Four weeks ago in Southern California, Santa Ana winds threatened to drive two fires in Simi Valley — the Porter Ranch and the Sesnon Fire — on a path through the sun-baked hills towards the ocean. But instead of building in strength and destructiveness, as the winds often have in the past, the Santa Anas died down after a couple of days, much to the relief of firefighters and homeowners in the area.
Could this diminution of wind be traced to global warming?
Climatologists analyze massive sets of data gathered over decades of observation, so a trend cannot cause a single weather event, in much the same way that medical researchers can document the risk of smoking, but doctors cannot link a patient’s lung cancer to a single cigarette. But climatologists are developing a new understanding of the factors that drive the Santa Anas, and a team of researchers at UCLA is reporting evidence that global warming has already brought down the risk of the most powerful of Santa Ana winds, which are perhaps the most dangerous annual natural event in Southern California.
Although the paper
has yet to be published in a peer-reviewed scientific journal, Alex
Hall, research spokesman, said his team expects to see it accepted in a
prominent journal next year, and his team is scheduled to present their
results at the state of California’s annual climate change conference
next September.
The UCLA researchers are confident in their findings because the underlying conditions that create the most powerful of Santa Ana winds have been well documented both observationally and in equations. As these fall and winter winds roar through mountain passes, in areas such as Simi Valley, they are powered by a spillover effect from a huge dome of high pressure air that builds up in the high desert area known as the Great Basin, in eastern California, Nevada and Utah.
The vast airflows are drawn to low-pressure areas to be found over the ocean off the coast near Los Angeles, like water in a stream moving downhill.
But it’s not just the difference in elevation and air pressure between the high desert and the ocean that fuels these winds. “Temperature forcing” also plays a crucial role, and as the land warms more quickly than in the past, in the fall and early winter, this forcing loses some of its power. With a physics calculation, Hall’s team finds that this factor has fallen by about one-third, resulting in a slow but steady downtrend in the most dangerous winds.
“This is not a small effect,” Hall said. “It’s a well-known fact that the cool air that forms over the desert at night is part of the Santa Ana condition, and so, as the interior of California warms, the difference between the desert and the ocean air pressures is reduced. That’s why we’re seeing fewer Santa Ana conditions over Southern California, and why we should continue to see fewer until the warming of the ocean catches up to the warming of the land, which won’t be until sometime in the 22nd century.”
Ventura County meteorologist Terry Schaeffer, who has been forecasting weather for local farmers since l976, buys Hall’s argument, although he cautions that the dataset may not be big enough to draw firm conclusions yet.
“It does make sense, and I think California in general is going to come out better than the desert southwest,” he said. “In recent years, the Santa Anas have definitely been weaker, and have been occurring later in the season. It’s a logical pattern, and the data points in that direction, but it’s too soon to know for sure yet.”
Hall points out that with this reduction of wildfire risk will come other changes that may not be so beneficial.
“When fires aren’t burning, which is most of the time, these winds bring pleasant summer-like weather to the area, and they improve air quality because they blow polluted air out over the ocean,” he said. “They also play a role in maintaining the productivity of marine ecosystems in the Southern California Bight by depositing mineral nutrients from the desert to the ocean surface. So I don’t view this as “good” or “bad” news.”
Will global warming lead to cool coastal summers?
Hall’s
team is preparing to present their results next year to a state-funded
scientific research group, the California Climate Change Research
Center, which hosts an annual conference. A paper presented at the
conference this year by a team led by Robert Bornstein at San Jose State, working with Prof. Jorge Gonzalez at Santa Clara, and grad student Bereket Lebassi, found solid evidence over the last 30 years that as the interior
of California warms more quickly in the summer, it pulls cool air from
the ocean inland more forcefully than in the past, resulting in
slightly cooler conditions along the coast.
This is another trend that is expected to strengthen.
“The world is warming, but it’s not a uniform process, not like an oven,” Bornstein said. “The warming will bring a redistribution of pressure systems and wind patterns, and one of those secondary mechanisms will be stronger sea breezes along the California coast in the summer.”
[Figure above: warming shown in red dots: cooling in blue dots. Note cooling in Bay Area.]
Bornstein readily admits that this is not a new idea. Weather experts have long known that the fogginess that develops along the California coast is driven by a thermal low born of the heat that develops in the summer in the San Joaquin and other interior valleys. As the winds circulate in a huge wheel counterclockwise from the land over the ocean, they drag on top of the ocean, bringing cold water from the ocean floor toward the surface. The onshore winds pass over this cold upwelling, forming fog. As the thermal low in the interior pulls the winds a little faster, the process is intensified, resulting in increased fogginess, which has already slightly dropped the maximum daytime temperature along the coast in the summer by about 1 degree Fahrenheit per decade.
“We had a hunch that something like this was happening already,” Bornstein said. “I’ve been showing graphs on this effect to my students for 15 years. But it wasn’t until we started mapping the temperature declines that the pattern along the coast popped out.”
