ENSO is generated by ocean-atmosphere interactions internal to the tropical Pacific and overlying atmosphere. Positive temperature anomalies in the eastern equatorial Pacific (characteristic of an El Niño event) reduce the normally large sea surface temperature difference across the tropical Pacific. As a consequence, the trade winds weaken, the Southern Oscillation index (defined as the sea level pressure difference between Tahiti and Darwin) becomes anomalously negative, and sea level falls in the west and rises in the east by as much as 25 cm as warm waters extend eastward along the equator. At the same time, these weakened trades reduce the upwelling of cold water in the eastern equatorial Pacific, thereby strengthening the initial positive temperature anomaly. The weakened trades also cause negative off-equatorial thermocline depth anomalies in the central and western Pacific. These anomalies propagate westward to Indonesia, where they are reflected and propagate eastward along the equator. Thus some time after their generation, these negative anomalies cause the temperature anomaly in the east to decrease and change sign. The combination of the tropical air-sea instability and the delayed negative feedback due to sub-surface ocean dynamics can give rise to oscillations (for a summary of theories see Neelin et al., 1998). Two of these feedbacks are schematically illustrated in Figure 7.8. Beyond influencing tropical climate, ENSO seems to have a global influence: during and following El Niño, the global mean surface temperature increases as the ocean transfers heat to the atmosphere (Sun and Trenberth, 1998).