Warm winter due to La Nina & Arctic Oscillation, cold air can't get to most of US say climate scientists-LA Times

1/27/12, "U.S. seems to have largely escaped winter," LA Times, E. Brown, "A combination of factors has trapped winter's cold air over Canada and Alaska, making for unseasonably warm weather in the Lower 48."

La Nina's influence on the jet stream has kept cold air in Canada and Alaska from reaching the lower 48. This has been reinforced by Arctic Oscillation...La Nina can persist for years.

"Throughout the continental United States, it's been a very warm winter....

The answer: A combination of factors has trapped the winter's cold air in the northern latitudes over Canada and Alaska.

"If you look at U.S. temperatures, you'd say, 'Wow, it was a warm winter,'" said Dan Cayan, a climate researcher at the U.S. Geological Service and the Scripps Institution of Oceanography in La Jolla. And you'd be right.

"But," he added, "in the coastal West, it's been cool."...

"Scientists said the cyclical cooling in the Pacific Ocean known as La Niña was a likely cause for dry conditions in California and across the nation.

There's an 82% probability of less-than-normal rainfall in a La Niña year, said Bill Patzert, a climate researcher at the Jet Propulsion Laboratory in La Cañada Flintridge.

Most of California has received less than half of its normal precipitation this winter, Cayan said.

According to the National Weather Service, downtown Los Angeles has had 5.06 inches of rain this water year, which began July 1. The average for that time period is 6.74 inches.

La Niña-related dryness might have helped California stay cool at night, Kittell said, because less rain means less water vapor in the air. Water vapor is a greenhouse gas that traps heat near the ground.

"When it's very dry, you kind of lose that extra layer and the ground cools like crazy," he said.

Cayan chalked up the cool temperatures on the West Coast to its position on the eastern edge of a La Niña-related high-pressure center over the Pacific Ocean that has created a dry, cool air flow in the region.

La Niña has also helped keep the jet stream on a west-to-east path over Canada, preventing cold Arctic air from dipping into the Lower 48 states, he said.

A phenomenon known as the Arctic Oscillation has reinforced that effect, Patzert said.

The oscillation is a pattern of pressure that wraps itself around the North Pole. When the pressure is low, as it has been for most of this winter, the oscillation captures the cool air that normally breaks out of the Arctic and moves into Canada.

The Arctic Oscillation shifted in January, leading some meteorologists to predict that cold air would soon dip farther south,

• allowing the winter to finally begin in earnest.

But since La Niña can persist for years, Cayan said he suspected it was unlikely California would catch up on rain and snowfall this year.

"We're so far behind right now," he said."



via Climate Depot