Bornstein believes that the intensified sea breezes will push deeper into Ventura County in the summer than in the past in areas below about 1,000 feet of elevation, and has already documented this effect in the Bay Area and in the Los Angeles basin.
Ventura county meteorologist Schaeffer finds this argument convincing as well.
“We haven’t had any really hot summers in years,” he said. “I can remember years in the past when it was actually hotter in the summer along the coast than in some inland areas like Piru.”
Bornstein’s paper has been submitted to Climate, the most prestigious scientific journal in the field, and has a good chance of being published in spring next year, but already has stirred up excitement in the field. He is meeting with state officials to work on new temperature estimate maps that he believes will show a reduced need for air conditioning in coastal cities such as Los Angeles, and a slowdown in the production of smog, which thrives on high temperatures.
Smog levels already falling sharply in Ventura County
Due to efforts to reduce emissions of smog-producing chemicals, Ventura County, already in the last 15 years, has seen a steady decline in smog levels. According to the Ventura County Air Pollution Control District, which was founded in l968, last year the air in the county was the cleanest ever in its recorded history. Despite doubling its population over the last 35 years, the number of days in which the county as a whole exceeded the Federal standards for lung-damaging ozone fell almost to zero, when many years in the l970s it exceeded that standard over 100 days a year.
“My primary concern has always been ozone,” said Mike Villegas, air pollution control district official. “Coastal areas such as Ventura and Oxnard have not exceeded the Federal standards for some time, but in our inland valleys, such as the Ojai, Conejo and Simi Valleys, it’s still an issue. One factor that drives the ozone reaction is temperature, so a slight cooling would have a modestly helpful effect.”
Bornstein’s team hasn’t looked closely at the entire state of California, but estimates that the slight cooling he has found in the Bay Area and in Los Angeles would be seen elsewhere in the state in the areas in which the sea breezes dominate the local climate. These are zones 22-24, according to the climate maps produced by the University of California for Sunset magazine, where the ocean influence prevails at least 85 percent of the time, and include Oxnard, Ventura and Camarillo, but not Ojai, Simi Valley or most of Moorpark.
Villegas is nonetheless confident that air pollution in the county will continue to decline due to ongoing efforts to reduce smog-producing emissions. As one example, Villegas points to new Federal standards to reduce emissions from oceangoing ships, which contribute a surprisingly high percentage of the nitrogen oxides (NOX) that help make smog when heated in the air — about 17 percent of the NOX found in Ventura County, according to his estimates.
Some good news about temperature, but what about water?
To climatologists, both these changes — the slightly cooler summers along the California coast, and the reduction of the worst of the Santa Ana winds — are considered local effects. Experts in the field stress that the larger effects of global warming on a regional level may overwhelm whatever good news it brings to a specific town.
Kevin Trenberth, an internationally recognized expert who leads a research team at the National Center for Atmospheric Research, and who testified before Congress earlier this year on global warming, has a phrase for the future of the American southwest — “the wets will get wetter and the dries will get drier.”
In an interview, he explained that “coastal regions may benefit from sea breezes, as they always have, but will be hard hit by rising sea level, occasional storm surges and erosion, perhaps associated with an El Niño event, and the rains can be exceedingly hard when they do come, owing to more evaporation and water vapor in the atmosphere.”
Ventura
County has had a relatively quiet couple of years on the storm front,
but will not soon forget the devastating rains of January 2005,
considered the worst in decades, which severely damaged roads and
highways throughout the county, and contributed mightily to the deaths
of 10 people in La Conchita, buried under a collapsing cliff.
Kelly Redmond of the Desert Research Institute in Reno, another highly regarded climatologist, said in an interview that those storms were “not inconsistent with” global warming, although he stressed that any one storm cannot be attributed to an overall trend.
Redmond agrees with Trenberth that the prospects for drought in the Southwestern region are alarming — not for one reason, but for many different reasons.
For one, most Californians depend on water from the snowpack that builds up in the winter in the Sierra Nevada and melts over the course of the spring and summer. Many climatological studies have shown that as the planet warms, that snow will melt faster. Worse, increasing amounts of precipitation in the mountains will likely fall as rain, or as rain on snow. This could lead to flooding in the spring, and water shortages in the long California summers and falls.
Separately, climatologists have found a long-term trend toward the cool dry La Niña pattern that tends to make for drought. Even more troubling, scientists who study climates of the past have found strong evidence of what they call “megadroughts”— droughts lasting as long as 150 years, also linked to La Niña. Richard Seager, who leads a research team at Columbia, published a paper in Science magazine in 2005 arguing that a long-term drought has already begun, and projecting that by 2050 the region as a whole will be as drought-stricken as Oklahoma was in the Dust Bowl.
What to do? Bill Patzert, the irreverent climatologist who works with NASA and is widely considered the best forecaster in Southern California, argues that the time has come to lessen stress on the system by reducing population growth and immigration, but admits, “I’m the only one talking about that.”
The answers, in other words, are still blowing in the wind.
