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The Niño3.4 SST anomaly index is an indicator of central tropical Pacific El Niño conditions. It is calculated with SSTs in the box 170°W - 120°W, 5°S - 5°N.

Anomalously warm conditions are associated with El Niño events, which peak in boreal fall/winter. NOAA/PMEL hosts an El Niño theme page with further information on the tropical Pacific.

Data source

The index is calculated using the Reynolds OIv2 SST analysis, made available through the IRI Data Library, and is updated weekly (last update 23-NOV-2016).   get series (netcdf 24kB) »  get error (netcdf 24kB) »


The anomaly is calculated relative to a climatological seasonal cycle based on the years 1982-2005. The weekly series was linearly interpolated to a daily series to calculate the climatological seasonal cycle with daily resolution. The leap day (29 February) was treated as a special case and linearly interpolated between the climatology of 28 February and 1 March. Spatial averaging of the gridded analysis was weighted by surface area. The standard deviation of the index over the period 1982-2005 is indicated on the plot.


The uncertainty in the estimate of the index is shown in gray, surrounding the zero x-axis, and is based on the uncertainty estimate of the analysis. For the surface indices, it is generally small compared to the value of the index.    more details »

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