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How is the strength of El Niño determined? | CLIMAS

How is the strength of El Niño determined?

There are actually several different indices or metrics used to track the state of ENSO (the El Niño-Southern Oscillation) across the Pacific Ocean and whether oceanic and atmospheric patterns reflect El Niño, La Niña or neutral conditions. Some of these indices look just at the atmosphere (for example Southern Oscillation Index), some just at the ocean (for example Niño3.4 index) and some the combination of both (for example the Multivariate ENSO Index). All have different strengths and weaknesses, but oceanic based indices tend to reflect the more slowly evolving part of ENSO, the shift in sea surface temperature patterns, which in turn can have an impact on global weather patterns over many months or seasons.

The NOAA Climate Prediction Center uses the Oceanic Nino Index (ONI) which is the running 3-month average of sea surface temperature anomalies (or departures from average) across a key part of the central equatorial Pacific from just west of the International Date Line back towards the east to 120W longitude.

Tracking sea temperatures in this region provides a good indication of whether or not tropical convection (which is driven by warm waters) is shifting or may soon shift unusually far east along the equator during El Nino conditions or far west during La Nina events. ONI values typically range from -2.5 which would be indicative of a strong La Nina to 2.5 which would be a strong El Nino event. The NOAA-CPC makes a distinction between the emergence of El Nino or La Nina conditions which would be ONI values climbing past the +0.5 (El Nino) or -0.5 threshold at the monthly scale as well as other atmospheric indications like shifts in wind and rainfall patterns versus a full blown event that lasts for many months or over seasons. An event requires ONI values to be greater than the +0.5 or -0.5 threshold for at least five consecutive months (more details at

A version of this FAQ was originally published on the CLIMAS blog on Aug 13, 2014