A fisherman casts his line from a jetty at Encinal Beach in Alameda on Thursday. Water temperatures in Alameda this month have ranged from 61 to 67 degrees, about 1 to 7 degrees warmer than a normal April, according to the National Oceanic and Atmospheric Administration.
Yalonda M. James/S.F. Chronicle
The largest marine heat wave in the world is lapping at the California coast and shattering ocean temperature records in the northeast Pacific.
It will have major implications for California’s weather — supercharging spring storms, setting the stage for an active North American monsoon and Pacific hurricane season, and potentially harming marine plants and animals that are accustomed to the cool, nutrient-rich water typical of the current that runs down the California coast.
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Summer may feel warmer and muggier than usual in Los Angeles, and perhaps even in San Francisco — a stark contrast to 2025, which was notably cool and foggy along the California coast. More Sierra Nevada thunderstorms are possible by the middle of summer as a result of an enhanced monsoon.
Cesar Cuyuch removes a hook from a nearly 28-inch halibut he caught Thursday from the Alameda Rock Wall at Encinal Beach.
Yalonda M. James/S.F. Chronicle
The marine heat wave first reached California’s shoreline in September and shrank in fall, a pattern typical of the past decade, said Andy Leising, a research oceanographer at the National Oceanic Atmospheric Administration. Then, however, it swelled again in December — an unusual occurrence, Leising said.
This week, the exceptionally warm waters extended from San Francisco to central Mexico, south of Hawaii to the equator and beyond the International Date Line. Sea temperatures were most anomalous from Point Conception (Santa Barbara County) to around Cabo San Lucas, Mexico meeting the National Oceanic Atmospheric Administration’s criteria for a “strong” to “severe” marine heat wave.
A “strong” marine heat wave, level 2 of 5 on NOAA’s scale, extended from the California coast beyond the International Date Line in mid-April.
NOAA
Sea surface temperatures near the Central and Southern California coast were “rivaled only by the wintertime anomalies seen during the strongest El Niño on record during 1997-1998, and the ‘warm blob’ years of 2014-2015,” Leising said in an April blog update.
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Cool water surrounding San Francisco usually stabilizes the lower atmosphere and limits the energy needed for thunderstorms. But temperatures are as much as 5 degrees above normal in San Francisco Bay and the Pacific coast; last weekend, they provided extra energy and moisture for lightning and downpours in San Francisco and Oakland during an unusually strong April storm.
“Extremely warm ocean temperatures of this kind of magnitude can certainly ‘juice up’ existing storms,” said Daniel Swain, a climate scientist at UC Agriculture and Natural Resources.
Swain pointed to a study that found similarly warm seas in 1998 increased potential energy for coastal California thunderstorms by about 26%. Further storms this spring are more likely to feature thunder and lightning as these anomalies persist, particularly south of Point Conception where water temperatures are even more unusual.
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Scripps Pier in San Diego has set daily ocean temperature records once every three days so far this year and is routinely 3 to 7 degrees above normal.
Sea surface temperatures at Scripps Pier in the San Diego area were above the 110-year normal every day so far in 2026, with daily records being set about once every three days.
UC San Diego
Warmer-than-normal seas threaten marine life that are accustomed to the cool, nutrient-rich water that rises from deep in the ocean to the surface, a process called upwelling. Winds that trigger upwelling usually kick into gear in spring in the ecosystem of the California Current, which flows north to south along the coastline. But it hasn’t happened in earnest yet this year, Leising said.
Storms along the California coast — especially in spring — suppress upwelling by reversing typical wind directions, allowing warm water to linger near the surface, said Nate Mantua, a climate scientist at NOAA.
Elsewhere in the Pacific, storms do the opposite, whipping up winds that draw cooler water up from deeper in the ocean, said Nick Bond, emeritus Washington state climatologist.
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Jesse Rodriguez, from left, holds a halibut caught by Cesar Cuyuch as Fernando Flores looks on during a fishing trip at Encinal Beach in Alameda. Bay waters are warmer than normal, as are the ocean waters off the California coast.
