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How Does Climate Change Impact California Ocean Temperatures? Cal Poly Study Investigates

A field of orange flowers on a cliff overlooking the Pacific Ocean and Cal Poly Pier.
Written By Nick Wilson

As unprecedented and unrelenting atmospheric heat waves ravage the globe, a team of Cal Poly student and faculty researchers recently published a study exploring how climate change impacts extreme ocean temperatures off California’s Central Coast.

A professor and student smile for a photo together on a gray morning with the ocean behind them.
Professor Ryan Walter, left, and student Michael Dalsin, right, both worked on the study. 

The Cal Poly team collaborated with scientists at the Virginia Institute of Marine Science to conduct the study, published July 31 in Nature Scientific Reports.

Their investigation looks at drivers of both marine heat waves and cold spells in the shallow waters along the California coast. It also analyzes conditions that lead to extreme water temperatures, and provides a better understanding of when, where and why these extreme marine events occur.

“One thing is clear,” said Ryan Walter, a coauthor and Cal Poly associate professor of physics specializing in physical oceanography. “These extreme temperature events are not going away, so it is critical that we continue to explore their drivers and consequences.”

The study found that certain environmental conditions and the state of the ocean (El Niño and La Niña years, for example) led to an enhanced risk for marine heat waves and cold spells, conditions that scientists and environmental managers will need to monitor to preserve and protect vital ecosystems critical to the California ocean or Blue Economy.

The researchers also investigated the Pacific Decadal Oscillation (PDO), a large-scale pattern of temperature in the North Pacific that varies over time scales of years.

“There is high confidence that, because of climate change, El Niño events will increase in frequency and intensity,” Walter said. “And so, if we have stronger El Niño events in the future, we expect to see more frequent and more extreme marine heat waves and all the consequences that come with it.”

El Niño conditions happen when the trade winds that usually blow from east to west along the equator of the Pacific Ocean weaken, as is forecast to occur this coming winter. This causes warm surface water to move east and causes warmer than usual water temperatures and reduced upwelling along the California coast. 

If the temperatures get too warm during one of these events, marine ecosystems can be severely impacted. Previous marine heatwaves have led to giant kelp forest loss, mass die-offs of seabirds and economically important fisheries, and harmful algal blooms.

The team analyzed ocean temperatures recorded from 1978-2020, taken in a fixed Central Coast location near Diablo Canyon Power Plant. In addition to variable ocean and weather conditions caused during El Niño and La Niña years and changes in the PDO, the study found short-term upwelling patterns can trigger the temperature spikes that can cause temporary and permanent damage to marine ecosystems.

Thermal stress, both hot and cold, can significantly affect aquaculture and fisheries. In the future, it will be increasingly important to understand how wind patterns and surface warming from climate change affect upwelling along California’s coast, according to Walter.

Climatologists expect that 2023 will be the hottest year on record. Just a few weeks ago, on July 24, a water sensor 5-feet underwater at Manatee Bay, Florida, reached 101.1 degrees Fahrenheit —eclipsing the previous sea surface record of 99.7 degrees in Kuwait Bay, set in 2020. These temperature extremes cause detrimental impacts on marine ecosystems and ocean-related ecology. 

It has long been known that coastal upwelling — the wind-driven transport of deep, cold water into shallow areas along the coast — has a strong cooling effect on coastal waters, creating foggy marine layers and stimulating marine productivity. 

Thanks to upwelling, the researchers also found that California coastal marine environments generally aren't warming as much as other parts of the world.

"If we didn’t have upwelling along our coast, we’d see far more heat waves,” Walter said. “So, the upwelling is cooling down nearshore regions along the coast and causes the climate-induced warming signal to be more muted. This also provides a thermal refuge for marine organisms.”

Upwelling helps maintain healthy fisheries and robust marine life. The cold waters also help buffer against rising water temperatures frequently found farther from shore and in other locations around the world like Florida, which does not experience strong upwelling. 

“Upwelling systems in general are among the world’s most productive ecosystems, including many of the world’s fisheries and beautiful kelp forests,” Walter said. “Because the deep upwelled waters are cold, they help mitigate some of the warm water extremes. Additionally, these deep, cold waters are full of nutrients — and when they upwell, they effectively fertilize the surface of the ocean and lead to strong biological productivity.”

Cal Poly undergraduate physics senior Michael Dalsin served as the lead author along with co-authors Walter and Piero Mazzini, an assistant professor at the Virginia Institute of Marine Science.

“This study lays the foundation for understanding how temperature extremes in our ocean will respond to climate change,” said Dalsin, an undergraduate who has won multiple awards for his work on the study, including an American Meteorological Society (AMS) Student Award for his oral presentation at the organization’s 2023 annual meeting. He also presented the article's findings at the 2023 California State University Student Research Competition. 

“One fascinating aspect of our research is that we can predict the likelihood of one of these extreme marine events given the state of our ocean," Dalsin said. "The state of the ocean, as determined by large-scale climate modes and local-scale upwelling winds, could be used to forecast heat waves and cold spells in the future.”

The research was supported by the William and Linda Frost Fund in the Bailey College of Science and Mathematics.


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