Pacific Ocean shift is reshaping California's weather patterns

A major ocean temperature index in the North Pacific has plunged to record low levels signaling a shift that tends to lock in coastal fog, delay California’s rainy season and reroute storms to the north.

This summer already bears the stamp of this setup. Around the Bay Area, mornings have been trapped in gray, afternoons unusually breezy, and inland heat has arrived in short, erratic bursts.

New research suggests this isn’t just a temporary phase. The persistent sea surface temperature anomalies driving this cool phase pattern may reflect a longer-term shift in the Pacific’s ocean and atmosphere, one that climate change appears to be reinforcing.

The Pacific Decadal Oscillation, or PDO, is a long-term index that tracks sea surface temperature patterns in the North Pacific. The index shifts between warm and cool phases roughly every 10 to 30 years. In its warm phase, coastal waters off California are typically hotter and temperatures across the basin are more evenly distributed, causing conditions that weaken the marine layer and support wetter winters.

In a cool phase, near-shore eastern Pacific waters turn colder while the central and western Pacific warms, sharpening the temperature contrast across the basin. That contrast tends to strengthen the marine layer, delay the rainy season and reduce precipitation in Southern California and the Southwest.

The PDO has been in a cool phase since fall 2019. But in July it plunged to record negative levels.

“A lot of what we’re seeing now, the deep negative PDO values, can be traced to widespread warming across parts of the North Pacific since 2014,” said Nate Mantua, a research scientist with the National Oceanic and Atmospheric Administration. 

At the same time as these ocean temperature anomalies are happening, a stronger than normal high-pressure system, the North Pacific High, has been parked between Hawaii and the West Coast. It’s driven persistent northwest winds along the California coastline, enhancing upwelling and keeping near-shore waters slightly cooler than average. That contrast, between warm water in the western and central Pacific and cold water hugging the coast, is what’s pulling the PDO deep into negative territory.

This pattern generated the summer Californians have been living through, with a thick and stubborn marine layer, muted inland heat, and gray mornings that stretch well into the afternoon. The temperature contrast in the ocean enhances the inversion layer near the coast, allowing the marine layer to deepen and persist.

“Because of that warm ocean temperature upstream, the air above the water has been warmed and moistened. Then it’s transported toward North America, and when it hits the cooler water on the California coast, it chills the air and creates very persistent stratus clouds,” said Nick Bond, a senior research scientist at the University of Washington. “The result is a lot of gray skies like we’ve seen in San Francisco this summer.”

In fall and winter, a negative PDO pattern in the North Pacific often nudges the jet stream northward, delaying the start of California’s rainy season and keeping Southern California and the Southwest drier than usual.

Now that setup may be reinforced. NOAA recently issued a La Niña watch for the fall, and the timing couldn’t be more aligned.

La Niña develops in the tropical Pacific, thousands of miles south of the PDO’s core region in the North Pacific. La Niña is defined by cooler than average waters in the eastern tropical Pacific and warmer waters to the west. Despite covering different parts of the ocean, the two patterns look similar in their sea surface temperature signatures and tend to produce similar atmospheric effects. When they occur together, they can often phase lock, reinforcing each other and locking in a cool-state Pacific for months or even years.

New research suggests that climate change may be making these La Niña and negative PDO Pacific patterns more frequent and more persistent.

In the mid-20th century, many factories released sulfate pollution. Sulfates reflected sunlight and cooled the air, changing the wind patterns over the Pacific. After clean air laws took effect in the 1970s, sulfate pollution decreased, leaving greenhouse gases like carbon dioxide and methane as the dominant pollutant in the atmosphere.

Unlike aerosols, greenhouse gases trap heat from the sun, causing the atmosphere and oceans to warm. But that warming doesn’t spread evenly through the ocean. Greenhouse gases cause the western Pacific to heat up more, which strengthens the east to west trade winds at the equator. Those winds push warm water west and pull up cold water in the east, making the Pacific even more unbalanced and locking in the La Niña and negative PDO pattern. 

The ripple effects reach into the North Pacific, where the ocean and atmosphere begin to reinforce one another with warm water in the western and central Pacific, cold water near California, and a jet stream displaced northward, the classic signature of a negative PDO.

This ocean temperature and pressure pattern in the Pacific  leads to longer negative PDO stretches and more frequent La Niña phases, a pattern that’s already emerging. Over the past 25 years, La Niña has occurred in 12 winters, outpacing El Niño, which appeared just eight times. 

With record low PDO values and a likely La Niña on the horizon, California is poised for another season of disrupted weather. A robust marine layer will probably persist beyond the summer and into the fall. Fire season could stretch deep into autumn as storm systems are delayed. And when rains do arrive, they may come in intense, fast-moving bursts causing the kind of precipitation whiplash that can overwhelm.  

Much like last year, the impacts won’t be uniform across the state. Southern California and the Southwest typically see the steepest rainfall drop under these conditions, stressing already limited water supplies. Northern California may still get rain, but in more erratic patterns, with a later start and longer dry gaps between storms.

“This pattern doesn’t just mean more fog or less rain, it means volatility,” Bond said. “It makes it harder to plan for fire and harder to know what kind of season we’re walking into.”