* Extreme La Ninas to happen once every 13 years - study
* Cooler twin of El Nino is linked to floods and droughts
By Alister Doyle
OSLO, Jan 26 (Reuters) - Extreme "La Nina" weather events that cool the Pacific Ocean and can disrupt weather worldwide will paradoxically happen almost twice as often in a warming world, an international team of scientists said on Monday.
Severe La Ninas, linked to both floods and droughts as well as more landfalls by Atlantic hurricanes, would happen on average every 13 years in the 21st century if greenhouse gas emissions keep rising, compared with once every 23 years last century, the researchers said.
"We show that greenhouse warming leads to a significant increase in the frequency of extreme La Niña events," said the report led by experts at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO).
La Nina is the opposite of the better-known El Nino weather event characterised by warmer waters in the tropical Pacific with knock-on effects that can cause billions of dollars of damage to food and water supplies around the globe.
La Ninas happen unpredictably every two to seven years, with the last extreme event, judged by a sharp cooling of Pacific surface waters, in 1998-99.
The findings, published in the journal Nature Climate Change, followed another study by lead author Wenju Cai last year that identified a link between El Nino and global warming.
"We found that extreme El Nino will double in frequency. We don't find evidence that the intensity will increase. The extreme events just occur more frequently," he told Reuters.
The study also indicated that La Ninas were likely to follow El Ninos more often, in a damaging double dose.
Writing in a comment in Nature Climate Change, Antonietta Capotondi, of the U.S. National Oceanic and Atmospheric Administration's Earth System Research Laboratory, said the findings were "plausible", but urged more study.
Capotondi said it seemed counter-intuitive that La Ninas, linked to cool waters, might happen more often in a warmer world.
Under La Nina conditions, however, heat over land in the Western Pacific including Australia and Indonesia would suck winds westwards off the ocean, in turn drawing waters from the depths and cooling the ocean surface.
Among other extremes coinciding with the 1998-99 La Nina, river floods and storms killed thousands of people in China and Bangladesh. In 1998, Hurricane Mitch killed 11,000 people in Honduras and Nicaragua. (Editing by Robin Pomeroy)
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