One Ocean, Two Numbers: Why CPC Says +1.1°C and IRI Says +1.7°C

On June 22, 2026, IRI published its Technical ENSO Update and reported that the latest weekly Niño 3.4 index, centered on June 17, had climbed to +1.7°C.

The weekly value this site cached from NOAA CPC for that identical week, centered June 17, 2026, was +1.1°C.

Neither is a typo, neither is stale, and neither institution is wrong. They are publishing two different indices that happen to share a name. The 0.6°C between them is the single most useful thing you can understand about El Niño coverage right now, because that gap is quietly sitting underneath the phrase "strongest El Niño ever recorded."

What Each Agency Is Actually Publishing

NOAA replaced the Oceanic Niño Index with the Relative Oceanic Niño Index on February 1, 2026. That change did not stop at the seasonal index. It went all the way down into the weekly figures CPC quotes in its monthly discussion, and it is stated plainly in the fine print rather than in the headline.

The figure captions in CPC's ENSO Diagnostic Discussion describe exactly what is being plotted: area-averaged relative sea surface temperature anomalies in the Niño regions, defined as the region anomaly minus tropical mean (20°N-20°S), with the relative indices "re-scaled to match the variance of traditional indices." The official probability forecast and the event strength probabilities carry the same definition.

That second step matters and is easy to skip past. The relative index is not simply Niño 3.4 minus the tropical mean. It is that difference multiplied by a variance-restoration scaling factor, so that the resulting index has the same statistical spread as the traditional one and the familiar ±0.5°C thresholds still mean what they used to. A practical consequence: the gap between the traditional and relative values is not equal to the tropical-mean anomaly. It reflects the subtraction and the rescaling together.

IRI states its own convention with equal clarity, in the opposite direction. Its Technical ENSO Update notes that "the primary metric used to monitor the El Niño-Southern Oscillation (ENSO) is the traditional Niño3.4 index (TONI), defined as the area-averaged sea surface temperature anomaly over the Niño3.4 region." No tropical-mean subtraction. Same box, same 1991-2020 climatology, different arithmetic.

Both organizations documented what they were doing. The problem is downstream, where the label falls off and only the number survives.

The Same Week, Measured Twice

Here is the relative weekly series this site pulls from CPC, alongside the two points where a published document lets us pin the comparison down.

All values in that table are relative, in degrees Celsius. Around the June 17 comparison, the relative value was 0.6°C lower than the traditional one. The size of that gap can change over time, so treat it as one measured comparison rather than a fixed offset you can apply to any week.

How to Tell Which Number You Are Looking At

There is a clean way to confirm which series CPC is quoting, and it does not require taking anyone's word for it.

CPC's July 9, 2026 discussion states: "The latest weekly Niño-3.4 index value was +1.2°C, with the westernmost (Niño-4) and easternmost (Niño-1+2) indices at +0.5°C and +2.7°C, respectively." The relative weekly table for the week centered July 1, 2026 reads +1.2, +0.5, +2.7. Three values, three exact matches.

Run the same test on the previous month. The June 11, 2026 discussion gives +0.7°C, +0.7°C and +2.1°C. The relative table for the week centered June 3 reads +0.7, +0.7 and +2.0, matching on Niño 3.4 and Niño 4 exactly and on Niño 1+2 within one revision-sized tenth.

So when CPC says +1.2°C, that is the relative index. It is not a lagging figure, and it is not a different week. It is a different quantity from the one IRI, the model plumes, and the daily SST trackers are reporting.

Practical checks for the rest of 2026. Note that these are about identifying the source, not about pattern-matching the magnitude:

• Anything sourced to the CPC Diagnostic Discussion, CPC's official ENSO probabilities, or CPC's strength categories is relative.
• IRI's current plume is presented in traditional Niño 3.4 terms.
• Individual NMME and C3S products may be traditional or relative, so the product documentation must be checked. CPC uses relative Niño 3.4 model guidance, and ECMWF also computes relative Niño indices.
• A dataset is not an index convention. Either index can be computed from OISST, so "OISST-based" on its own does not tell you which one you are reading unless the tracker states whether it is adjusted.
• A number with no index named and no source linked is not a number you can use.

