Anticipating the extremes: Birch Aquarium lecture looks at atmospheric rivers and California’s climate

The first of three virtual “Perspectives on Ocean Science” lectures presented by Birch Aquarium in La Jolla focused on atmospheric rivers and the impact they may have on California’s climate.

Meteorologist Alexander Gershunov discussed “The Art and Science of Atmospheric Rivers and the Changing Hydroclimate of the West” on Feb. 8.

He said he studies “how regional weather patterns and extreme weather events are related to climate change and how to use those relationships to predict weather and climate better” and that he is also interested in the effects of extreme weather on society.

An atmospheric river is a “filament of very moist and windy conditions that is longer than it is wide,” Gershunov said.

“There are anywhere from three to seven of these rivers around the globe at any time. They transport, in the form of water vapor, the equivalent of several Mississippi Rivers instantaneously,” he said. “And they can last for a few days but wiggle around a lot. The topography of California is ideally oriented to squeeze water from these features.”

Seemingly a two-sided coin, atmospheric rivers “cause a lot of extreme events associated with floods, but they also contribute a heck of a lot to our water resources” in California, Gershunov said.

“What’s interesting about the hydroclimate of California is how variable it is … it is an extremely volatile region in terms of water resources. It’s typical that we have drought years followed by flood years and get floods during a drought year like we are today,” he said.

As for the future of California’s climate, scientific models disagree. “Some predict wetter conditions and some predict drier conditions. So what is going on?” Gershunov said.

He explained that low-intensity rain, such as drizzle, is not expected to change. Middle-of-the-road precipitation is expected to decrease, heavy precipitation is expected to increase and “extreme precipitation sees a huge increase by the end of the century, specifically in California.”

“Extreme events” include atmospheric rivers. “Atmospheric rivers in California contribute about 40 to 50 percent of the total precipitation we get,” he said. “If we know the atmospheric rivers are responsible for the increase in extreme events and the other types of storms are responsible for the decrease in frequency of precipitation … this has implications for all kinds of aspects to our environment, including snowpack and wildfire.”

“The main takeaways is that our very volatile hydroclimate, which is so variable because precipitation is so infrequent in this region, is becoming more volatile as that precipitation becomes less frequent and more and more of the annual total [rainfall] depends on the few big storms of the year,” Gershunov said. “It becomes more challenging to manage because the breakdown between atmospheric rivers and precipitation in general is changing from beneficial events that feed our moisture supply to damaging events that increase flood risk.”

Each year the “Perspectives on Ocean Science” series brings presentations on research conducted worldwide by scientists from the Scripps Institution of Oceanography. The lectures traditionally are held at Birch Aquarium but have been online since last spring.

“The new virtual format ... has allowed us to reach audiences all over the country in a way we never have before,” said Harry Helling, executive director of Birch Aquarium. “This is an exciting opportunity for people to learn how Scripps researchers are tackling tough questions about the impacts of climate change in our own backyard and how this research is helping inform adaptation and mitigation strategies. These talks make meaningful climate science accessible to our audience.”

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The next lectures are “Getting Warmer? Ocean Temperatures off the California Coast” with oceanographer Katherine Zaba at 6 p.m. Monday, March 8, and “Fire, Extreme Rainfall and Debris Flows: Cascading Disasters in a Changing Climate” with meteorologist Nina Oakley at 6 p.m. Monday, April 12.

Lectures are free and available to the public through Zoom. Registration is required. For more information, visit aquarium.ucsd.edu/experiences/programs/pos-lectures. ◆