If 1997-98 is the classic textbook Super El Niño, 2015-16, nicknamed the "Godzilla El Niño" by some forecasters at the time, is the modern benchmark. Whether it is the record holder depends entirely on which index you ask, and that is not a technicality.
Record Holder Depends on the Index
Measured on the Oceanic Niño Index, the raw Niño 3.4 anomaly, 2015-16 is the strongest event since 1950, peaking around +2.6°C in the November-December-January season and narrowly surpassing 1997-98.
Measured on the Relative Oceanic Niño Index, which subtracts the tropical-mean anomaly and has been NOAA's official standard since February 2026, the picture changes. CPC's own published RONI table puts the 2015-16 peak at +2.4°C for NDJ 2015. And in RONI terms the record holder is not 2015-16 at all: Zeke Hausfather's analysis identifies 1982-83 as the strongest event on that index.
The reason is straightforward once stated. 2015-16 occurred on top of a warmer background ocean than 1982-83 did. ONI counts that background warmth as part of the event. RONI does not. Same ocean, same observations, two defensible answers to "which was strongest," and they disagree about which decade holds the record.
This site quotes RONI as its default, because that is what NOAA now uses to classify events and therefore what determines how the current event enters the historical record. For the full mechanics of the difference, see our RONI vs ONI explainer. The practical upshot for reading coverage: any "strongest on record" claim that does not name its index has skipped the step that decides the answer.
2015-16 By the Numbers
Similarities to 2026
Like the current event, 2015-16 built on top of an already-warming global ocean, and like 2026, model ensembles converged on a "very strong" classification well before the event actually peaked, though forecasters at the time noted the convergence happened somewhat later in the year than what's being seen now. Both events also featured a strong subsurface heat signal months ahead of the surface peak, the same kind of Kelvin wave buildup currently being tracked in the equatorial Pacific.
Where They Diverge
One notable difference: 2015-16 developed more gradually through the year, with its most dramatic ECMWF probability jumps coming later in the season than what's been observed in 2026's faster-moving forecast escalation. If that pace continues, 2026 could either be on track to match or exceed 2015-16's strength; or the extra speed could mean an earlier peak and earlier decay. Both are plausible outcomes at this stage, and it's exactly the kind of detail the next few CPC monthly updates should start to clarify.
Also worth remembering: 2015-16's regional impacts weren't uniform even within a single strongly-correlated region. Southern California got the expected wet winter, but Northern California's rainy season underperformed expectations, a reminder that "strong El Niño" doesn't mean "guaranteed outcome everywhere," even in the most reliable corner cases.