Niño 3.4

Niño 3.4 is a rectangle of open ocean — 5°N to 5°S, 170°W to 120°W, straddling the equator in the central Pacific — that serves as El Niño's thermometer. When this site's headline number reads "+1.8°C," it means the sea surface in that box is running 1.8 degrees Celsius above its long-term average for the date.

Why that box? Early researchers defined several index regions (Niño 1+2 near South America, Niño 3, Niño 4 to the west). Region 3.4, overlapping 3 and 4, won out as the standard because its temperature couples most tightly to the atmosphere: water there sits near the convection threshold, so an anomaly reliably moves tropical rainfall — the step that turns warm water into global weather. Niño 1+2, by contrast, matters most for Peru's local floods and fishery.

Two versions of the number circulate. Weekly values (from the OISST satellite-blended dataset) are fast and slightly noisy — those drive our dashboard and gauge. The seasonal, smoothed version is the Oceanic Niño Index, the official record. Mid-June 2026's weekly reading near +1.7°C — strong-event territory before summer's end — is what put this event on super-El-Niño watch.