The tools scientists have relied on for decades to track El Niño and La Niña are breaking down, and the reason is straightforward: the ocean is warming so fast that the old measuring sticks can no longer tell the difference between a genuine climate oscillation and the rising baseline temperature of the tropical Pacific. In response, major weather agencies on two continents have overhauled their monitoring systems, while a cluster of peer-reviewed studies now challenges the prevailing scientific consensus that El Niño’s basic character would remain unchanged in a hotter world. Together, these shifts amount to the most significant rethinking of the El Niño-Southern Oscillation, or ENSO, since it became a household term in the 1990s.
Why NOAA Retired Its Flagship El Niño Index
For years, NOAA’s Climate Prediction Center relied on the Oceanic Niño Index, or ONI, which compared sea surface temperatures in the central Pacific’s Niño-3.4 region against a rolling 30-year average. That approach worked well enough when background ocean temperatures were relatively stable. But as tropical sea surface temperatures climbed, the 30-year baseline began to lag behind real-time warming, making it harder to detect true ENSO events in a timely way. The result was a growing risk of misidentifying ordinary warming as an El Niño signal, or missing a genuine La Niña because the threshold had drifted.
The replacement, called the Relative Oceanic Nino Index, or RONI, takes a different approach. Instead of comparing Nino-3.4 temperatures to a fixed historical period, RONI subtracts the broader tropical ocean average (spanning 20 degrees South to 20 degrees North) from the Nino-3.4 reading, then adjusts for variance, according to NOAA’s methodology documentation. This design reduces dependency on any single base period and isolates the ENSO signal from background warming. The operational tables now reclassify historical El Nino and La Nina episodes from 1950 to the present under this new framework, and recent values remain subject to revision as the underlying ERSSTv5 dataset is filtered. Australia’s Bureau of Meteorology independently adopted a similar relative index for the same reason, warning that traditional indices can misrepresent how often ENSO events actually occur when the ocean keeps getting warmer.
New Research Challenges the IPCC’s “No Change” Consensus
The measurement overhaul would matter less if ENSO itself were behaving the same way it always has. But a growing body of research suggests it is not. The most recent assessment from the Intergovernmental Panel on Climate Change, published as part of the AR6 Working Group I report, stated that ENSO would remain the dominant mode of tropical climate variability and reported no model consensus for systematic change in ENSO sea surface temperature variability. That framing, however, is now under pressure. A study published in Nature Climate Change argues that century-scale ENSO SST variability increases under multiple emission scenarios when comparing 20th-century and 21st-century model runs, directly challenging the AR6 conclusion.
The tension between these positions is real, not just academic. If ENSO’s temperature swings are growing more intense, then the downstream effects on global weather patterns, from drought in Australia to flooding in South America, are being systematically underestimated in current planning frameworks. The AR6 did note that amplification of ENSO-related rainfall variability is “very likely” by the second half of this century under higher-emissions scenarios, per the IPCC’s own language. But acknowledging wetter and drier extremes while holding that the underlying ocean temperature oscillation stays the same creates an internal contradiction that newer studies are now forcing into the open.
Warming Rewires ENSO Phase Transitions
Beyond the question of whether El Nino events are getting stronger, scientists are finding that the sequence and timing of ENSO phases is shifting in ways that amplify global temperature spikes. Research published in Science Bulletin found that the transition from a multi-year La Nina directly into a strong El Nino, historically a rare event, becomes more likely under transient greenhouse warming. This matters because such transitions do not simply add warmth on top of an already warm baseline. They concentrate heat release from the tropical Pacific into a compressed window, producing temperature jumps that exceed what either the background trend or the ENSO event alone would generate.
That appears to mirror what happened during the 2022-to-2023 period. A prolonged La Nina gave way to one of the strongest recent El Nino episodes, according to analyses from the University of Washington. An attribution-style analysis published in Atmospheric Chemistry and Physics concluded that the resulting 2023 global warming spike was driven by this ENSO sequencing, and that the magnitude of the temperature jump was rarer than models would predict from internal variability alone but became far more probable when conditioned on the specific La Nina-to-El Nino transition. In plain terms, the order in which ENSO phases occur now matters as much as their individual strength, because rapid swings from cool to warm states can supercharge short-term global temperature records.
Paleo Records and Models Point to a More Volatile Future
Modern satellite and buoy records only capture a few decades of ENSO behavior, which makes it hard to distinguish long-term climate shifts from natural ups and downs. To fill that gap, researchers increasingly turn to paleoclimate evidence—corals, sediments, and other archives that encode past ocean conditions. A reconstruction study in Climate of the Past used such proxies to show that ENSO variability over the last millennium fits within a broad natural range, but it also demonstrated that climate models can reproduce that range while still projecting larger swings under continued warming. In other words, the past does not guarantee stability in the future; it simply provides a baseline against which emerging changes can be measured.
This convergence between paleoclimate reconstructions and forward-looking simulations strengthens the case that ENSO is entering a more volatile regime. When models that correctly simulate historical variability also project stronger or more frequent extremes in a high-emissions world, it becomes harder to dismiss recent observational trends as mere noise. Combined with the new work on phase transitions and the updated indices from operational agencies, the paleo record suggests that ENSO’s sensitivity to greenhouse forcing has been underestimated, and that the system may now be crossing thresholds that were rarely, if ever, reached in the preindustrial climate.
Forecasting, Risk, and the Limits of Old Assumptions
The scientific rethink around ENSO is not just an academic debate; it is reshaping how weather services issue forecasts and warnings. In the United States, the National Weather Service, whose mission is outlined on the agency’s organizational pages, depends on accurate ENSO monitoring to anticipate seasonal risks like Atlantic hurricane activity, Western drought, and flood potential across the South. As traditional indices lose reliability in a rapidly warming ocean, forecasters must lean on updated metrics like RONI, coupled climate models, and experimental tools that better separate natural oscillations from the long-term trend.
Those changes are already filtering into public-facing products. Routine outlooks on the National Weather Service website increasingly emphasize probabilistic ranges and scenario-based language, reflecting both improved modeling and heightened uncertainty about how ENSO will behave in a hotter climate. At the same time, the broader National Oceanic and Atmospheric Administration, described on its main information portal, is investing in ocean observations and data assimilation systems that can keep pace with rapid warming. The underlying message is that the old assumption—that El Nino and La Nina would remain essentially the same, just riding on top of a gradually warming world—no longer looks tenable. Policymakers and communities that still plan around that assumption risk being blindsided by sharper swings, more abrupt transitions, and compound extremes that strain infrastructure and disaster-response systems beyond what past experience would suggest.
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*This article was researched with the help of AI, with human editors creating the final content.
