El Niño-Southern Oscillation (ENSO)

The El Niño-Southern Oscillation (ENSO) is a change in the surface ocean temperatures that occurs in the tropical Pacific Ocean. During the El Niño phase, surface waters are warmer than average, while they are cooler in the La Niña phase. Because the area of the ocean that is affected by the ENSO is so large, the impact of the ENSO is felt globally, and is a major contributor to short-term climate variability. Currently, we do not have a strong understanding of what drives the ENSO.

The oscillation between the two states of the ENSO is apparent in throughout the tropical Pacific. In its cool phase, warm water is pooled in the western Pacific, as cold, deep water upwells near the coast of Central and South America. During the warm phase, this upwelling is reduced, and warm surface water spreads across the entire tropical Pacific. The change in ocean surface temperatures is accompanied by changes in the atmosphere, including temperatures and rainfall patterns. Because of the massive size of the tropical Pacific, changes in weather patterns are observed on all continents except Europe.

Historically, the El Niño phase of the ENSO has been associated with years that are relatively warm throughout the globe. Thus, many of the previous years with record high global temperatures have included warm El Niño periods. In contrast, the La Niña phase is associated with temperatures that are generally cooler than those surrounding them. These changes occur on top of the general warming trend, however. Thus, a warm El Niño year from the 1960s is now cooler than an average year from the most recent decade; similarly, La Niña years have recently been sufficiently warm to be among the hottest years in the instrument record.

Although ENSO is the dominant influence on short term changes in the climate, it is not the only one, and its impact can be overridden by other climate forcings, such as large volcanic eruptions.