The Scripps Pier in La Jolla. Courtesy of Scripps Institution of Oceanography

Climate researchers at Scripps Institution of Oceanography have identified the reason why water temperatures off San Diego reached their highest point in at least a century in 2018, the institution reported Wednesday.

Three times during August 2018, a new record high water temperature was established, shattering a record that had stood for nearly 90 years, according to data that Scripps Oceanography has collected since 1916.

Research led by Xinyue Wei and Kaiyuan Li, who served as interns with Shang-Ping Xie, a climate scientist at Scripps Oceanography, found that weather events thousands of miles away from California caused the record warming.

The two found that in the week leading to the record heat wave, tropical storms several hundred miles south of San Diego stifled the flow of deep, colder water to the surface, and thickened the thermocline, the layer of the ocean that is nearest to the surface and warmed by sunlight.

The conclusion from Xie’s group highlights the large-scale environment in which the coastal marine heat wave occurred. Scripps colleagues had earlier identified a lack of cloud cover and curtailed winds as a source of the warm ocean off San Diego in a 2020 study.

The two results reflect the different approaches to observing the natural world between seagoing oceanographers more concerned with direct ocean measurements and climate scientists who look for global connections of climate events.

“Coastal waters off California are usually cold even in summer,” said Xie, who directed the research. “This study shows that they can be as warm as Hawaii when waves from the equator and the relaxed winds conspire to shut off the upwelling that pumps cold water from beneath.”

James Fumo, now a graduate student at the University of Hawaii, led the 2020 study and said its goal was to describe the phenomenon and document that a long-term trend of warming caused by human activities such as fossil fuel burning raises the baseline for extreme heat events. Fumo and colleagues concluded that with so much heat energy added to the oceans in general, any warming episode has a substantially greater chance of setting records.

Xie said he agrees with that premise and that his work illustrates how events in one region influence events in other regions even over great distances.

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Chris Jennewein

Chris Jennewein is Editor & Publisher of Times of San Diego.