Why This Matters for "Strongest Ever"

This is where the 0.6°C stops being pedantry.

Zeke Hausfather's July 13, 2026 analysis pooled 667 ensemble members across 14 seasonal forecast systems and put the multi-model median peak at 3.6°C, measured as detrended Niño 3.4. That is a genuinely extraordinary number, roughly 0.8°C above the prior record. It is also the reason "strongest ever by a mind-blowing margin" traveled as far as it did.

Hausfather ran the same ensemble both ways, and the two framings do not say the same thing:

• In traditional Niño 3.4 terms, about 91% of members exceed the prior record at their peak.
• In relative terms, about 77% do.
• The record being challenged also changes. In traditional terms it is 2015-16 at a peak monthly 2.75°C. In relative terms it is 1982-83 at 2.69°C, and 2015-16 is not the benchmark at all.

Both numbers point the same direction. A 77% chance of a record is still a remarkable forecast. But 91% and 77% are different claims, and coverage that quotes the first while implying the second is overstating the confidence by a meaningful margin. Hausfather stated the distinction explicitly. Most of what syndicated from his post did not carry it.

The same split runs through the strength probabilities. CPC's 81% chance of a very strong El Niño during October-December is computed on the relative index. IRI's finding that 13 of 24 models reach very strong intensity in SON 2026 uses a traditional-index threshold of +2.0°C. Those are two different bars at two different heights, and they are not interchangeable.

What This Does Not Mean

It does not mean the event is being oversold. El Niño is present, CPC carries an El Niño Advisory, and the coupled indicators are aligned: negative SOI, low-level westerly anomalies, enhanced convection over the central and east-central Pacific, suppressed convection over Indonesia, and a substantial subsurface heat reservoir. Niño 1+2 reached +3.0°C on the relative basis for the week centered July 22, which is a pronounced east-weighted SST pattern at that point in the event. None of that is softened by the index question.

It does not mean the relative index is a way of making El Niño look smaller. It exists because the traditional index measures Niño 3.4 against its applicable 30-year climatology but does not subtract contemporaneous warming across the wider tropics, which increasingly mixes the ENSO signal with the background trend. Subtracting the tropical mean and restoring the variance is an attempt to measure the same physical phenomenon consistently across eras, which is precisely what you need if you want to say the words "on record" and have them mean something.

And it does not mean model agreement settles the outcome. As Hausfather noted directly, no ensemble has ever forecast and then verified a 3.6°C event, because one has never happened. Agreement across models is not the same as demonstrated skill in territory none of them has been tested in.

What to Watch

• 13 August 2026: the next CPC Diagnostic Discussion, and the direct test of the callout above.
• The MJJ and JJA seasonal RONI values. As of this writing the published relative seasonal series runs through AMJ 2026 at +0.47, against a traditional ONI of +0.98 for the same season. That 0.5°C seasonal gap is the same effect as the weekly one.
• Whether the gap widens or narrows. Around the June 17 comparison the relative value was 0.6°C lower, and the gap can change over time. A widening gap would mean the broader tropics are warming alongside the Pacific, which pulls the relative reading down further even as absolute temperatures climb.
• Niño 1+2 versus Niño 4. Niño 1+2 at +3.0 and Niño 4 at +0.3 for the week centered July 22 is a widening east-west contrast, which matters for teleconnections independently of which basin-wide index you prefer.
• Whether outlets start labeling. The simplest fix is a four-word parenthetical naming the index. Watch for who adopts it.

If you take one thing from this: the number is meaningless without the index name attached. Ask which one. It is a fair question, and right now it is the difference between a 77% claim and a 91% one.