What is El Niño?

Weather
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The El Niño-Southern Oscillation (ENSO) cycle refers to a large-scale ocean-atmospheric interaction linked to either a warming or cooling phase in sea surface temperatures (SSTs) across the east-central Equatorial Pacific. El Niño and La Niña are opposite phases of ENSO, and can each impact large-scale ocean processes and global weather and climate. With Equatorial Pacific SSTs continuing to warm this Fall, it is likely an El Niño will develop in the Northern Hemisphere.

The latest visual of above normal SSTs across the Equatorial Pacific through late November 2018:

How is El Niño studied and monitored?
Scientists track the sea surface temperature (SST) anomalies across four regions in the Equatorial Pacific: Niño-4, Niño-3.4, Niño-3, Niño-1+2. SST anomalies are departures from the 1981-2010 base period weekly SST normals.

The Oceanic Niño Index (ONI) is based on SST departures from average in the Niño 3.4 region specifically, and is a principal measure for monitoring, assessing, and predicting ENSO through the coming winter. By historical standards, to be classified as a full-fledged El Niño, the ONI must be equal or greater to +0.5ºC for a period of at least 5 consecutive overlapping 3-month seasons. 

ENSO phases can have large-scale effects on global and local weather.
Typical El Niño effects are likely to develop over North America during the upcoming winter season. Those include warmer-than-average temperatures over western and central Canada, and over the western and northern United States. Wetter-than-average conditions are likely over portions of the U.S. Gulf Coast and Florida, while drier-than-average conditions can be expected in the Ohio Valley and the Pacific Northwest. The presence of El Niño can significantly influence weather patterns, ocean conditions, and marine fisheries across large portions of the globe for an extended period of time.

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