- Mounting evidence suggests that after a brief summer hiatus La Niña may be back.
- The second year in back-to-back La Niña events is often drier than the first.
This doesn't bode well for the region.
"The bigger the droughts are, the longer they last," said Klaus Wolter, research scientist at the Climate Diagnostics Center at the University of Colorado. "I think when you have a big drought it can perpetuate itself."
Another La Niña would only exacerbate the situation.
In July, the CPC issued a La Niña Watch, indicating favorable conditions for the development of another La Niña event in the next six months. With colder-than-average waters once again upwelling in the tropical Pacific Ocean, La Niña appears to be re-forming.
"Temperatures below the sea surface have decreased quite markedly in the last few months," said David Unger, meteorologist at the CPC.
In addition, he said, the Climate Forecast System model – a state-of-the-art climate model that integrates interactions between the Earth's oceans, land, and atmosphere – has been impressive in its prediction capabilities in the last few years and has been increasingly more confident in the development of a La Niña this winter.
"It's close to even odds right now that La Niña or neutral conditions will develop," Unger said. "It's pretty trivial chances that El Niño will form."
The International Research Institute for Climate and Society, or IRI, also indicates increasing odds for a return of La Niña. Basted on statistical and dynamicla models and conditions that developed in the last week, there is a 43 percent chance that La Niña will develop during the October-December period, an increase from 25 percent assigned last month to this period.
Historically speaking, back-to-back La Niña events are not surprising. The climate system tends to have a more difficult time shedding a moderate or strong La Niña event than a weaker one. An intense La Niña tends to persist for multiple years; one even lasted for 34 consecutive months between 1954 and 1957.
A La Niña event may return the following fall season even if it weakened or disappeared during the summer, as was the case this summer, Wolter said.
His insight, detailed in his experimental forecast discussion, lies in looking at past La Niña events that, like last winter's event, had rapid onsets and were associated with very cold sea surface temperatures anomalies. Dynamical and statistical climate models are now starting to agree with Wolter, but he thinks this event will be less intense than last winter's.
"My expectation is that this winter's (La Niña) will be weaker," Wolter said. "Last winter was the third strongest event, and it will be hard to beat that; it's an opinion based purely on statistics."
A weaker event doesn't necessarily bring wetter conditions than a stronger event, however. Wolter recreated the MEI back to 1870 and found that for the 10 historical cases in which La Niña lasted at least two consecutive years, eight generated lower flows in the Colorado River Basin in the second year.
Lower flows are very likely to happen this year because record snows packed the Upper Colorado River Basin last winter.
Last winter "was the first time since 1917 that the Colorado River had a big runoff year-with more than 20 million acre-feet-in a La Niña event," Wolter said.