Science and CSU

Spring Fashion Tip: What goes with La Niña?




Cool waters in eastern Pacific usually portend drier (but not in north), chillier weather for California, CSU scientists report

By Sean Kearns, CSU Public Affairs

Before updating my spring wardrobe to go with this year’s La Niña weather cycle, I asked a meteorologist, “Do I go for sweaters or shorts?

“Definitely sweaters,” Steve LaDochy told me. A geography professor at California State University, Los Angeles, he bases his advice on what he calls “one of the best predictors of California temperatures”: the temperature of the surface waters of the eastern Pacific Ocean.

Steve LaDochy, Cal State L.A.This year – and, by definition, in any La Niña period – they are cooler. The current La Niña condition strengthened in January but shows signs of weakening come March. It made last summer in Southern California cloudier and chillier than normal, and the waters off shore have remained relatively cold. “My surfer daughter was complaining all year,” said LaDochy.

He is among several meteorologists, oceanographers, biologists and others in the California State University with their eyes on water temperatures more than half an ocean away. (In January at the American Meteorological Society’s annual meeting, LaDochy, Cal State L.A. faculty colleague Pedro Ramirez and others presented research on “California temperature and precipitation trends: climate variability or global warming.”)

For example, the CSU’s Moss Landing Marine Laboratory (MLML) provides timely data regarding ocean and meteorological conditions to the Central and Northern California Ocean Observing System, which develops predictive weather models. MLML and other CSU researchers also explore the impacts of ocean-driven weather patterns on seabirds, fish, invertebrates and marine mammals.

Californians and other inhabitants along the west coast of the Americas have long known of La Niña’s counterpart: El Niño – the periodic warm-weather phase created by unusual months-long warming of the eastern Pacific’s surface waters. When those waters warm – and scientists are not exactly sure why they do – the interchange between the ocean and atmosphere causes the jet stream crossing over the Pacific to alter its course and intensity.

El Niño, La Niña and California

NOAA-Big Sur coast

According to Toby Garfield, a physical oceanographer and director of San Francisco State’s Romberg Tiburon Center for Environmental Studies, when an El Niño develops, the trade winds slacken or even stop. With various dynamic gradients of temperature, pressure and moisture in play, a massive “blob” of warm water flows eastward along the equator and hits the Americas. There the water mass splits, heading both northward and southward along the coast.

With El Niño’s currents restricting the coastal upwelling of nutrients from ocean depths, a food-chain of events ensues that sends fish populations elsewhere. Meanwhile, El Niño generally brings seasons of warmer temperatures and heavier rains to California’s onshore populations, humans included.

But during La Niña, with cooler sea-surface temperatures in the eastern Pacific, the jet stream takes a different course, wind-patterns shift, upwelling resumes, and conditions return to “normal,” said Garfield.

Recent ocean temperatures suggest that Southern California could expect weather cooler and drier than normal; meanwhile, the Pacific Northwest and Northern California might anticipate cooler and wetter.

“But, it varies year-to-year. You can’t say for sure what’s going to happen,” Garfield said.

The long view of Pacific Decadal Oscillation

El Niño or La Niña periods might last months to a year or more. While they come and go, a much longer cyclic pattern – called the Pacific Decadal Oscillation (PDO) – also drives the climate.

As LaDochy describes it, the PDO is a complex, roughly 50-year cycle in which the eastern Pacific ocean surface is warmer than usual for about 20 to 25 years, and then, for another couple of decades, the pattern reverses and the surface is, over the long term, generally cooler.

NASA image from December 2010 shows sea-surface height with higher (warmer) than normal as yellow and red, lower (cooler) as blue and purple.“Right now, we’re in the cool phase of the PDO, which does not bode well for our rain supply in the Southwest. More or less, we have been in the cool phase since 1998, so chances of El Niños and wetter weather would be less overall in the next 10-15 years.”

Back to the forecasts – with caveats

In 2004, LaDochy was quoted in a news story, “Over the next several years there is going to be a tendency toward dry and cooler temperatures in the southern U.S. West Coast. It is very difficult to forecast day-today here on the West Coast, but we can say with some confidence that over the next five years, we’d better start saving water.”

Well? Was he close?

Here’s his update: “In 2004, we had our only one wet year since 1998 – and it was a very wet year. But overall, PDO has stayed cool and Southern California has been cool and dry, including the driest year on record, 2006-07.”

Yet the climatic theme can vary. John Monteverdi, a professor of meteorology at San Francisco State University who has examined the ocean’s long-term history of oscillations, said, “It’s not unprecedented for Southern California to have a wet year during a La Niña.”

Sometimes during a cool phase the jet stream shifts south, picking up moisture as it travels toward California along the latitude of 35 degrees north, he said. This creates an “atmospheric river” that delivers a low-latitude storm. This so-called “Pineapple Connection” brought historic floods to California in 1955 and 1964, both La Niña years. (During an El Niño, the jet stream “locks in” along 32 to 33 degrees latitude, he said.)

“Maybe one out of every five or 10 La Niñas is a particularly wet one,” Monteverdi said.

Then it’s time for sweaters and a slicker.

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