El Nino Watch
NOAA's official ENSO index since 1 February 2026: the Nino 3.4 anomaly minus the average anomaly of the whole tropical ocean (20° S-20° N), as a 3-month mean.
Indices
Why it exists: the whole tropical ocean has been warming. With the classic ONI, part of that general warming shows up as "El Nino-like" warmth, and the 30-year baseline struggles to keep up in real time.
What matters for the weather is whether the Nino 3.4 region is warmer than the rest of the tropics, because that contrast is what moves the rain and the winds. RONI measures exactly that:
The thresholds are unchanged: +0.5 °C for El Nino, -0.5 °C for La Nina, 5 consecutive overlapping seasons for a historical episode. NOAA made RONI official through NWS Public Information Statement 26-05, effective 1 February 2026.
Read it like the ONI. RONI is usually a bit lower than ONI when the whole tropics are unusually warm (2023-24: ONI peak +2.0, RONI peak +1.4).
NOAA now states El Nino strength and forecasts in RONI (for example, the 75% chance of a RONI of +2.5 °C or more in October-December 2026). Note: the app's ENSO factor still uses ONI and the weekly Nino 3.4 value, which run higher than RONI this year (JJA 2026: ONI +1.80, RONI +1.36).