El Niño is a natural phenomenon in which warm water in the western Pacific ocean near the equator spreads eastward, until much of the equatorial Pacific is warmer than usual. When that happens, it affects temperature and rainfall patterns worldwide in a profound way. For one thing, the global average surface temperature is usually a few tenths of a degree higher, mainly because all that warm water is spread out where it can give off some of its heat to the air above. Perhaps more important are the changes in rainfall patterns, bringing drought to some areas and flooding to others.

The ocean region scientists most often use to characterize this phenomenon is the NINO 3.4 region, the area in a rectangle from latitudes 5°North to 5°South, and from longitudes 170°West to 120°West, as outlined in this map:

The map, from the Copernicus Climate Change Service, shows sea surface temperature anomaly, which at this time happens to be very high in the NINO3.4 region (exceptionally so in fact). That’s why everyone is talking about the “monster el Niño” that has arrived.

We can compute the average temperature anomaly in the NINO3.4 region at any point in time, and graph how it has changed over time, as here:

Big el Niño events in the past stand out as peaks in 1982-1983, 1997-1998, and 2015-2016, and tend to peak around Christmastime of the first year given, while global temperature peaks in the second year. The highest point of all is happening right now, so presumably in 2027 we’ll see record high global temperature. We can also plot this index as a function of day of the year (plotting multiple years over each other), and if we highlight this year (2026) in red it becomes obvious just how different it is from its predecessors. One can easily get the impression that this el Niño isn’t just big, it’s truly a monster.

But there’s a catch. The worldwide impact of el Niño depends less on how hot the anomaly is in the NINO3.4 region, than on how much hotter it is than the rest of the world’s equatorial ocean. Because the global equatorial ocean has slowly warmed over the years (global warming!), we need to compare the NINO3.4 anomalies to an evolving baseline.

Therefore we define a new quantity, the relative Oceanic Nino Index (RONI), as the difference between the NINO3.4 temperature anomaly, and the average for the global ocean from latitudes 20°North to 20°South. The result is then re-scaled so that it has the same variance as the original Oceanic Nino Index. And here are the relative values:

The final peak (the curren one) is no longer the highest of all. On the graph by day-of-the-year, this year (shown in red) is one among several others which follow a similar path — the previous strong el Niño events.

Using the relative Oceanic Nino Index, the current one is clearly big, but no bigger than other big el Niño events of the past.

I conclude that it is possible for this el Niño to behave much like previous ones. Its impact may be dramatic in some regions, but for Americans the most notable thing might be that people actually notice it because it’s getting so much press these days. As for tales of apocalypse from the monster, the mother of all el Niño events, the world might not notice dramatic differences between this year’s model and others from our own living memory. Then again, some have suggested that as the climate changes, the very nature of the el Niño phenomenon might change, and so too could its worldwode impacts, but as far as I know this is just speculation.

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