Expect More Rainfall Variability for California

A research team led by Berkeley Lab has determined that a tropical weather phenomenon impacting the West Coast will dramatically alter California rainfall by century’s end, leaving the state to contend with a 50% increase in winter rainfall variability by the year 2100.

The Madden-Julian Oscillation (MJO) is a poorly understood rainfall pattern that covers a quarter of the globe when moving eastward from the Indian Ocean. It lasts 30 to 60 days and has a vast reach, influencing monsoons in India and tropical cyclones in the Western North Pacific. In North America, the MJO influences the occurrence of flooding, cold weather, and drought.

Berkeley Lab faculty scientist Da Yang and colleagues attempted to quantify the impact of environmental change on this obscure weather phenomenon, operating under the assumption that global greenhouse gas emissions will rise as expected in coming decades. In effect, the findings suggest that when the MJO is in a cycle that brings more rain to the West Coast, a region of California that currently receives an average of 20 inches of rain could instead receive 30 inches — over just a two-month period. Or, if the MJO is in a drying pattern, the same California region could receive just 10 inches of rain over one to two months.

“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 the University of California, Davis. “This may have substantial implications for agriculture, flood control, and water resource management.”

The MJO was first identified in 1970; but the physics behind its behavior has not been pinned down. Yang has a theory that is among four that are being widely debated. Part of the challenge, he notes, is that many state-of-the-art global climate models fail to incorporate MJO behavior. As a result, he and lead author Wenyu Zhou, a postdoctoral fellow in Yang’s lab, averaged the results using the best 10 models for the analyses. The modeling assumed high emissions of greenhouse gases would continue, leading to an estimated 4 degree C temperature increase globally by 2100.

The findings, published July 6 in Nature Climate Change, not only revealed the weather phenomena’s future California impact, but provide a step toward defining MJO behavior. In particular, their analyses suggest that the MJO is able to transfer its precipitation-related atmospheric conditions to the West Coast of the United States via changes in the strong, high-atmosphere winds of the subtropical jet stream.

“The precipitation change isn’t just due to the MJO itself,” Yang said, “but is mainly due to changes in the jet stream’s wind distribution.”

The MJO affects California weather by exciting a group of atmospheric waves—ripples in the air—that can travel through the jet stream and reach the West Coast. The wave propagation is most effective at the exit region of the jet stream, where the wind speed decreases sharply. In a warmer climate, this jet exit shifts eastward, bringing the trajectory of planetary waves, and storms or droughtcloser to California.

“We now know that the MJO can impact California weather by exciting these atmospheric waves,” Yang said, noting that he hopes the findings will enhance interest in understanding MJO behavior better and in incorporating the phenomenon into climate models.

In the future, Yang and colleagues will further investigate the MJO’s impact, such as whether the eastward shift of the jet stream exit will produce more atmospheric rivers in California. In addition, his lab will look deeper into the physics of the MJO.

The current research was supported by Berkeley Lab’s Laboratory Directed Research and Development funding for the project “Toward Accurately Predicting California Hydroclimate by Cracking the Tropical Storm King” affiliated with the Department of Energy CASCADE Scientific Focus Area, and by a Packard Fellowship for Science and Engineering through UC Davis.

The material in this press release comes from the originating research organization. Content may be edited for style and length. Want more? Sign up for our daily email.