A Giant Ocean Wave Is Heading Toward California's Coast

A special kind of ocean wave is slowly making its way up the coast of Mexico toward California. Scientists call it a 'coastally trapped Kelvin wave,' and it is connected to the powerful El Niño weather pattern happening right now. When it arrives — expected in early October — it could raise the sea level along California's coast by about six inches. That might not sound like a lot, but experts say it could make flooding and beach erosion much worse.

This wave is nothing like the waves surfers ride. You would not even be able to see it or feel it if you were swimming. But it is a real and powerful force in the ocean. Dillon Amaya, a scientist at North Carolina State University who studies these waves, says it could raise sea levels for months.

El Niño is a weather pattern that starts when ocean water in the Pacific gets unusually warm. That warming changes weather around the world. Right now, scientists say this El Niño is one of the strongest ever recorded. Government forecasters say there is a 75% chance it will be stronger than any El Niño since 1950.

Here is how a Kelvin wave is born: During El Niño, the normal winds along the equator slow down or even reverse direction. This lets huge amounts of very warm water slide eastward across the Pacific Ocean. When that warm water reaches South America, the wave splits into two parts. One part heads north, toward California, hugging the coastline the whole way.

The wave travels an incredible distance — about 15,500 miles — from the western Pacific Ocean all the way to the Gulf of Alaska. The entire journey takes about 120 days. Right now, the wave is near the western coast of Mexico, south of Puerto Vallarta. Scientists expect it to enter the Gulf of California around September 19.

After the wave travels around the Gulf of California, it will head toward the southern tip of the Baja California peninsula, called Cabo San Lucas. From there, it will take about seven days to reach San Diego. Then it will take another two to three days to reach Los Angeles, and a few more days after that to get to San Francisco.

California's coast is already in rough shape. The state has seen about one foot of sea level rise over the past 125 years because of climate change and melting ice caps. On top of that, a separate marine heat wave — not related to El Niño — has also been warming the ocean and pushing sea levels higher. Jonathan Warrick, a research geologist with the U.S. Geological Survey, pointed to recent storms as proof of the danger. 'When the water level is extra high, a foot or more higher than we expect it to be, then it can really wreak havoc,' he said.

When you add all of these things together — El Niño, the Kelvin wave, and the marine heat wave — scientists say California is experiencing roughly another foot of extra sea level rise on top of what climate change has already caused. This is a big deal because even a small rise in sea level can lead to major flooding when a storm hits. Beaches can lose huge amounts of sand, and homes near the shore can be badly damaged.

These kinds of Kelvin waves are not new — scientists have known about them for decades. But computers were not powerful enough to track them in real time until recently. Now, scientists like Amaya can watch the waves move across the ocean as they happen. This is especially important because the current El Niño is so strong that the Kelvin waves it creates are expected to be among the largest ever seen.

This is not even the first Kelvin wave of this El Niño season. One already traveled from the western Pacific to the Gulf of Alaska between May and June. During the very strong El Niño of 2015 and 2016, there were five or six of these coastally trapped waves. The strongest ones came in winter and caused record amounts of erosion along the California coast. Amaya believes the worst waves from this El Niño are still to come.

On top of all this, hurricane season in the eastern Pacific is not over yet. El Niño makes it more likely that hurricanes and tropical storms will form there. A tropical storm named Norbert was already spinning southwest of Baja California. UC climate scientist Daniel Swain has warned that strong El Niño conditions increase the chances of tropical storms sending dangerous winds and heavy rain toward California between August and October. For people living near the California coast, the coming months are ones to watch closely.