El Nino in the 1990s

The evolution of the unusual warming events of the first half of the 1990s is discussed based on the National Center for Environmental Prediction ocean re-analysis data, covering 1980-1995. The warmings occurring in 1991-1992, 1993, and 1994 are viewed as individual episodes, rather than as one long El Nino. It is shown that the warm episodes of the 1990s differ from previous El Ninos in two important ways. First, subsurface 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 instrumental 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 skill of dynamical atmosphere-ocean models in predicting the recent warmings is related to a weakened delayed oscillator mechanism.