After three straight dry winters and nine subnormal rainfall years in the last 10, water managers and many people in Southern California and the West are ready for a wet winter.
The question is, is the Pacific?
The short answer is, it's probably too early to tell.
But there is a wide range of intriguing possibilities – everything from an average winter, to several more years of potentially devastating drought, to a good, drenching El Niño.
“The system is preconditioned for a big one,” said Mike McPhaden, a senior scientist at the Pacific Marine Environmental Laboratory in Seattle. “But whether or not it occurs is kind of a crapshoot.”
Spring is not the best time for long-range forecasters to peer into their meteorological crystal balls. It's a time of low confidence, when forecasters hit what they call their “predictability barrier.”
During the spring, conditions in the central and western Pacific, which can have a huge impact on global weather patterns, are often making the transition in or out of cold or warm phases.
When sea-surface temperatures are colder than normal for several months, a condition known as La Niña, fewer and weaker storms tend to reach Southern California. La Niña ruled from last summer through early this spring.
San Diego's winter started off normal, but things went bone dry after mid-March, when La Niña flexed its muscles. The city has had 7.02 inches of rain since July 1; normal through today is about 10.65 inches. More importantly, California's snowpack was much smaller than normal this year. Many farmers have had water deliveries reduced by 30 percent, and homeowners and businesses are being asked to cut water usage.
But conditions could be changing drastically in the western Pacific. An El Niño, when waters in the equatorial Pacific turn abnormally warm, looks like at least a possibility. If it develops, it could temporarily transform the outlook for California and the West.
El Niño years have brought severe coastal erosion and flooding inland in California, but overall, studies have shown the phenomenon is a financial plus for North America. Copious precipitation tends to replenish Western water supplies, and changes in upper-level wind patterns often reduce the number and severity of hurricanes that hit the Atlantic Seaboard and Gulf Coast.
Waters at the surface in the equatorial Pacific remain cooler than normal, but there is an unusually large amount of heat energy currently stored in the upper 1,000 feet of the western portions of the ocean, McPhaden said.
The surplus heat is the highest measured since 1980, when it first became possible to track Pacific heat storage, he said. It is 20 percent higher than the previous record set just prior to the onset of the monster 1997-98 El Niño.
No guarantees
But those warm subsurface waters alone are no guarantee an El Niño will develop, McPhaden said. Several things must happen in the ocean and the atmosphere before El Niño takes hold.
Something must trigger the release of that warm water. If there were to be a relaxation of the trade winds, which blow toward the west near the equator, pent-up warm water could move eastward.
If that happened, a big pool of warm water would collect at the surface in the central Pacific along the equator, and that could start a feedback loop. Warmer temperatures would slacken the winds, which would warm the waters more, which would weaken the winds more, and so on.
Eventually ocean temperatures would climb markedly enough, McPhaden said, to alter the position of storm tracks over North America. Southern California and the Southwest often get pounded with storms under those conditions.
As long as the sea-surface temperatures remain cool, however, it's unlikely those trade winds will weaken sufficiently. But if surface temperatures rise a little, it could start a feedback loop. Warmer temperatures could slacken the winds, which would warm the waters more, and that would reduce the winds even more, and so on. Eventually, ocean temperatures would climb markedly, McPhaden said.
But that trigger is very difficult to foresee, said David Pierce, a climate researcher at the Scripps Institution of Oceanography in La Jolla.
“It's important to understand that there are processes that can affect El Niño that are unpredictable,” Pierce said. “The main one is westerly wind bursts, related to tropical storms in the Pacific.”
Such bursts could reverse the trade winds and give El Niño a kick start, Pierce said, but there's no way to know if those storms will develop.
In 1997, the trade winds did relax suddenly about this time of year, McPhaden said. Things “cascaded from there,” he said, and El Niño got rolling.
Conflicting forecasts
Long-term computer forecast models are split on what will happen this year. A handful of the models see an El Niño taking hold; an equal number envision a return to La Niña; and even more models expect neutral conditions come winter. San Diego's winters have varied widely under neutral conditions.
“Our forecasters are kind of scratching their heads trying to figure it out,” said Mike Halpert, a meteorologist with the Climate Prediction Center in Camp Springs, Md. “We really don't know what to make of these (computer) forecasts.”
Whatever develops this winter, it will take a back seat to an even bigger atmospheric cycle known as the Pacific Decadal Oscillation, according to Bill Patzert, a research oceanographer and long-range forecaster at NASA's Jet Propulsion Laboratory in Pasadena.
The oscillation, the PDO for short, is “10 times bigger and 10 times longer” than the El Niño/La Niña cycle, Patzert said. He said many long-range forecasters incorrectly look solely at the El Niño/La Niña signal, which focuses on the equatorial region.
The PDO is entering a negative phase that will likely bring many more years of subpar rainfall to the West, Patzert believes. Under the negative phase, a horseshoe-shaped pattern of below-normal sea-surface temperatures stretches from the northern Pacific, along the West Coast of North America, then westward along the equator. Under the negative phase, El Niños are suppressed, and La Niñas are enhanced.
“We're going into a significant PDO, and that's what will dominate,” Patzert said. “El Niño and La Niña won't play much of a role.”
McPhaden believes it's too early to tell if the PDO is reigning, or if the Pacific has simply been under the influence of La Niña.
“It's very difficult to separate them out,” McPhaden said. “I'm of the opinion we need to wait another five years” to tell if the bigger PDO or the smaller La Niña has been the main player.
In two or three months, all of the long-range forecasters will have a better idea of what the coming winter will hold: continued dry times, or perhaps a significant El Niño that will fill Western watersheds and ease the drought.
“Everything is sort of delicately balanced in the spring season,” McPhaden said. “In the summer, patterns set up that tend to persist for a couple of seasons.”
Visit the Weather Watch blog at Weatherwatch.uniontrib.com. Have a question or comment about the weather? Write to Weather Watch, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or send e-mail to rob.krier@uniontrib.com.
