NASA’s Sentinel-6 Michael Freilich satellite tracked a wave of warm water stretching 14,500 kilometers across the Pacific this spring. The wave raised sea levels near Peru by more than 15 centimeters. On June 11, NOAA declared that El Niño had formed.
Forecasters now say the event could grow into one of the strongest on record.
How a Kelvin Wave Works
Trade winds normally blow from east to west across the Pacific. They pile up warm water near Indonesia and let cold water rise near South America, a process called upwelling.
Sometimes those winds weaken and reverse. Warm water then slides back east in a slow-moving swell called a Kelvin wave.
The warm water expands and pushes the sea surface higher. It also pushes down the thermocline, the boundary between warm surface water and cold depths below. That shuts off the upwelling that normally cools the eastern Pacific.
Severine Fournier, a sea level researcher at NASA’s Jet Propulsion Laboratory and deputy project scientist for Sentinel-6, said conditions in the western Pacific in early June resembled those before the historic 1997 El Niño. “For now, it looks like it’s going to be a big one — more so than I would have said last week — but we still need more observations to know what’s going to happen,” Fournier said.
From Micronesia to Peru
Sentinel-6 first picked up a small Kelvin wave near Micronesia in January. That wave faded by mid-February.
A larger wave formed in March and moved east. By mid-May, sea levels near Peru sat more than 15 centimeters above their long-term average.
NOAA declared El Niño on June 11, after Pacific sea surface temperatures stayed at least 0.5 degrees Celsius above average for several months in a row.
Odds of a ‘Super’ El Niño
NOAA forecasters put the odds at 63 percent that sea surface temperatures in the tropical Pacific will climb past 2 degrees Celsius above average. NOAA classifies any event that crosses that line as “very strong,” on par with 1997 and 2015.
El Niño tends to peak in winter. NOAA expects this one to strengthen through the fall and into early 2027.
What It Could Mean
Strong El Niño events have historically brought wetter weather to the southwestern United States and drought to parts of Indonesia and Australia. They also tend to raise global average temperatures for six to twelve months, on top of the warming already caused by greenhouse gas emissions.
Fournier’s team will keep watching the satellite data in the coming weeks to see whether this El Niño catches up to 1997 levels. The ocean has already shown its hand. Now scientists wait to see how far it goes.
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