EL Nio And The SouTHERN oSciLLATioN

In the next few sections we describe the phenomenon (and what a phenomenon it is!) known as El Niño, or sometimes as El Niño and the Southern Oscillation (ENSo). The Southern oscillation is the atmospheric part of the phenomenon, and El Niño, the oceanic component; ENSO is the combination. However, unless we need to be particularly precise, we often just use the term El Niño, for this has a pleasant euphony lacking in the acronym ENSO.2 El Niño is the largest and most important phenomenon in global climate variability on interannual timescales. In brief, it is an anomalous warming of the surface waters in the eastern equatorial Pacific, most pronounced just off the coast of Peru but extending westward to the dateline.

In the next few sections we describe in a little more detail what El Niño is, what mechanisms give rise to it, and what effects it has, or is perceived to have, on weather and climate throughout the world.

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