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Scientists Say: Expect More Rainfall Variability for California

Tropical weather phenomenon likely to bring greater rainfall – and drought – by 2100

A new study by researchers from the Climate and Ecosystem Sciences Division at Berkeley Lab has shown that a tropical weather pattern will bring a 50% increase in winter rainfall variability to California by the year 2100. (Credit: Shutterstock)

California’s winter precipitation is expected to become 50% more variable by century’s end, based on a Berkeley Lab-led study of the impact of future greenhouse gas emissions on the Madden-Julian Oscillation (MJO), a rainfall pattern that covers a quarter of the globe.

When active, the MJO influences whether precipitation occurs for 30 to 60 days, and is already known to affect North America’s weather when it moves eastward from the Indian Ocean (sometimes driving, for example, the Pineapple Express, which brings heavy rainfall to the Pacific coast). To see how much global emissions increase would influence the MJO, Berkeley Lab faculty scientist Da Yang and postdoctoral fellow Wenyu Zhou, and colleagues at UC San Diego and Nanjing University used the 10 computer models that best capture MJO behavior to study the emissions’ impact on it. By averaging the models’ results in the paper published recently in Nature Climate Change, the researchers identified the 50% increase in winter precipitation variability throughout California by century’s end.

“I was quite surprised to see such a huge effect, knowing that even a small change in rainfall statewide could have a significant impact,” said Yang, who is also an assistant professor of atmospheric sciences at UC Davis. “This may have substantial implications for agriculture, flood control, and water resource management.”

Moreover, their analysis suggests that the MJO is able to transfer its precipitation-related atmospheric conditions to the West Coast of the United States via changes in strong, high-atmosphere winds of the subtropical jet stream.

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