In the coming days, sea levels off Southern California are expected to begin rising by half a foot or more and then stay elevated for months, as an enormous pulse of wave energy generated by El Niño travels up the coast, lifting the water as it goes.

"It's one of the first, if not the first indication that El Niño is here," said Dillon Amaya, an assistant professor at North Carolina State University. This pulse, known as a Kelvin wave, will boost the ocean's height across an area extending 12 to 18 miles offshore. It is coming on top of several other factors that are raising the seas off California and increasing threats from coastal flooding, erosion and saltwater intrusion.

With so many elements stacking on top of one another, it's "highly probable" that California will soon witness its highest sea levels ever recorded, said Mark Merrifield, director of the Center for Climate Change Impacts and Adaptation at Scripps Institution of Oceanography. Last week, Gov. Gavin Newsom declared a state of emergency to prepare the state for El Niño, directing agencies to position equipment like sandbags and pumps.

This year's El Niño is predicted to be the strongest on record, meaning its influence on temperatures, rainfall and storms could be especially stark. For California, the boost to sea levels began nearly half a world away, as warm water in the western Pacific starts spilling eastward toward the Americas in what oceanographers call Kelvin waves.