The evolution of the unusual warming events of the first half of the 1
990s is discussed based on the National Center for Environmental Predi
ction ocean re-analysis data, covering 1980-1995. The warmings occurri
ng in 1991-1992, 1993, and 1994 are viewed as individual episodes, rat
her than as one long El Nino. It is shown that the warm episodes of th
e 1990s differ from previous El Ninos in two important ways. First, su
bsurface anomalies are more strongly coupled to the mixed layer across
the equatorial Pacific in the 1990s than in earlier years. Second, a
persistent warm SST anomaly in the central Pacific has been instrument
al in initiating warm events. Both of these factors interfere with the
dominant mode of variability in the tropical Pacific air-sea system,
namely the delayed oscillator mechanism. We postulate that the low ski
ll of dynamical atmosphere-ocean models in predicting the recent warmi
ngs is related to a weakened delayed oscillator mechanism.