series analysis are applied to a 1882 to 1995 s easonal Darwin sea level pressure (DSLP) anomaly time series to explor e the variations in El Nino-like anomaly occurrence and persistence ov er the period of record. Return periods for the duration of the 1990-9 5 event are estimated to be considerably smaller than those recently o btained by Trenberth and Hear 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 decadal- to centennial-scale variability and was nearly as high at the end of the last century as i t 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 sugge st that conclusions that the 1990-95 event may be an effect of greenho use gas-induced warming be tempered by a recognition of the natural va riability in the system.