Central Pacific's Niño 3.4 region passes 30C for the first time as El Niño strengthens

An absolute temperature is a poor way to rank an El Niño. Timothy Osborn, director of the Climatic Research Unit at the University of East Anglia, told Live Science that it is not the standard measure of strength ^[7]. Researchers compare Niño 3.4 with its own average conditions, or with sea surface temperatures across the rest of the tropics ^[7]. The second method is the control. It removes the skew that climate-driven heating of the oceans as a whole puts into the data ^[7].

That control matters this year. This El Niño is being amplified by human-driven warming that has raised background ocean temperatures ^[12]. A first 30C day in Niño 3.4 combines two signals: the event itself, and the warming of the ocean around it ^[7][12]. Jin-Yi Yu, a professor of Earth system science at the University of California, Irvine, said the data indicate "an exceptionally warm state for this part of the tropical Pacific" ^[2].

On the warming-adjusted measures, the event is still large. Osborn said that by those measures the event already nearly matches the strongest El Niño on record, and that he expects it to overtake earlier records soon because forecasts show it strengthening further ^[8]. "For some measures, it is already record-breaking for the time of year," he said ^[9]. Osborn and Mat Collins, head of mathematics and statistics at the University of Exeter, each called the reading "very significant" ^[4].

The anomaly gives the effect size. Measured against the top of the typical range, the current anomaly is 1.34C higher, about 1.7 times the usual upper limit ^[18][19]. "This indicates that a very strong, or what is sometimes called a 'super,' El Niño is developing," Yu said ^[11]. Forecasters have suggested the anomaly could reach 4C by November, 0.66C above the Oct. 6 value ^[17][20]. "The Niño 3.4 anomaly is very likely to remain extremely large and may increase further," Yu said ^[14].

Strength matters because of what it does to the atmosphere. "An El Nino with record strength is very significant because it greatly disrupts the atmospheric circulation which controls the weather patterns including patterns of rainfall," Osborn said ^[15].

Live Science framed the larger question as the world nears a critical warming threshold: whether conditions like these are a new normal for future El Niños ^[21]. The thing this doesn't tell you is whether the current event is a new type. One El Niño, however strong, is a single case. A new kind of event would have to show up as a pattern across many events on the warming-adjusted index. The measurements so far describe one event that has not yet peaked ^[13].