Yalonda M. James/S.F. Chronicle
Less storminess this winter across most of the northeast Pacific basin, followed by a series of spring storms in California, helped drive the anomalous sea surface temperatures everywhere.
“Climate models used for seasonal predictions are quite confident that the anomalies will persist or even strengthen with time over the summer”, Bond said.
Mantua said the California Current behaves differently than elsewhere in the Pacific, meaning ocean temperatures may be harder to predict. An emerging El Niño may reduce upwelling.
San Francisco’s windy fog — the city’s natural air conditioning — probably won’t be as cold this summer due to the elevated marine heat levels. That doesn’t mean there will be less fog, as there isn’t definitive proof that warmer seas decrease fog frequency, but days and nights may be a few degrees warmer than usual.
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Danelle Schmitz of Alameda returns to shore with her 11-year-old son, Tavish Schmitz, after wing foiling at Robert Crown Beach in Alameda on Thursday. “I have noticed a temperature change. It is warmer, for sure,” she said. “No, I don’t think it’s great for the climate or anything that lives in the water that’s used to a certain temperature.”
Yalonda M. James/S.F. Chronicle
Ocean temperatures this elevated have never been observed ahead of an anticipated El Niño, a pattern which favors stronger and more frequent northeast Pacific hurricanes — and could draw even more strength from warmer waters.
“This event is strong enough and the background pattern has so much warming in it, it may be a totally novel situation,” said Mike Crimmins, a climate scientist at the University of Arizona.
In August 2023, unseasonably warm water and an emerging El Niño fueled Tropical Storm Hilary, which dropped torrential rains in parts of Southern California and Arizona. Acapulco, Mexico was hit by Hurricane Otis two months later, the most powerful storm to ever make landfall in the eastern Pacific. Otis, and other rapidly intensifying hurricanes that result in billion-dollar damages, are made 60% more likely by marine heat waves, according to a recent study.
During the strongest El Niño on record in August 1997, Tropical Storm Ignacio brought precipitation as far north as Oregon, said Kim Wood, a climate scientist at the University of Arizona.
Tropical storms and hurricanes are more common in western Mexico and are wild cards for Southwestern U.S. precipitation. A more direct consequence of the warm water may be a stronger North American monsoon, which draws tropical air toward the Four Corners region and occasionally California in summer — potentially triggering more Sierra storms, and perhaps dry lightning elsewhere in California, including the Bay Area.
San Francisco Bay as seen from the Alameda Rock Wall at Encinal Beach. Warmer-than-normal waters in San Francisco Bay are dampening the effects of the marine layer that typically shields San Francisco, Oakland and other adjacent cities from thunderstorms.
Yalonda M. James/S.F. Chronicle
NOAA predicts a “robust monsoon” by July.
Warm water in and around the Gulf of California may prime the Southwest for an active monsoon. However, monsoon onset depends on additional factors like ocean currents and atmospheric wind patterns that cannot be predicted this far in advance, said David Mitchell, an atmospheric scientist at the Desert Research Institute.
Ocean temperatures weren’t just elevated in the northeast Pacific. The past three years have been the warmest on record for global oceans — a consequence of the greenhouse gas warming in the atmosphere, which sends heat into the oceans.
“March 2026 had its second-warmest global sea surface temperature on record,” Copernicus Climate Change Service said in a press release Wednesday. “Strong or very strong El Niño conditions could contribute to another record-breaking year, with warmer surface waters across the equatorial Pacific releasing additional heat into the atmosphere.”
Past marine heat waves had devastating effects on California’s ocean ecosystems. During a record-breaking event from 2014-15 dubbed “the blob,” kelp forests suffered and died, swordfish and tuna traveled unusually far north and toxic algal blooms threatened large mammals and crab fisheries. As much as 20% of some seabird populations starved to death, Mantua said.