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Marine Heatwaves and Cold Spells

Cal Poly ocean physics researchersCal Poly physics Professor Ryan Walter (left) and Michael Dalsin (Physics, ’23) with an acoustic Doppler current profiler used to measure ocean currents. Photos by Joe Johnston


APRIL 2024
by nick wilson

Michael DalsinThe hottest year on record took place in 2023. As global temperatures continue to increase due to human-caused greenhouse gas emissions and boosts from El Niño cycles, a Cal Poly oceanography team is taking a unique approach to understanding, and perhaps predicting, changing ocean conditions.

"California is a distinctive place biologically, and that means climate impacts are distinctive as well.

Michael Dalsin

Physics, ‘23,
2024 National Science Foundation Graduate Research Fellow
(Dalsin will join Stanford University's physical oceanography doctoral program in fall 2024.)

Ryan WalterThe team published the first-ever study of drivers of marine heatwaves and cold spells in nearshore waters off California's Central Coast using long-term measurements of ocean temperatures. While still a student, Cal Poly alumnus Michael Dalsin (Physics, ’23) was the lead author on the paper, published in the journal Nature Scientific Reports. Dalsin worked with co-authors Ryan Walter, a physics professor and faculty affiliate in Cal Poly’s Center for Coastal Marine Sciences, and Piero Mazzini, a professor at the Virginia Institute of Marine Science.

"These extreme temperature events are not going away, so it is critical that we continue to explore their drivers and consequences.

Ryan Walter

Physics faculty researcher

Marine heatwaves — prolonged periods of unusually warm ocean temperatures — have become more frequent, longer lasting, and more intense worldwide over the last four decades. But those trends in extreme events are more muted, the study found, in the shallow waters along California’s Central Coast. Thanks to a unique dataset recorded near Diablo Canyon nuclear power plant from 1978 to 2020, the researchers analyzed how California’s ocean conditions help mitigate some warm water extremes, while still facing environmental stressors due to a variety of climate factors. Long-term datasets from the very nearshore such as this one are quite rare and offer uncommon insights.

A combination of factors affects when, where and why marine heatwaves and their opposite, marine cold spells, occur in nearshore California waters, the team found. These factors include: the overall global temperature; El Niño and La Niña conditions; the Pacific Decadal Oscillation, a large-scale climate pattern in the North Pacific that varies over time scales of many years; and coastal upwelling, the wind-driven transport of deep, cold water into shallow areas along the coast.

“California is a distinctive place biologically, and that means climate impacts are distinctive as well,” said Dalsin, who won an American Meteorological Society (AMS) Student Award for his work on the study and represented Cal Poly at the California State University Student Research Competition in 2023.
Coastal upwelling proved to be a major driver affecting marine heatwaves in the Golden State’s coastal waters. Upwelling has a strong cooling effect, creating foggy marine layers and helping maintain healthy fisheries and robust marine life. Upwelling systems generally are among the world’s most productive ecosystems, including many of the world’s fisheries and beautiful kelp forests. 

“Because the deep upwelled waters are cold, they help mitigate some of the warm water extremes,” Walter said. “Additionally, these deep, cold waters are full of nutrients, and they effectively fertilize the surface of the ocean and lead to strong biological productivity.”

These cold, upwelled waters, along with the large-scale ocean circulation, help buffer against rising water temperatures frequently found elsewhere, such as in Florida, where similar upwelling doesn’t occur and 2023 peak sea surface temperatures topped 100 degrees Fahrenheit.

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

Ryan Walter and Michael DalsinWalter and Dalsin review research data.

California’s coast can still be affected by other climatic events such as El Niño years, when marine ecosystems can be severely impacted if the temperatures get too hot. Typically, trade winds blow west along the equator of the Pacific Ocean pushing warm surface waters towards Asia, piling them up in the western Pacific. But during El Niño conditions, they reverse, leading to warmer-than-normal waters off the U.S. West Coast, creating warming trends, and reducing cold water upwelling.The resulting higher ocean temperatures can sometimes lead to giant kelp forest loss, mass die-offs of seabirds and economically important fisheries, and harmful algal blooms. In 2015-16, a major El Niño event collapsed areas of the species-rich kelp forests in parts of California.

“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 heatwaves and all the consequences that come with it.”
La Niña years have the opposite effect, increasing risk for marine cold spells, or prolonged periods of extreme cooling, that can also negatively impact marine ecosystems.

In addition, the study found that short-term upwelling patterns and changes in the Pacific Decadal Oscillation can also trigger extreme temperature events that can cause temporary and permanent damage to marine ecosystems, affecting aquaculture and fisheries.

Moving forward, it will be increasingly important to understand how wind patterns and surface warming from climate change affect upwelling along California’s coast, and this research could contribute to climate models that would help predict the likelihood of extreme marine events.

Isabelle Cobb presenting researchPhysics student Isabelle Cobb presenting the team's research at the Ocean Sciences Meeting 2024 in New Orleans. Photo by Ryan Walter

Cal Poly physics student Isabelle Cobb recently joined the team to explore how various coastal ocean locations experience different heatwave conditions and better understand how upwelling mitigates the effects of heatwaves and leads to a thermal refuge for organisms.

"This research could inform how marine organisms adapt and respond to changing temperatures along our coastline as a result of climate change,” Cobb said.
Cobb and Dalsin presented work at the Ocean Sciences Meeting in New Orleans in February.

“One thing is clear,” Walter said. “These extreme temperature events are not going away, so it is critical that we continue to explore their drivers and consequences.”


Funding contributors to this research:

  • William and Linda Frost Fund

  • Santa Rosa Creek Foundation


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