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This Year’s La Nina Likely To Bring Cold, Moist Jet Streams Into Northwest This Winter
Posted on Friday, October 29, 2010 (PST)

The winds and general drenching across much of the Columbia River basin in recent days are likely just the start, according to climate experts who gathered in Vancouver, Wash., last week for the 14th annual “Washington/Oregon Climate and Water Meeting: 2011 Water Year.”

 

The strength of current “La Nina” climate signals, and the fact that it transitioned quickly and strongly from stout El Nino conditions, make even more likely that the Pacific Northwest will experience a wetter and colder winter than is the average, the experts say. That means that streamflows fed by mountain snowpack would also most likely to be higher than average.

 

Mother Nature can be contrary. La Nina prevailed going into the winter of 2000-2001. But the result was a meager snowpack and one of the lowest runoff volumes on record.

 

“It defied the odds,” Nathan Mantua of the University of Washington’s Climate Impacts said. That is highly unlikely this year.

 

“The La Nina is stronger this year,” Mantua said of below normal sea surface temperatures in the equatorial Pacific sea surface temperatures and other signals.

 

“This is a very strong La Nina event,” the strongest since the 1950s by some measures, he said. The last time ocean and atmospheric conditions, as measured by the Multivariate ENSO Index, were as low as they are now was in 1954. Low indicates La Nina and high indicates El Nino. ENSO stands for El Nino/Southern Oscillation, the major source of inter-annual climate variability in the Pacific Northwest. El Nino conditions increase the likelihood of drier, warmer winters; La Nino ups the odds of having a wetter, cooler winter.

 

Of the climate signals monitored, “many are getting stronger,” Mantua said.

 

The Pacific Decadal Oscillation, the predominant source of inter-decadal climate variability in the Pacific Northwest, has also slipped into a cool phase, Mantua said. That condition most often supports those increased La Nina odds. The PDO is characterized by changes in sea surface temperature, sea level pressure and wind patterns.

 

The two indexes “continue to be our best indicators for winter climate,” Mantua said. “There are strong odds for a wet fall.” And those La Nina conditions are expected to persist through the winter.

 

A typical climate pattern in La Nina years is that moist jet streams pummel the northwest, either directly from the West or by looping up through Alaska and then down. The latter pattern “can bring very cold weather,” Mantua said.

 

“There are many different flavors of these things,” said Alan Hamlet, but the swift and strong transition from last winter’s El Nino conditions to La Nina like this late spring and summer have most often meant a wet, stormy winter is looming.

 

He said that water year pairs with ENSO transitions from warm to cool phase in a single year are strongly associated historically with above average flows in Columbia River in the second year. And historically such paired (warm/cool) events are more likely to occur when a strong (above 1.0 standard deviation) warm event occurs.

 

The existence La Nina has often “biased” Northwest weather toward producing higher flows.

 

Dating back to 1900 there have been 14 years with a transition as abrupt as the flip-flop that took place this past spring and summer as equatorial sea surface temperatures shifted from well above normal to well below normal. In 12 of those years the April-September streamflows as now measured at The Dalles were 108 percent of average or higher, and in only one year were streamflows below normal. In nine of 14 years streamflows were 114 percent of average.

 

Hamlet said the region is “likely to be in very wet territory, or certainly above average” flow conditions next spring though, again, Mother Nature can be fickle.

 

Still, building knowledge about how ENSO influences the region improves the reliability of long-term predictability.

 

“Thus increased odds of above average flow are predictable with very long lead times of up to 24 months under certain conditions. In June 1997, with a strong warm ENSO event clearly under way, elevated odds of high flows in warm season 1999 would have been predicted, for example,” Hamlet included.

 

The annual climate and water meetings are sponsored by CIG to provide a forum for discussing the upcoming winter's climate forecast and how it may affect streamflow conditions in the Columbia River Basin and coastal drainages in Washington and Oregon in 2011. The meeting also highlights new research, decision support tools, and other information useful to managing for Pacific Northwest climate variability and climate change.

 

CIG is an interdisciplinary research group studying the impacts of natural climate variability and global climate change (“global warming”) and informing the region about those potential impacts.

 

Presentations from the meeting have been posted on the meeting website in PDF format:

http://cses.washington.edu/cig/outreach/workshopfiles/vancouver2010/index.html

 

The Idaho Climate and Water Meeting for the 2001 Water Year provides the opportunity to hear how forecasted climate conditions for the winter 2010-2011 may affect regional climate and streamflow conditions in the Columbia and Snake River Basins in 2011. More information on the Idaho meeting is available at:

 http://cses.washington.edu/cig/outreach/workshopfiles/boise2010/index.html

 

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