February 12, 2003
The word California invokes many images: miles of sunny beaches, streets lined with swaying palm trees, mountains that touch the great blue sky and cold nights filled with thick, dense fog. What was that? Cold? Fog?
Since the "beachgoer's dream" summers that preceded 1998, California has experienced consistently cooler and foggier summers than usual - not exactly grade-A performance for the coast that's supposed to have the most. The question many sun worshipers may be asking is, "Why?"
"History seems to be repeating itself every 50 years," said Dr. William Patzert, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The weather switches from warmer temperatures, wetter winters and less fog to cooler temperatures, drier winters and more fog, and back and forth."
Patzert and colleagues Dr. Steve LaDochy and Jeff Brown from California State University, Los Angeles, studied the factors responsible for variable coastal temperatures and fog frequencies along the southern California coast from 1948 to 2001. They are presenting their findings at the 83rd Annual Meeting of the American Meteorological Society, being held through February 13 in Long Beach, Calif.
From the mid-1940s to the 1970s, temperatures (as measured at Los Angeles International Airport and Los Angeles Civic Center, among other locations) were generally cooler, winters were drier and fog levels were fairly high. In the 1980s and 1990s, temperatures warmed up, winters were wetter and the number of foggy days each year was halved. Now, it seems that since the 1997-98 El Niño, we've returned to the previous prolonged "June Gloom" pattern.
"'June Gloom,' when unusually heavy coastal fog and cooler temperatures hug the coast, occurs in the early stage of summer," Patzert said. "The dismal weather results from an extreme contrast between warming land and cool ocean temperatures, when the temperature of the land rises quickly, but the temperature of the Pacific Ocean, which covers about one third of the earth's surface, takes longer to heat up. Fog, or vapor condensed into fine particles of water, is basically a set of clouds on the ground produced by this land-sea interaction. During some decades, the large-scale climate shifts in the Pacific Ocean keep the California current ocean temperatures cooler. This gives southern California more heavy fog days. The switch to a warmer pattern results in fewer foggy days."
Patzert said studies have shown relationships between sea-surface temperatures in the tropics and weather in the United States, but several other oceanic measurements show even stronger connections to Southern California temperatures. The Pacific Ocean appears to be the most obvious factor affecting coastal temperatures and moisture. By comparing various studies and indexed measurements of Pacific Ocean sea surface temperature variability, Patzert and his colleagues found strong correlations between north Pacific Ocean climate cycles and coastal temperatures and heavy fog frequencies. Warmer eastern tropical Pacific Ocean temperatures, for example, give us less fog. They therefore deduced that large-scale, long-term temperature patterns in the northern and tropical regions of the Pacific Ocean are useful in predicting year-to-year fog conditions.
The researchers also found that while fog has been more noticeable during the past four years, heavy fog has declined significantly since 1950, in part because of urban influences. For instance, downtown Los Angeles temperatures have increased by approximately 2 degrees Fahrenheit (1 degree Celsius) since 1950, and particulate air pollution has declined by about 50 percent. This increase between the land in the Los Angeles basin and Pacific Ocean temperatures could explain the long-term trend in decreasing fog. This also suggests that future urban growth, global climate change and continuing Pacific Ocean cycles may significantly affect future coastal weather in Southern California.
Although Patzert forecasts a prolonged "June Gloom" for this summer, he says, "There's no need to escape to the tropics just yet. As with any cloud, there is usually a silver lining. While the past has held a record of long-term gloomy periods, uncharacteristic mood swings have been known to occur on a sporadic basis. Case in point: last month, in the middle of winter, it was warm and clear enough outside for a beach barbecue."
The California Institute of Technology in Pasadena manages JPL for NASA.
Contacts: JPL/Charli Schuler (818) 354-3965