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NatureScience

La Niña Set to Return After Brief Hiatus

Many of the leading climate services are predicting that La Niña return this winter, after a brief hiatus.

La Niña, one of the two El Niño Southern Oscillation phases, played a massive part in creating a record-dry winter in the Southwest of America in 2010, and now mounting evidence from multiple quarters suggests that 2011 will see her return, increasing the likelihood that drought conditions in the Southwest will continue and, possibly, deepen.

Less than 2 percent of the October-July periods since 1895 have been drier than they are currently for all of Texas and many parts of New Mexico. These areas experienced either their driest or second driest October-July periods in the last 117 years. Less than 6 percent of the October-July periods have been drier than current conditions in southeastern Arizona. (Source: Western Regional)

A double La Niña is not unheard of, but there is good news in that, statistically, it should be weaker than its predecessor.

“My expectation is that this winter’s (La Niña) will be weaker,” said Klaus Wolter, research scientist at the Climate Diagnostics Center at the University of Colorado. “Last winter was the third strongest event, and it will be hard to beat that; it’s an opinion based purely on statistics.”

Sadly, a weaker La Niña does not necessarily make it a wetter La Niña. 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.

The NOAA-Climate Prediction Center, or CPC, assigns less than a 3 percent chance that moisture in the upcoming four months will be sufficient to erase drought conditions in southern Arizona and New Mexico where drought conditions are most severe.

“The bigger the droughts are, the longer they last,” said Wolter. “I think when you have a big drought it can perpetuate itself.”

The CPC issued a La Niña Watch in July, which indicated favourable conditions for the development of another La Niña in the following six months, noting that the colder-than-average waters once again upwelling in the tropical Pacific Ocean seem to indicate La Niña is reforming.

“Temperatures below the sea surface have decreased quite markedly in the last few months,” said David Unger, meteorologist at the CPC.

Additionally,  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 reliable in predicting La Niña and El Niño over the past few years, and is again predicting that a La Niña is likely for 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.

Source: The University of Arizona




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