Is El Nino changing?

We review the characteristics of El Niño ocean surface anomalies over the past 50 years. Using a composite analysis the statistically significant features are identified and those which are present in every event or all-but-one event are selected. From these features a new multi-variate index, the Bjerknes ENSO Index (BEI) is constructed. We present this and other time series that have been used to monitor El Niño and note some interesting differences between them. In particular, the indices differ substantially in the picture they paint about the extent of unusual El Niño conditions during the early 1990s. The challenges of detecting statistically significant change in the characteristics of events are reviewed. The existing data do not support the interpretation that El Niño is quantitatively different since, e.g., the mid-1970s, or that conditions present during the early 1990s were unprecedented.