El Niño could bring highest sea levels ever recorded to the California coast

A strengthening El Niño could bring the highest sea levels ever recorded to the California coast this year, raising the risk of widespread and damaging flooding.

It’s the latest effect forecast for what’s expected to be an historically strong event, one forecasters warn could also lash Southern California with powerful rain and broil both land and sea with punishing heat waves.

“We’re expecting likely the highest sea levels ever recorded on the California coast associated with this event,” said Mark Merrifield, director of the UC San Diego Scripps Institution of Oceanography’s Center for Climate Change Impacts and Adaptation.

During a recent online town hall, Merrifield presented a NASA satellite image showing sea levels already more than half a foot higher in the eastern Pacific Ocean compared to the reference state, and said by this fall and winter, “we should be seeing sea levels far above the average seasonal cycle.”

According to NASA, El Niño’s warmer ocean water displaces colder water close to the sea surface. Warmer water causes a rise in the sea surface height because water expands as temperature rises.

Data collected by scientists over the years also have shown how El Niños contribute to “enormously strong” waves, which can cause beaches to “erode and to narrow,” Merrifield said. “It’s well documented that during El Niño events, beaches across the entire state tend to move landward by about 30 meters [98 feet] or so.”

With less of a coastal buffer, any large waves that come in “will have even greater impact on the shoreline, with more likelihood of flooding and undercutting of cliffs,” he added.

“That combination — high waves, high sea levels, high tides, and … high rainfall will contribute to considerable coastal flooding, intrusion of salt water into the groundwater, and coastal erosion,” Merrifield said. “Flooding … along the Southern California coast is likely to be a major issue for the winter season.”

This has proven true in recent editions of the climate pattern. In 2023-24, significant flooding struck portions of Ventura, Los Angeles and San Diego counties. During one memorable moment, a powerful wave crashed over a sea wall in Ventura, injuring eight people.

A sneaker wave hits the beach at the end of Seward Avenue in Ventura on Dec. 28, 2023. (Ventura Police Department and the city of Ventura)

“Usually the entire California coast sees these higher waves as the storm track is perturbed by the El Niño,” Merrifield said during the town hall, which was hosted by state Sen. Mike McGuire, a Democrat representing the North Coast.

Any El Niño-influenced rise in sea level would only add to a wider trend influenced by human-caused climate change. In the San Francisco Bay Area, where sea levels have risen over the last century, notable peaks have been observed during the El Niño seasons of 1982-83, 1997-98 and 2023-24, according to Merrifield.

The average sea level at La Jolla Pier was significantly above average during the last two “very strong” El Niños, in 1997-98 and 2015-16, according to data shared by Merrifield. And this year’s El Niño could be similarly powerful in a matter of months.

A slide shows how sea levels have risen in San Francisco Bay over the last century.

(Mark Merrifield / Scripps Institution of Oceanography at UC San Diego)

Sea level rise is expected to increase in all sorts of areas, from the San Francisco Bay to San Diego, Merrifield said, and impact the Sacramento-San Joaquin Delta.

“Historically, these kind of events have led to, in San Francisco, anywhere from half a foot to a foot of anomalously high sea level,” Merrifield said. “That typically starts to peak up in November, December.”

El Niño is a climate pattern characterized by warmer ocean waters in the central and eastern tropical Pacific Ocean, coupled with changing atmospheric conditions, in which the typical east-to-west trade winds weaken or even reverse.

That shift can train a veritable fire hose of tropical moisture at Southern California, potentially leading to significant rainfall across the region — albeit at the cost of drier-than-usual conditions elsewhere.

A large wave crashes over the rocks as viewed from the Redondo Beach Pier on Dec. 30, 2023.

(Mel Melcon / Los Angeles Times)

There’s a high chance that this El Niño could be “very strong,” perhaps one of the strongest on record, forecasters say.

“We’ve never seen ocean temperatures in the eastern tropical Pacific as warm as they are today in the region that matters for El Niño,” UC climate scientist Daniel Swain said during the Tuesday night town hall.

“The eastern tropical Pacific Ocean right now today is warmer [for the calendar day] than in any previous recorded year. So this now exceeds ’82-’83 and ’97-’98 in the tropical Pacific,” Swain said. “We have not seen an El Niño event already on the trajectory that this one is on.”

The World Meteorological Organization said on July 31 that a “strong El Niño is developing” and was expected to continue strengthening through early autumn.

“El Niño is not just on our doorstep — it is inside the house and turning up the heat. And this is only a warmup act,” United Nations Secretary-General António Guterres said in a statement.

While commonly thought of as a rainmaker for Southern California, scientists say that’s not a given even during a “very strong” or “super” El Niño like this one is shaping up to be.

Notably, the very strong 2015-16 El Niño did not result in above-average rain for Southern California.

Still, experts say El Niño does tilt the odds for a rainier-than-normal winter for Southern California, and that officials across the state should be preparing for the prospect of a stormy winter.

The strength of this El Niño “is going to significantly shift the odds in favor of a particularly wet winter, especially in Southern California. But for an event this big, probably Northern California as well,” Swain said.

“What it really does is increase the likelihood of particularly wet conditions starting in about late December and into January, February and March,” Swain said. “It’s not so much on the first third of the rainy season, but it has more to do with the peak, more consistent, reliable rainy season months.”

In anticipation of a busy winter storm season, Swain said he’s clearing his calendar “from January through March this coming year to be available if things get a little bit wild.”

El Niño also is expected to lengthen the marine heat wave off portions of the coast, which is not yet being influenced by the climate pattern.

“The El Niño is going to bring even warmer waters, and so the question of a long-duration heat wave is a possibility for this year,” Merrifield said. “That has strong impacts on on the marine ecosystem and various parts of the food web.”

Warming suppresses upwelling — which normally allows colder, deeper and nutrient-rich ocean water to rise closer to the surface.

Among the impacts the California Department of Fish and Wildlife will be monitoring, according to McGuire, are levels of the neurotoxin domoic acid, which in late 2015 forced the postponement of the opening of the Dungeness crab fishery season.

“The state of California, working with California’s crab fleet, will be launching domoic acid testing here in about 60 days as we approach Dungeness crab season,” the state senator said.

State officials are “anticipating higher-than-average domoic acid blooms because of the warmer ocean temperatures,” McGuire said. The neurotoxin can be produced by an algae called Pseudo-nitzschia, and “the warmer the ocean is, the more algae,” McGuire said.

Domoic acid can accumulate in shellfish and cause nausea, diarrhea and illness in humans when consumed at low levels. At high levels, the toxin can cause short-term memory loss and seizures and be fatal in rare cases, state officials say.

It also can wreak havoc on wildlife, with numerous pelicans and sea lions being poisoned in recent years.

El Niño’s delivery of warmer ocean temperatures may add yet more stress to California’s kelp. Past marine heat waves caused Northern California’s bull kelp habitats to decline by 90% since 2014, which trickles down to other species, including endangered white abalone, according to Nate Jaros, the Aquarium of the Pacific’s vice president of animal care for fish and invertebrates.

Even without El Niño, ocean temperatures have soared off parts of the coastline — a factor in the recent rounds of uncharacteristic muggy humidity.

“In parts of Southern California where it’s been foggy recently, it’s not a damp fog by any means. It’s a very humid, warm, muggy air mass,” Swain said. “We’re starting to see the warming off the coast of Northern California too. So that might continue to make for a fairly different coastal experience the rest of this fall than we would typically be used to.”