The 1997-98 El Niño remains, by most measures, the strongest event of the 20th century and one of the strongest in the full instrumental record dating to 1950. It's the reference point every "Super El Niño" comparison gets measured against, including the one currently developing.
How It Unfolded
1997's event developed unusually fast, building from neutral conditions in early spring to a clearly established El Niño by summer, a speed of onset some forecasters have compared to what's been observed in 2026. By autumn, Niño 3.4 anomalies were already well past the "strong" threshold, and the event peaked above +2.0°C in the November-December-January window, qualifying as "very strong" by NOAA's current classification standards.
Globally, the impacts were severe: catastrophic flooding across Peru and Ecuador, devastating drought and wildfire conditions across Indonesia and parts of Southeast Asia, an unusually mild but wet winter across the southern United States, and a measurable bump in global average temperature that helped make 1998 the warmest year on record at the time.
Why Forecasters Keep Citing It
1997-98 set the template for what a "textbook" Eastern Pacific-dominant Super El Niño looks like: rapid spring onset, a clean winter peak, and a fast decay the following spring. When NASA scientists say 2026's western Pacific sea-surface-height pattern "looks similar to 1997," this is the event they're referencing, and it's exactly why the eastern Pacific's slower pace this year (see our Sentinel-6 coverage) is being watched so closely. If 2026 catches up to 1997's eastern Pacific pace, it's in rare company.
Which Index Are We Ranking On?
Calling 1997-98 "the strongest of the 20th century" requires naming the index, because the answer changes.
On the Oceanic Niño Index, the raw Niño 3.4 anomaly, 1997-98 peaked around +2.4°C for NDJ and was narrowly surpassed by 2015-16. On the Relative Oceanic Niño Index, which subtracts the tropical-mean anomaly and became NOAA's official standard in February 2026, the ordering shifts because 1997-98 occurred on a cooler background ocean than 2015-16 did. In RONI terms the strongest event in the modern record is 1982-83.
None of these events moves. What moves is the question being asked. For the full mechanics see our RONI vs ONI explainer, and for the head-to-head, 2015-16 versus 2026.
What 1997-98 Actually Cost
The impacts are usually described qualitatively, which undersells why this event became the reference case. It is also the event underpinning the economic literature now being quoted about 2026.
Callahan and Mankin, writing in Science in 2023, attributed roughly $5.7 trillion in reduced global economic growth to the 1997-98 event in the years that followed, using a method that measures foregone growth rather than physical damage. That figure is the anchor for the $10 trillion projection now circulating about the current event, which we examined in The $10 Trillion Number.
Why Analogs Mislead
A historical analog is a similarity, not a forecast, and 1997-98 is the most over-used analog in the field.
Two events can share a peak amplitude and produce very different weather, because the shape of the warming matters as much as the size. 1997-98 was strongly east-weighted. 2015-16 reached comparable amplitude with substantially more warmth in the central Pacific, and its regional outcomes differed accordingly, most visibly in a Northern California season that disappointed relative to expectations built on 1997.
The current event is showing a pronounced east-weighted structure of its own, which we track in the east-west gradient piece. That makes 1997-98 a more useful structural reference than 2015-16 this time. It still does not make it a forecast, and the background ocean in 2026 is warmer than 1997's by an amount that changes what the same index value means.