Good, we should learn this stuff. This webpage:

* NOAA, [Equatorial Pacific sea surface temperatures](https://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php).

says:

> El Niño (La Niña) is a phenomenon in the equatorial Pacific Ocean characterized by a five consecutive 3-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region that is above (below) the threshold of +0.5°C (-0.5°C). This standard of measure is known as the Oceanic Niño Index (ONI).

The Niño 3.4 region is the rectangular region surrounded by thick lines here: