A research team led by a Scripps climate scientist has found evidence that the risk of hazardous weather is increasing throughout the Southwest and Southern California.
“What we were working with was 70 years of weather records that went back to 1949,” said Kristen Guirguis with the Climate, Atmospheric Sciences and Physical Oceanography Department of Scripps Institution of Oceanography, University of California, San Diego.
“We looked at weather changes in Southern California with a focus on precipitation in the wintertime associated with atmospheric rivers. Climate model projections for the future indicated we are likely to experience more frequent dry days, and more intense precipitation days.”
Guirguis said the likely consequence of that trend is that “we will be expending more of our water resources that come from these really intense atmospheric river storms, which are becoming stronger due to climate change. The implication is that if these weather patterns occur when the vegetation is dry, the threat of wildfires will increase.”
Atmospheric rivers are seasonal streams of moisture in the sky, which are prevalent in California in the winter. The state already has the most volatile water resources in the country.
Scripps scientists have discovered that the state’s precipitation, as it becomes less frequent but stronger due to atmospheric rivers, will fluctuate even more between extremes of drought and flooding, as a consequence of climate change.
The mountain snowpack, which historically has portioned out water from melting snow in California during summer, will be less of a water resource in the future because of atmospheric rivers.
Atmospheric rivers are warm storms with high snow levels. They are becoming wetter as they warm, causing even more precipitation to fall as rain and less as snow. It will fall in progressively less frequent – but more extreme – bursts requiring more adaptive reservoir management based on better atmospheric rivers forecasting.
The Scripps study also revealed the prospect of worsening droughts.
“This study suggests that weather patterns are changing in a way that enhances hot, dry Santa Ana winds while reducing precipitation frequency in the Southwest,” said Guirguis. “These changes in atmospheric circulation are raising the risk of wildfires during California winters.”
The study, “Winter wet—dry weather patterns driving atmospheric rivers and Santa Ana winds provide evidence for increasing wildfire hazard in California,” was published in the journal Climate Dynamics on July 17, 2022.
The research team identified 16 recurring weather patterns that are created daily with the interaction of the dominant atmospheric circulation patterns over the Pacific Ocean, and how they interact with each other. One product of the work was a summary of California weather patterns from 1949 to 2017.
The study revealed the patterns associated with the formation of dry gusty Santa Ana winds that often stoke Southern California fires are becoming more frequent. Meanwhile, patterns associated with what might be considered “normal” rainfall are decreasing in the Southwest, thus promoting drought.
However, patterns associated with extreme precipitation and strong atmospheric river episodes have remained steady over the study period. The researchers noted that while the patterns associated with heavy precipitation and strong atmospheric rivers have not changed in frequency, a warmer atmosphere is capable of holding more water. So these storms are becoming more damaging.
Gurguis said the results of their climate study are “more evidence of increasing volatility in California’s climate,” while adding “there has been other research showing this to be the case.”
The climate scientist warned of the growing threat of a combination of weather disasters.
“In any given winter, the potential for wildfires and floods to occur back-to-back just seems to be something that is increasing in likelihood,” she said. “That presents the possibility that if you have a fire, followed by an atmospheric river, then you can create the possibility of a mudslide – which can be quite devastating.”
Concerning climate change, Guirguis characterized the evidence as being “overwhelming.” She noted it is important for the government to “take dramatic steps to reduce greenhouse gas emissions,” as well as to “think about adapting and preparing for the types of extreme weather that we are going to be experiencing. We should just take action now while we can.”
