Four boxes along the equatorial Pacific where sea temperature is averaged to track ENSO. Nino 3.4 is the main one.
Indices
Scientists average the sea surface temperature over fixed boxes of ocean:
Nino 1+2: off the coast of Peru and Ecuador.
Nino 3: the eastern equatorial Pacific.
Nino 3.4: the east-central equatorial Pacific. The reference for El Nino.
Nino 4: the central and western equatorial Pacific.
The boxes are drawn on the Map page.
How to read it
When the eastern boxes (1+2, 3) are much warmer than Nino 4, the event is "eastern Pacific" type, like 1997-98 and 2026. When Nino 4 is warmest, it is a "central Pacific" event.