Hazardous waves up to 16 feet in size are returning to the California coast this week, driven by the strong El Niño pattern in the Pacific Ocean.

The latest event is unlikely to be as dramatic as the waves and swell that slammed the California coast last week, but such massive waves are occurring more often than they used to researchers say, as global warming amplifies extreme weather events and pushes sea levels higher. They say that means greater coastal flooding in the future.

A swell with big waves and coastal flooding is expected along the West Coast this week. Then a seasonal king tide could bring even more water shoreward in about 10 days, said Laura Engeman, a coastal resilience specialist with California Sea Grant. “If there are large waves those days, we may see more flooding.” 

While this latest round of waves isn't expected to be as strong as the late December event, it's the El Niños of the future and the increase in coastal flooding fueled by the warming climate that alarm those who study the coast.

El Niño brings larger, more energetic waves, more impactful winter storms and higher sea levels, said William Sweet, an oceanographer with the National Oceanic and Atmospheric Administration. That means greater coastal flooding, higher astronomical high tides and more severe erosion.

The current El Niño provides a peek into the future, Sweet said. As sea levels continue rising, the damage from such events is expected to grow.

That’s especially a concern in California, where so many houses teeter on the edge of the ocean and the beachfront was developed on a “razor thin margin,” said Patrick Barnard, a research geologist with the U.S. Geological Survey’s Pacific Coastal Marine Science Center in Santa Cruz. 

“It’s such a dynamic environment. We’ve basically built ourselves in harm’s way up and down the coast,” Barnard said. “We didn’t anticipate (sea level rise) when we built out the California coast in the 50s and 60s. But you know, we're going to get three feet of sea level rise by the end of the 21st century.”

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What's the impact from El Niño?

Big, powerful waves from winter El Niños are well known to surfers. One of the world's most prestigious surf competitions is linked to such conditions off Hawaii, Sweet said. That’s the Eddie Aikau Big Wave Invitational at Waimea Bay, California. 

The last El Niño saw wave energy increase by about 50% over a typical winter, Barnard said. And it's not uncommon for sea levels to rise up to a foot higher along parts of the California coast during El Niño.

But conditions are changing. A growing body of research shows the chances are increasing for more powerful waves and high tide flooding events, Barnard said. 

Today, the swell of water and massive waves arrive on top of sea levels driven higher by climate change, and years of tide gauge data show an increase in West Coast flooding events during El Niños.

The ocean has risen roughly 10 inches along the West Coast over the last century, and that trend is accelerating, Barnard said. “We’re riding on a higher baseline when these larger waves come in, so you tend to have larger coastal impacts, more erosion and more flooding.” 

A study published last summer looked at data back to the 1930s and found the average height of winter waves along the California coast has increased with climate change. Barnard said some evidence has suggested El Niños are becoming more extreme, but the research isn't yet definitive.

The 2015/2016 El Niño left record erosion along many California beaches, researchers concluded in studies afterward. When sea levels are higher, the largest waves have even more of an impact, Sweet said.

NOAA’s monthly high tide flooding outlook takes daily high tides and El Niño into consideration, but not storm events. Its data shows high tide flooding occurs nearly three times more often today than it did in 2000. It continues to accelerate, particularly along many Atlantic and Gulf Coast locations, Sweet said.

In the current year, El Niño may add anywhere from one to eight days of high tide flooding above the 2000 average in locations along the West Coast, NOAA projects. That’s on top of the increase in flood days added by sea level rise. When sea levels are higher, larger waves can become more damaging, penetrating farther up the shore.

El Niño can temporarily elevate sea levels along the nation’s West Coast for several months at a time and can bring a decade’s worth of sea level rise in just one season, Sweet said. “That's your future decade in a season. You’re getting snapshots to where you’re headed.” 

Even the more frequent mild and moderate flooding “starts to eat away at the resilience of our coast,” Engeman said. “This means that when we do experience these bigger storm events that often occur during El Niño seasons, then our shoreline is already stressed and likely to incur great damages and impacts.”

What about the wave that hit Ventura?

Weather forecasters had warned of waves 15-20 feet high at Ventura on Dec. 28, and onlookers had gathered to watch. But a huge wave sent water soaring over a sea wall, with people fleeing in panic as the water surged inland.

Colloquially it's been called a rogue wave, although researchers have a more precise vocabulary. Coastal experts and meteorologists told USA TODAY the wave was likely a low frequency, gravity-induced wave from offshore that created a huge runup under rough surf and high water conditions.

Among the higher tides and other massive waves that occurred along the coast, one surfer died and at least 13 people were injured, according to National Weather Service reports. 

The surfer’s death was at least the third attributed to ocean conditions on the California coast since Nov. 25, when 5-year-old Naretzi Navarrete and her grandfather were swept out to sea in Martins Beach, about 30 miles south of San Francisco. The weather service had warned of sneaker waves and rip currents due to astronomical high tides.

Ariel Cohen, meteorologist in charge of the National Weather Service office Los Angeles/Oxnard, described the Ventura incident as a surge from a long period, highly energetic swell that arrived at high tide.

Scientific research shows such huge wave run ups occur more often during winter storms juiced up by El Niño.

But it's not just the big waves that cause concern. Even the more frequent mild and moderate flooding “starts to eat away at the resilience of our coast,” Engeman said. When bigger storm events occur, the shoreline is "already stressed and likely to incur great damages and impacts.”

Explaining the difference in big waves

Here are the terms to know when looking at wave impacts, according to NOAA and the U.S. Geological Survey.

Sneaker waves – Deadly, larger-than-average swells that arrive suddenly, often with no warning after long periods of quiet surf and smaller waves. Without warning, they can surge dozens of feet higher up the beach than expected, breaking over rocks and rolling logs. They are blamed for killing more people on West Coast beaches than all other weather hazards combined. 

Long waves – Also called infragravity waves, these are generated by wind on the open ocean and can grow slowly but steadily when traveling into shallower water. Other long waves, such as an astronomical tide, are generated by gravitational pull on the ocean.

Rogue waves – Generated by wind and shorter waves, they often occur in the open ocean and during stormy conditions, but also can occur in coastal waters. The technical definition is a wave more than twice as high as the waves around it. 

Tsunamis – A very long wave of seismic origin, generated by earthquakes, volcanoes and landslides. In the open ocean tsunamis have small wave heights, but in shallow water closer to the coast wave height can grow dramatically to 30-60 feet.

Meteotsunamis – Typically smaller than tsunamis, they’re caused by air pressure disturbances in fast moving storms. 

Wave runup – When a wave breaks and pushes water onto the beach.

Wave setup – When waves continually break onshore and water piles up because it can't get back out to sea.

Even scientists don’t always agree about the origin of many extreme wave events, and they’re sometimes studied for years. Researchers also attribute extreme waves or runup events to waves trapped in shallow areas and the transfer of energy from one type of wave to another. 

Dinah Voyles Pulver covers climate and environment issues for USA TODAY. Reach her at [email protected] or @dinahvp.