The 2017 Thomas Fire, fanned by Santa Ana winds, burned an area nearly as large as Los Angeles.
David McNew/REUTERS
The howling hot Santa Ana winds that funnel into Southern California every fall are a fixture of the West Coast experience. Underneath cloudless skies, winds arrive from the high desert mountains to the east, sending temperatures soaring, flattening the surf, and, so the stories go, turning human behavior erratic. The dry winds also fan the region’s deadliest wildfires, including one of the largest in California’s modern history, burning land nearly equal in size to Los Angeles itself.
Until recently, the future of the Santa Ana winds was thought to be one of the few good-news stories of climate change. Scientists had predicted rapid inland warming would weaken one of the winds’ primary drivers and reduce their frequency. But a new study is casting doubt on that projection, finding that bouts of hot Santa Ana winds are not declining. They could even be increasing.
Like other downslope, or katabatic, winds, the Santa Anas form because of differences in temperature and topography. As fall comes around, the high desert of Nevada and western Utah, found in an elevated basin between the Sierra Nevada and Rocky mountains, cools starkly in comparison with the coast. Every so often, a heavy mass of high pressure air plants itself above the basin, suppressing cloud formation. The pressure difference between the basin and the coast means the clockwise-rotating air mass kicks the cold desert air west over the Sierras and down their slopes.
The winds accelerate as they flow down through mountain passes like a river. “In the elevated backcountry, these winds really scream,” says Alexander Gershunov, a climate scientist at the Scripps Institution of Oceanography and lead author of the new paper. And as they descend, they squeeze and heat the air below, much as a bicycle tire warms when inflated. By the time the winds hit Los Angeles, the winds can be 10°C warmer.
The Santa Anas tend to strike first in October, when the rainy season is just beginning. Often, it hasn’t rained for 5 months. “The fuel is ready to burn,” Gershunov says. The cloudless days that come with the Santa Anas mean nature provides little lightning. But humans, through arson, accidents, or arcing powerlines, can provide plenty of sparks. Once started, “that fire is almost unstoppable,” says Scott Capps, head of Atmospheric Data Solutions, a small Los Angeles firm that forecasts the winds. “It’s just going to go until it runs out of wildfire fuel.”
Previous work from Gershunov’s group, published in 2019, had suggested the Santa Anas would decline with warming. But as his team began to look deeper into heat waves tied to days with Santa Ana conditions—an index they have compiled going back to the late 1940s—they noticed an oddity: Although the winds were often the cause of extreme heat, they were also associated with some of the region’s most severe cold spells. “That’s when it got interesting,” Gershunov says.
Downslope desert winds drive both the “hot” and “cold” varieties of the Santa Anas, but they form under different large-scale conditions. The hot variety tended to form in the traditional manner, but the cold species, which made up about one-third of the overall bouts of Santa Ana winds, were tied to passing cold fronts that dump snow in the high desert basin and cause a deep freeze. After dropping its snow, the storm system morphs into a high pressure pattern that kicks the basin’s superchilled air down to California. And even though compression warms these winds, they still arrive frigid.
Gershunov and his colleagues found that coldest Santa Ana winds have become less frequent since the late 1940s. Maximum temperatures during “cold” Santa Ana days in January, February, and March grew by 3.5°C, with similar warming and declining snow cover in the Great Basin. The hot, fire-fanning Santa Anas that start in the fall, meanwhile, showed no clear trend.
But Gershunov suspects a redo of their 2019 projections that accounts for these two species of winds would show that the cold winds will decline in the coming decades. The hot winds, meanwhile, along with the rainfall season, should lag deeper into the fall, allowing fuel to dry even further. “In that case, we’re not seeing any positive news in terms of future wildfire seasons,” says Gershunov, whose team published its new findings last month in Climate Dynamics.
It’s a plausible forecast, but the finding is still speculative without the modeling to go with it, says Leila Carvalho, an atmospheric scientist who studies downslope winds at the University of California, Santa Barbara. And although it’s long been known that some downslope winds can run cold, the paper does a good job exploring the fine differences between their formation and should aid forecasting efforts, she adds.
Although the hot winds are hard to forecast deep into the future, the overall climatic stage is not: California is getting hotter and drier because of humanity’s release of greenhouse gases. And with drought gripping the state again, if the Santa Anas come before the rain this fall, conditions could be set for another historic fire season.
Source: Science Mag