There’s something unusual and far-reaching lurking in the waters off the coast of California, stretching more than 800 miles southwest from the Baja Peninsula.
This widespread region is experiencing a large-scale, long-lasting, and record-breaking marine heatwave that is predicted to continue to intensify, altering weather conditions on the West Coast and negatively impacting the marine food chain.
This heatwave is comparable to a heatwave on land and could have widespread effects on marine life. That’s because warm-water species, such as hammerhead sharks and bluefin tuna, move into areas where they aren’t normally found, while cold-water species move deeper north.
Marine heatwaves could have far-reaching effects on the weather in the West, increasing the likelihood of extreme heatwaves like those in March, causing intense downpours, and even potentially causing tropical systems to move northward and reach California.
Scientists at the Scripps Institution of Oceanography monitor ocean temperatures along the California coast, and records go back more than a century. Especially in recent weeks, they have been setting one hot ocean record after another.
From January 1 through last weekend, there were 36 days when sea surface temperatures at the Scripps Pier in La Jolla, California, set records for the highest water temperatures ever recorded for that day. This is important because daily data for that location goes back to 1916.
Scientists at Scripps are using robotic oceangoing vessels to study water temperatures below the surface. The researchers found that the unusually hot water extended to great depths, comparable to conditions typically seen during major El Niño events.
El Niño is the occurrence of abnormally hot seawater near the equator of the Pacific Ocean, causing major changes in global weather patterns and the Earth’s climate.
However, although a potentially strong El Niño is predicted to occur by late summer or early fall, no El Niño has occurred at this time. This is a warning of how much water temperatures in the region are likely to rise in the coming weeks and months.
Dillon Amaya, a research scientist at the National Oceanic and Atmospheric Administration, says climate cycles that are precursors to El Niño are now joining California’s typical marine heatwave, with a much wider area of abnormally hot ocean water extending into Indonesia. (The climate cycle is known as the Pacific Meridian Mode.)
Together, these two entities make up one of the largest ocean heatwaves on Earth today.
Computer model projections show that over time, this broad, warmer-than-average region will transform into an El Niño along the equator, while a smaller, closer ocean heat wave will continue to bake the waters off California, potentially into the fall and winter, Amaya said.
California’s marine heatwave is already impacting the Pacific food web. Tammy Russell, a marine ornithologist at Scripps, said seabirds are particularly affected and this is a warning that other species could be more severely affected. Russell studies seabirds in detail and how they interact with the broader marine ecosystem.
“For several months now, we have been seeing an increase in the number of seabirds arriving at rehabilitation facilities and with carcasses washing up on the coasts of southern and central California,” she said in a statement. “Most of the birds were weakened and tested negative for HPAI (avian influenza). Therefore, we concluded that starvation was the main cause of this mortality.”
As cold-water fish move northward and deeper, seabirds may have difficulty finding their typical food sources, Russell said. Numbers of distressed brown pelicans, blunts and murres showing up up and down the coast to San Diego are increasing, she said.
The ocean heatwave was particularly intense from Mexico’s Pacific coast northward to Southern California and San Francisco, with sea surface temperatures reaching above average 4 degrees Celsius (about 7.2 degrees Fahrenheit) in many areas. This is an “astonishing” scale for a marine heatwave, Amaya said.
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He traced the origin of the heat wave to the same area of high pressure that caused record heat waves across the West in March and April. Amaya said this weather feature created unusually weak surface winds off the coast, reduced ocean cloud cover, and increased ocean temperatures as they absorbed more solar energy.
“Ocean heat waves of this magnitude and spatial extent would not occur without some kind of atmospheric driving force,” he said, pinpointing the culprit to the western high-pressure ridge. He added that this situation is likely to continue over the coming months due to other factors such as El Niño.
Human-driven climate change, caused by the accumulation of climate pollution in the atmosphere, is a major factor in the increased prevalence and intensity of marine heatwaves in recent years.
“Because of global warming, each ocean heatwave will be warmer than the last,” Amaya said.
Daniel Swain, a climatologist at the University of California’s College of Agriculture and Natural Resources, said the heat wave is already changing California’s weather conditions and could have significant effects in the coming months.
For example, warmer-than-average water lurking along the coast could lead to wetter heat waves on land this summer.
Amaya said many Californians aren’t used to humid heat.
Rising water temperatures could make California more vulnerable to tropical cyclones.
Typically, the cold ocean waters near California prevent tropical storms and hurricanes in the eastern Pacific Ocean from threatening the state. But Amaya said that could change this summer, as warmer waters make it easier for storms and their debris to hit Southern California.
One recent storm to have a significant impact on Southern California was Tropical Storm Hillary in 2023, whose moisture pushed inland and caused flooding.
The bottom line, Swain and Amaya stressed, is that this heatwave has had and will continue to affect all areas of land and sea.
“Hey, what do you think? It looks like we’re going to have a very severe marine heatwave heading into the summer,” Swain said.
