By now you've probably seen the headlines. ECMWF calling this a potential "strongest El Niño ever recorded." Models projecting Niño-3.4 SST anomalies above +3°C by December. NOAA's own probability tables putting a 63% chance on a very strong event. The ocean is hot, and it's getting hotter.
But there's a quieter counterpoint worth paying attention to, one that climate analysts who work closely with RONI — the Relative Oceanic Niño Index — have been raising since the Advisory dropped. The question isn't whether the ocean is warming. It clearly is. The question is whether the atmosphere is going to respond with the same intensity. And historically, those two things don't always move in lockstep.
Two Numbers, Two Stories
Right now, 2026 is telling two different stories depending on which index you use.
Models project +2.5–3.0°C+ by Dec 2026 — which would rival or surpass 2015-16 as the strongest on record.
RONI analogs from the past 30 years suggest a moderate atmospheric El Niño — not the record-smasher the raw SST implies.
That gap is not a contradiction. It's a feature of how RONI works — and why NOAA switched to it as the primary index in February 2026.
What RONI Actually Measures
The traditional ONI measures how warm the Niño-3.4 region of the Pacific is relative to a fixed historical baseline. The problem: the entire ocean has been warming for decades due to climate change. A reading of +1.5°C today is not the same climate event as +1.5°C in 1983, because the background ocean is warmer now than it was then.
RONI corrects for this by subtracting the tropical mean SST anomaly (the average warming across the whole tropics, 20°N–20°S) from the Niño-3.4 reading. What's left is the ENSO-specific warming — the Pacific heat that's actually above and beyond the global trend. That's the signal that drives the Walker Circulation, the atmospheric engine of El Niño.
Put simply: RONI measures how anomalous the Pacific is relative to the rest of the tropics. The raw SST measures how anomalous it is relative to 30 years ago — which increasingly includes a warming baseline that isn't ENSO.
This distinction matters enormously for 2026. The tropical oceans broadly are running warm right now — not just the Pacific. When you subtract that global baseline from the Niño-3.4 reading, you get a more modest number. That's not the Pacific "failing" to warm. It's the index doing exactly what it was designed to do: isolating the ENSO signal from the background noise of a warming planet.
The RONI Analog Problem
Here's where the forecast gets genuinely uncertain. When analysts at Climate Impact Company ran RONI-based analogs — looking at past years where the RONI profile most closely matched 2026's trajectory — they found something that cuts against the "record El Niño" narrative:
That's a meaningful divergence. The same models projecting record-hot Pacific SSTs are also the ones driving the "very strong" probability. But if the atmospheric response — the actual breakdown of the Walker Circulation, the westerly wind anomalies, the convection shifts — tracks the RONI analog rather than the raw SST model, the weather impacts across North America could be considerably more moderate than the headlines suggest.
Why the Atmosphere Might Not Keep Up
El Niño's weather impacts don't come directly from warm water. They come from what warm water does to the atmosphere. The sequence goes like this: elevated SSTs in the central and eastern Pacific → enhanced convection and rainfall over that region → disruption of the Walker Circulation (the east-to-west tropical wind pattern) → shift in the jet stream → downstream weather anomalies across North America, Australia, and elsewhere.
If the SST anomaly is partly a product of global background warming rather than pure ENSO forcing, the atmospheric response can be muted. The ocean is warm, but the tropics broadly are warm too — the contrast that drives convection anomalies is smaller than the raw SST number implies. The atmosphere is responding to the gradient, not the absolute temperature.
This is exactly why Weather West's Daniel Swain — while calling 2026 a likely very strong event — has been careful to note that even with this much oceanic heat, "it is impossible to know, this far in advance, whether California will see a major flood event during winter 2026-2027." The ocean loading up doesn't guarantee the atmospheric teleconnection fires cleanly.
Two Plausible Scenarios for Winter 2026-27
| Scenario | Driver | Pacific SSTs | Atmospheric Response | U.S. Winter Impact |
|---|---|---|---|---|
| S1: Record Event | Raw SST models (CFS v2, ECMWF) are correct | +2.5–3.0°C+ by NDJ | Strong Walker Circulation breakdown, clear jet stream displacement | Classic very strong pattern: wet Southwest/California, warm North, dry PNW. High confidence in teleconnections. |
| S2: Moderate Event | RONI analogs are more representative | High in absolute terms but RONI peaks moderate | Muted atmospheric response despite warm ocean; Walker Circ partially intact | Weaker or delayed teleconnections. California may not see the flooding rains many are forecasting. Pattern less locked-in. |
Neither scenario is fringe. Both are consistent with known ENSO physics. The uncertainty isn't about whether El Niño exists — the Advisory is clear on that — it's about which feedback loop dominates between now and December.
What to Watch Over the Next 90 Days
The key signals that will tell us which scenario is playing out:
- Southern Oscillation Index (SOI). Sustained strongly negative SOI (below −8 or so) means the atmosphere is fully coupling with the ocean. A weakly negative or erratic SOI into August would signal the muted-response scenario.
- Westerly wind anomalies. Are WWAs extending across the central Pacific and holding? That's atmospheric engagement. Episodic bursts that don't sustain point toward a weaker coupling.
- Subsurface heat content trend. The current warm anomaly in the upper 300m is the "fuel tank." If it depletes faster than expected — as downwelling Kelvin waves spread heat upward and it escapes to the atmosphere — peak SSTs may come earlier and lower than models project.
- The July 9 CPC discussion. NOAA will have another full month of data including the JJA seasonal window. If the RONI seasonal average crosses +0.5°C clearly, confidence in the strong-event scenario increases.
The RONI Takeaway
This is the core argument for why NOAA switched to RONI in the first place. In a warming world, raw SST anomalies overstate ENSO strength because they include a rising baseline. RONI strips that out and asks: how much of this Pacific warmth is actually El Niño, versus how much is just the ocean getting warmer everywhere?
For 2026, the honest answer is: we don't know yet which story is more true. The oceanic case for a very strong event is compelling — subsurface heat content is enormous, the SOI has gone strongly negative, the CPC has issued its Advisory. But the RONI analog case for a more moderate atmospheric response is also grounded in real physics and real historical precedent.
The most useful frame right now: plan around a strong event. Don't panic about a record one. Keep watching the atmospheric coupling signals — they'll tell the real story before the models do.