The 1990 { 1995 El Ni ~ no - Southern Oscillation Event : Longest on recordKevin

The tendency for more frequent El Ni~ no events and fewer La Ni~ na events since the late 1970's has been linked to decadal changes in climate throughout the Paciic basin. Aspects of the most recent warming in the tropical Paciic from 1990 to 1995, which are connected to but not synonymous with El Ni~ no, are unprecedented in the climate record of the past 113 years. There is a distinction between El Ni~ no (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Ni~ no region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Paciic are better linked to the SO and remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal-timescale variation. One test ts an autoregressive-moving average (ARMA) model to a measure of the SO given by the rst hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are unexpected given the previous record, with a probability of natural occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.