The evolution of oceanic and atmospheric anomaly fields for the period prior to and during the 1986–87 El Niño/Southem Oscillation (ENSO) is presented. A comparison is made between the 1986–87 ENSO and other warm episodes that occurred during the last 20 yr. In addition, for the first time, an ocean general circulation model was run in a real-time diagnostic mode. The model output provided detailed information about the evolution of subsurface features throughout the Pacific basin. A slow trend towards warm episode (ENSO) conditions in the Pacific was evident throughout the period 1985–86 in certain atmospheric and oceanic variables. Atmospheric and oceanic fields changed much more rapidly during late 1986 as enhanced atmospheric convection developed in the equatorial Pacific near the date line. Thermocline depths rapidly increased (decreased) in the eastern (western) equatorial Pacific as low-level westerlies developed in the western portion of the basin. A remote response to those westerlies wa... Abstract The evolution of oceanic and atmospheric anomaly fields for the period prior to and during the 1986–87 El Niño/Southem Oscillation (ENSO) is presented. A comparison is made between the 1986–87 ENSO and other warm episodes that occurred during the last 20 yr. In addition, for the first time, an ocean general circulation model was run in a real-time diagnostic mode. The model output provided detailed information about the evolution of subsurface features throughout the Pacific basin. A slow trend towards warm episode (ENSO) conditions in the Pacific was evident throughout the period 1985–86 in certain atmospheric and oceanic variables. Atmospheric and oceanic fields changed much more rapidly during late 1986 as enhanced atmospheric convection developed in the equatorial Pacific near the date line. Thermocline depths rapidly increased (decreased) in the eastern (western) equatorial Pacific as low-level westerlies developed in the western portion of the basin. A remote response to those westerlies wa...