NASA Altimetry Stacks the 2026 El Niño Against 1997 and 2015

NASA Earth Observatory has placed this year's El Niño side by side with the two strongest events in the satellite altimetry record, publishing maps of sea surface height anomalies from October 1997, October 2015 and October 2026. According to the agency, sea levels in October 2026 are higher than normal across the central and eastern equatorial Pacific — a signature of El Niño — in a pattern that closely resembles the one observed in October 1997.

The Image of the Day feature, published 23 October 2026, draws on three different spacecraft covering three different decades: TOPEX/Poseidon for 13 October 1997, Jason-2 for 12 October 2015, and Sentinel-6 Michael Freilich for 12 October 2026. The data were processed by scientists at NASA's Jet Propulsion Laboratory, and the maps were made by NASA Earth Observatory's Lauren Dauphin.

Why sea surface height tracks El Niño

NASA explains that altimetry satellites measure the height of the sea surface, and that because water expands as it warms, higher-than-normal sea levels in the central and eastern equatorial Pacific indicate the buildup of heat associated with El Niño. On the published maps, red shading marks sea levels above average, white marks normal conditions and blue marks lower-than-average levels. The agency notes that seasonal cycles and long-term trends have been stripped out so that short-term anomalies, including those tied to El Niño, stand out.

The agency describes the underlying mechanism as a feedback loop between ocean and atmosphere: trade winds normally hold warm water piled up in the western Pacific, and when those winds weaken, the warm water pulses eastward, further weakening or even reversing the winds and allowing still more warm water to migrate east.

"What's unique about altimetry is that we have a 35-year record from nearly identical satellites flying almost the same ground track, with each mission overlapping the next," said Josh Willis, project scientist for sea level missions at JPL, in the NASA article. "That lets us look back in time and compare this El Niño to what happened in 1997. And right now, this one is on track to be even bigger."

Three events, three characters

NASA says each El Niño has its own character, shaped by where in the Pacific the warmth accumulates and how the atmosphere reacts. "The search for the 'typical' El Niño is very elusive," Willis said. "They're all different in terms of where they land and how their impacts play out."

On the agency's reading, the 2026 event arrived far more suddenly than the 1997–98 one, but as of October the two look similar in character: sea level tilted low in the west and high in the east, with high, warm water reaching all the way to South America. On that basis, NASA reports, Willis thinks the impacts could be similar too — in 1997–98 that included heavy rainfall in California and across the southern tier of the United States.

The 2015–16 event, which NASA also classes as very strong, looked different. According to the agency, less of the elevated water reached the South American coast that year, and rainfall impacts that winter concentrated more over Central America and the southeastern United States, with significant flooding in Mexico.

NASA frames El Niño as part of a natural climate pattern recurring roughly every two to seven years, and notes that the 1997–98 and 2015–16 events brought worldwide consequences, from fires in Indonesia and the Amazon to drought-driven crop failures in southern Africa and heavy rain and coastal flooding in Southern California. The current event, the agency says, rivals those predecessors.

The forecast, and the caveat

NASA Earth Observatory cites NOAA's El Niño outlook of 8 October 2026, which called for an 83 percent chance of a strong-to-very-strong El Niño through the first three months of 2027. How the impacts unfold in the 2026–27 winter and beyond, the agency says, remains to be seen.

Willis noted that El Niño's influence travels through a chaotic atmosphere and that the signal can become muddled as it spreads beyond the tropics. "There's always a bit of a dice roll here," he said. "Just because this El Niño looks like 1997 doesn't mean it will be exactly the same — it's just our best guess at the moment."

More background on measurement methods is available on our science and glossary pages.