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Weather: El Niño, or La Niña’s ghost?

El Niño still struggling to develop

Will a negative Pacific Decadal Oscillation influence Colorado’s winter weather? Graphic courtesy NASA.

The three-month precipitation outlook from the Climate Prediction Center.

By Bob Berwyn

SUMMIT COUNTY — Climate experts are still hedging their bets when it comes to an outlook for the coming winter, with the official outlook from NOAA’s Climate Prediction Center showing no strong trend toward above- or below-average precipitation.

A somewhat murky El Niño outlook is clouding the picture, with sea surface temperatures in the equatorial Pacific ranging above average, but cooling down from just a month ago.

“It’s vexing … the models are just not up to the task,” Wolter said. Overall, he said he’s “guardedly optimistic” that Colorado will see at least close to an average snowfall year, which would would be critical to maintaining water supplies in the state’s depleted reservoirs.

Based on the current pattern and comparison with analogous situations in previous years, Wolter said he wouldn’t be completely surprised to see some decent moisture in October, though conditions look dry through the first half of the month.

“We’re going to have a pretty unusual combination of a negative PDO and an El Niño,” said meteorologist Klaus Wolter, with NOAA’s Earth Systems Research Laboratory, referring to a long-term Pacific Ocean temperature cycle that is similar to the El Niño-La Niña pattern, but affects a broader area over a longer period of time.

“That’s not necessarily bad for the West Slope – it might just combine with the El Niño in the fall to give at least an OK winter,” Wolter said.

A strong El Niño or La Niña signal is one of the best ways to develop regional seasonal climate outlooks. In the absence of that signal, meteorologists look at other patterns, including the PDO (Pacific Decadal Oscillation).

According to a NASA website, a negative PDO is when cooler sea surface temperatures prevail in the east and center of the Pacific, surrounded by warmer temperatures in the north, west and south that form a “horseshoe” shape — a pattern similar to La Niña, only much bigger.

Although the oscillation hasn’t been studied as much as El Niño, climate researchers say the cycles last about 10 to 20 years. One of the more recent negative phases was from 1945 to 1979, followed by a positive phase from 1975 to 1995. A negative PDO can cause the jet stream to stay a little farther north, similar to the pattern during La Niña years.

Wolter said he’s not ready to write off this year’s El Niño completely yet, even though the Climate Prediction Center said in its Oct. 4 update that the trends toward El Niño slowed in September. Overall, Pacific basin conditions reflect borderline neutral conditions.

A developing wind burst in the western Pacific is working into the central Pacific, and that could reinforce the weak El Niño conditions — “But we’re talking about a six-week delay … we’re looking at this Coke bottle glasses,” Wolter said.

Conditions in the north Atlantic could also be a factor. A positive phase of the North Atlantic Oscillation (a shift in the relative intensity of the Azores High and the Iceland Low) could also help set up a storm track that favors Colorado, he explained.

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