A new study by Stanford University scientists reveals that atmospheric patterns connected to droughts in California have occurred more frequently in recent decades, pointing to the state moving towards more extreme weather. The team came to their conclusions by analyzing large-scale circulation patterns that have occurred during California's historical precipitation and temperature extremes over the years.
"The current record-breaking drought in California has arisen from both extremely low precipitation and extremely warm temperature," said Noah Diffenbaugh, senior author of the study. "In this new study, we find clear evidence that atmospheric patterns that look like what we've seen during this extreme drought have in fact become more common in recent decades."
Diffenbaugh and his team examined atmospheric pressure patterns that were similar to those that occurred during California's historical weather events in order to determine if they have been occurring with a different frequency in recent decades.
The team used historical climate data from United States government archives to reveal changes during California's "rainy season" from October to May, focusing on identifying the specific North Pacific atmospheric patterns associated with the extreme temperature and precipitation seasons that took place between 1949 and 2015.
The results revealed a significant increase in the occurrence of atmospheric patterns associated with the aforementioned extreme temperature and precipitation seasons over the 67-year period that the study examined. In particular, the team found increases in patterns closely related to those observed during the latter half of California's ongoing drought.
"California's driest and warmest years are almost always associated with some sort of persistent high pressure region, which can deflect the Pacific storm track away from California," said Daniel Swain, first author of the study. "Since California depends on a relatively small number of heavy precipitation events to make up the bulk of its annual total, missing out on even one or two of these can have significant implications for water availability."
With this atmospheric "high" deflecting winter storms away from California, the state is missing on a crucial water supply - approximately 75 percent of the state's precipitation comes in the coldest months. In addition, this blocking pushes temperatures higher on land and in the coastal ocean.
Despite this drought, California has not experienced a decline in wet atmospheric patterns.
"We're seeing an increase in certain atmospheric patterns that have historically resulted in extremely dry conditions, and yet that's apparently not occurring at the expense of patterns that have historically been associated with extremely wet patterns," Swain said. "We're not necessarily shifting toward perpetually lower precipitation conditions in California - even though the risk of drought is increasing."
In other words, extreme weather in California isn't dying down, it's increasing, and residents should prepare themselves for "more warm and dry periods, punctuated by wet conditions."
The findings were published in the April 1 issue of Science Advances.
