This diagram shows the El Niño phenomenon. Observations have shown that the frequency and intensity of El Niño and la Nina has increased.
The strongest natural fluctuation of climate on interannual time-scales is the El Niño-Southern Oscillation (ENSO) phenomenon, and ENSO-like fluctuations also dominate decadal time-scales (sometimes referred to as the Pacific decadal oscillation). ENSO originates in the tropical Pacific but affects climate conditions globally. The importance of changes in ENSO as the climate changes, and its potential role in possible abrupt shifts have only recently been appreciated. Observational and modelling results suggest that more frequent or stronger ENSO events are possible in the future.
Technically, 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 trade winds 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 spread 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 again. The combination of the tropical air-sea instability and the delayed negative feedback due to sub-surface ocean dynamics can give rise to oscillations. 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). (TAR - Climate Change 2001, The Scientific Basis. 7.6.5)