Fall and winter outlook still murky
By Bob Berwyn
FRISCO — With no strong El Niño or La Niña on the horizon, forecasters are struggling even more than usual to develop seasonal outlooks for the western U.S. The periodic El Niño-La Niña cycle is a large-scale shift in the Pacific involving a complex interplay of winds, ocean currents and sea surface temperatures.
In the U.S. the warm El Niño phase is associated with wetter than average conditions in the Desert Southwest and California, and can result in below average precipitation in the Pacific Northwest.
La Niña, on the other hand, has been linked with Southwestern drought conditions and heavy precipitation in the Pacific Northwest. That persistent moist flow off the northwestern Pacific can also favor parts of Colorado with good winter snows, but the ENSO climate signal is more marginal in Colorado than in other areas.
In fact, some emerging research suggests that the El Niño-La Niña cycle may not be as big a factor as thought, said Colorado State Climatologist Nolan Doesken.
“During this extended neutral period, we start to look at other factors, including what’s happening in the Atlantic,” Doesken said. Fluctuations in Atlantic Basin pressure differences may play a bigger role in shaping Colorado’s weather than previously believed, he added.
For the next three months, at least, sea surface temperatures in the equatorial Pacific are projected to stay near neutral values and for the winter, there are signs that a third consecutive La Niña may develop, with cooler than average ocean temperatures playing a role in steering Pacific storms across the U.S.
Extended La Niña periods are not unprecedented, said NOAA climate researcher Michelle L’Heureux, explaining that the current cycle may fall well within the range of natural variability.
“In general, our reliable, historical ENSO record lasts about 60 years,” L’Heureux said. Before 1950, data from across the Pacific region is sparse, so NOAA’s official ENSO index starts in 1950.
Based on that record, there area about 15 neutral periods that have been longer than this one, she said.
“And I should add that 60 years is not a particularly long record from which to measure these sort of intervals. In other words, with another 500 years of data , we might find that that the frequency of long-length Neutral periods is actually much different,” L’Heureux said via email.
Having a strong ENSO signal helps weather experts hone seasonal outlooks.
“In the absence of ENSO … confidence decreases considerably and we need to rely on other forecast tools,” said NOAA seasonal forecaster Jon Gottschalck. “We utilize statistical forecast techniques to relate patterns of certain variables in both the Tropics and Extratropics to observed temperature and precipitation patterns in the past,” Gottschalck said via email.
“Sometimes, some signals are deemed statistically significant using these purely statistical methods. For example, sometimes we can relate reliably the patterns of sea level pressure in the north Pacific Ocean 3 months in the past to what the observed temperature is in the U.S. 6 months in the future based on analyzing these relationships over the past 50 years or so,” he said.
Decadal trends are also part of developing seasonal outlooks, he explained:
“For example, how does the average temperature over the past 10 years compare to that in the overall 30 year climatological or normal period? This consistent or linear change with time can be used as a prediction.”
Soil moisture and snow cover also factor into forecasts. Drier soil moisture conditions can lead to a feedback of warmer than average temperatures and vice versa for wet soil moisture conditions or abnormally high snow cover, he explained.
And it’s not clear if California will see any relief this winter.
“Unfortunately, there is no evidence in any of the forecast tools we have available (traditional statistical or dynamical models) of any reliable climate signals that would point to elevated odds of above-average precipitation this coming autumn or winter to improve things in California,” Gottschalck concluded.
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