What's Up In Climate Change? Risk of extreme La Niña events may double due to global warming
Credit: NASA's Jet Propulsion Laboratory
Friday, January 30, 2015, 7:27 PM - As the planet continues to warm due to the release of greenhouse gases, scientists are finding many unforeseen consequences of all the added energy in the Earth's weather and climate system. According to the latest research, one such consequence could be a two-fold increase in the number of extreme La Niña events.
ENSO, the El Niño Southern Oscillation, is a back-and-forth shifting of water and heat in the equatorial Pacific Ocean, marked by two different states - El Niño and La Niña. During normal or 'neutral' ENSO conditions, strong air currents blow along the surface of the ocean in a westerly direction, pushing the surface waters and the heat they contain from regions west of equatorial South America, all the way to Australia and southeast Asia. During an El Niño event, these persistent air currents break down into several smaller 'cells' and without the strong friction pushing that water westward, it sloshes back to the east. This flux of heat into the eastern equatorial Pacific can have a big influence on weather patterns over the Americas, especially since the shift tends to take place during the northern winter. The reverse of this is the La Nina, when the westerly winds kick it up a notch and push even more warm water, producing a wide swath of cold surface water in the eastern Pacific.
Every once in awhile, both El Niños and La Niñas can turn into super-sized versions of themselves, with consequences on weather patterns even more extreme than usual. As a result of the El Niño that developed in 1997-98, which became a 'super El Niño', 1998 was recorded as the hottest year on record, until it was surpassed by 2005 (and 2010, and now 2014). The 'super La Niña' that developed in the wake of the 1998 El Niño has been linked to several extreme weather events.
"The 1998 extreme La Niña, the strongest in the 20th century, caused disastrous extreme weather in many parts of the world," Dr. Wenju Cai, lead author of the new research and ocean climate researcher at CSIRO (the Commonwealth Scientific and Industrial Research Organisation) in Australia, said in a press release.
"South-western United States experienced severe drought, Bangladesh experienced one of the most destructive flooding events in modern history, with over 50 per cent of the country's land flooded, and in China floods and storms displaced over 200 million people," Dr. Cai added. "The 1998 North Atlantic hurricane season saw one of the deadliest and strongest hurricanes (Mitch) in the historical record, claiming more than 11,000 lives."
A similar extreme La Niña repeated in 2010, resulting in the sea surface temperature pattern shown to the right.
In a new study, published in the journal Nature Climate Change, Dr. Cai and a team of international colleagues ran state of the art climate models to determine how the pattern of La Niña events would be impacted by global warming, and found that extreme La Niñas showed up nearly twice as often as they have in the past.
"The frequency of extreme La Niña events is projected to increase from one event in 23 years over the 1900-1999 period to one event in 13 years in the 2000-2099 period," Dr. Cai said, according to CSIRO.
Study co-author Mat Collins, from the University of Exeter in the UK, added the following comments in a university press release: "Our previous research showed a doubling in frequency of extreme El Niño events, and this new study shows a similar fate for the cold phase of the cycle. It shows again how we are just beginning to understand the consequences of global warming."
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