Guide
Niño 3.4 index: current reading and what it means
The Niño 3.4 index is the sea surface temperature anomaly averaged over the Niño 3.4 region of the equatorial Pacific, between 5°N and 5°S and between 170°W and 120°W. NOAA publishes two weekly versions of it: a conventional anomaly and a relative anomaly. Both are shown below with their own observation dates, which can differ because they are separate products.
| Reading | Value | Week centred | Dataset |
|---|---|---|---|
| Relative weekly anomaly | +2.0 °C | 9 September 2026 | Relative OISST v2.1 |
| Conventional weekly anomaly | +2.9 °C | 9 September 2026 | OISST v2.1 |
| Sea surface temperature | 29.6 °C | 9 September 2026 | OISST v2.1 |
Open the relative Niño 3.4 chart →
Open the conventional Niño 3.4 chart →
What the Niño 3.4 region covers
The Niño 3.4 box sits in the central and eastern equatorial Pacific, straddling the equator from 5°N to 5°S and spanning 50 degrees of longitude from 170°W to 120°W. It overlaps the western half of the Niño 3 region and the eastern part of Niño 4. NOAA chose it as the primary ENSO monitoring region because sea surface temperature changes there are closely coupled to the shifts in tropical rainfall and trade winds that define El Niño and La Niña. Both the seasonal Relative Oceanic Niño Index and the older Oceanic Niño Index are built from Niño 3.4 temperatures.
Anomaly versus absolute sea surface temperature
An anomaly is a difference, not a temperature. The conventional weekly anomaly is the observed sea surface temperature in the region minus the 1991–2020 average for the same time of year. As a labelled hypothetical example: if the long-term average for a week in early September were 27.0 °C and the observed value were 28.2 °C, the conventional anomaly would be +1.2 °C. The conventional dataset reports both numbers, which is why a sea surface temperature appears in the table above. The relative dataset does not include absolute temperatures, so this site never derives one from a relative anomaly.
Why the relative and conventional readings differ
The conventional anomaly compares Niño 3.4 only with its own past. The relative anomaly additionally subtracts the temperature anomaly averaged across the wider tropics for the same week. When the whole tropical belt is warmer than the 1991–2020 baseline, part of the conventional Niño 3.4 anomaly reflects that shared warmth rather than a shift in the Pacific temperature pattern. The relative version removes that shared component, so it isolates how much warmer or cooler Niño 3.4 is compared with the tropics around it. In a warming climate the two values drift apart, and relative values are generally lower than conventional ones during warm periods. NOAA now uses relative anomalies for its official ENSO monitoring; see RONI vs ONI for the seasonal version of the same change.
What one weekly reading can and cannot say
A single weekly anomaly describes one week in one region. It cannot establish an ENSO episode. NOAA’s reference threshold of ±0.5 °C is a guide for reading the index, and its historical classification of El Niño and La Niña episodes requires the seasonal index to stay beyond that threshold for at least five consecutive overlapping three-month seasons. The operational assessment also looks for a matching response in the atmosphere. So a weekly Niño 3.4 value of +0.8 °C does not by itself mean El Niño, and a week near zero does not mean an event has ended. The current phase shown on the homepage comes from NOAA’s dated ENSO Diagnostic Discussion, never from a chart reading. Weekly Niño 3.4 values are also not the Oceanic Niño Index, which is a three-month average of a different monthly dataset.
How to use the chart
The homepage chart opens in relative mode by default. The comparison view overlays years by calendar date so you can see whether this year’s weekly values are running warmer or cooler than selected past years at the same point in the season; the record view shows the whole weekly series from 2 September 1981 onwards. Switch between relative and conventional with the basis buttons, or use the two links above to open the chart already set to the definition you want. Missing weeks appear as gaps rather than zeros.
The chart also offers the three other Niño regions from the same NOAA files. Niño 4 (5°N–5°S, 160°E–150°W) lies further west, Niño 3 (5°N–5°S, 150°W–90°W) extends east of Niño 3.4, and Niño 1+2 (0–10°S, 90°W–80°W) covers the coastal waters off Peru and Ecuador, where anomalies are often largest and most variable. Niño 3.4 remains the region NOAA’s ENSO indices are based on.
Sources
- NOAA CPC relative weekly Niño region anomalies (Relative OISST v2.1) ↗
- NOAA CPC conventional weekly Niño region anomalies and SST (OISST v2.1) ↗
- NOAA CPC description of the Niño regions ↗
- How ENSO Tracker handles these datasets