Scientists say they’ve pinpointed the super-warm undersea structure responsible for this year’s El Niño weather pattern, which they fear will be one of the worst warming events in recorded history.

Called a Kelvin wave, scientists have identified a massive pool of warm water in the Pacific carrying temperatures up to 13.5 degrees Fahrenheit above average in similar parts of the ocean. As the Wall Street Journal notes, that’s a major heat wave as far as the ocean is concerned, as deep water temperature patterns take much longer to shift than they would on land.

Kelvin waves are fueled by abrupt changes in wind force, such as the westerly bursts that push the superheated waters from the west Pacific to the east. That shift in wind forces a blob of warm water to stretch much farther into the Pacific than it might otherwise, creating the El Niño conditions that roil weather patterns around the world.

That said, the magnitude of a Kelvin wave ahead of El Niño can tell us a lot about the warming period to come.

“The current Kelvin wave is impressive and, by some measures we look at, it is rivaling the one we saw in 1997,” Michelle L’Heureux, a researcher with the National Oceanic and Atmospheric Administration’s Climate Prediction Center told the WSJ.

That event precipitated one of six recorded “super El Niños,” which happens when ocean surface temperatures in the Pacific rise a minimum of 3.6 degrees Fahrenheit above the long-term average.

The first super El Niño on record happened in 1877, a cataclysmic event which resulted in widespread droughts and famines that killed an estimated 50 million people across the world. The latest super El Niño, which lasted from 2015 to 2016, resulted in climate anomalies which directly contributed to outbreaks of diseases like the Zika virus, cholera, hantavirus, chikungunya, and even the plague.

How heavy-handed El Niño is this time around remains to be seen, but with global ocean temperatures significantly higher than they were a decade ago, it’s not going to be pretty.

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