When heavy rain falls over the Indian Ocean and Southeast Asia and the eastern Pacific Ocean, it is a good indicator that temperatures in central California will reach 100°F in four to 16 days, according to new research.
Heat waves are common in California’s Central Valley, a 50-mile-wide oval of land that runs 450 miles from just north of Los Angeles up to Redding. The valley is home to half of the nation’s tree fruit and nut crops, as well as extensive dairy production, and heat waves can wreak havoc on agricultural production. The dairy industry had a heat wave-induced economic loss of about $1 billion in 2006, for instance. The ability to predict heat waves and understand what causes them could inform protective measures against damage.
“We want to know more about how extreme events are created,” says corresponding author Richard Grotjahn, professor in the land, air, and water resources department at the University of California, Davis.
“We know that such patterns in winter are sometimes linked with areas of the tropics where thunderstorms are enhanced. We wondered if there might be similar links during summer for those heat waves.”
The scientists analyzed the heat wave data from June through September from 1979 to 2010. Fifteen National Climatic Data Center stations located throughout the valley collected the data. From these data, the researchers identified 24 heat waves. They compared these instances to the phases of a large, traveling atmospheric circulation pattern called the Madden-Julian Oscillation, or MJO.
The MJO manifests as heavy rain that migrates across the tropical Indian and then Pacific oceans, and researchers have shown that it influences winter weather patterns.
“It’s well known that tropical rainfall, such as the MJO, has effects beyond the tropics,” says the paper’s first author Yun-Young Lee of the Asia-Pacific Economic Cooperation Climate Center in Busan, South Korea. “So a question comes to mind: Is hot weather in the Central California Valley partly attributable to tropical rainfall?”
Lee and Grotjahn found that, yes, enhanced rainfall in the tropics preceded each heat wave in specific and relatively predictable patterns. They also found that hot weather in the valley is most common after more intense MJO activity in the eastern Pacific Ocean, and next most common after strong MJO activity in the Indian Ocean.
“The more we know about such associations to large-scale weather patterns and remote links, the better we can assess climate model simulations and therefore better assess simulations of future climate scenarios,” Grotjahn says.
The National Science Foundation, the National Aeronautics and Space Administration, the Department of Energy Office of Science, the United States Department of Agriculture’s National Institute of Food and Agriculture, and the APEC Climate Center in the Republic of Korea funded the research.
The work appears in Advances in Atmospheric Sciences.
Source: UC Davis