Human-caused climate change will double the frequency of La Niñas by the end of the century, resulting in floods, droughts and other extreme weather events, finds a new study based on climate modeling.

La Niñas, which are the cooler cousins of El Niños, are weather patterns associated with a temperature drop in the central Pacific Ocean. In the past, extreme La Niñas have caused droughts in the southwestern United States, flash flooding in Venezuela, flooding in China and the deaths of thousands of people.

Research by Wenju Cai, an atmospheric scientist with the Commonwealth Scientific and Industrial Research Organisation in Australia, and his colleagues suggests that extreme La Niñas would occur once every 13 years with climate change in 2100, an increase from the once-in-every-23-years frequency seen in the 1990s. The study was published in Nature Climate Change.

Three-quarters of the events would be preceded by an extreme El Niño, the scientists found.

Studies that project the occurrence of La Niñas are critical for the southwestern United States, said Park Williams, an assistant research professor at the Lamont-Doherty Earth Observatory of Columbia University, via email. He was not affiliated with the scientists of the present study.

These regions have been suffering from drought conditions nearly continuously for the past 15 years, he said.

"If the La Niña events are really going to become increasingly strong, and this trend is to be super-imposed upon a greenhouse warming trend, then this would represent a real double-[whammy] on southwestern North America," Williams said. "Not only is the trend toward warmer, but the dry years will be much drier."

The study is the third in a series by the scientists who are trying to tease out global warming impacts on individual extreme weather phenomena. Cai and his colleagues have previously found that global warming would make extreme El Niños more frequent, as well as Indian Ocean Dipoles, which are El Niño-like weather patterns that play out in the Indian Ocean (ClimateWire, Jun. 12, 2014).

Alternating deluges and droughts
To arrive at the results, the scientists used 21 climate models to simulate La Niñas and assumed that nations would continue emitting carbon dioxide at present-day levels.

As many as 17 of the 21 models found that La Niñas would become more frequent. They aggregated the results to assign a frequency of occurrence.

The finding surprised Cai. Most scientists had previously assumed that extreme La Niñas would decrease in frequency with climate change, he said.

Now, it appears extreme El Niños and La Niñas would both become more frequent than in the 1990s. This means an alternating regime of huge droughts followed by huge floods in a nation like Australia, which is the driest continent on Earth. The flip-flop in rainfall was seen in 1987 and 1988 in Australia, with disastrous consequences, Cai said.

"Either way, you suffer," he said.

Cai would like to hone his results by using a single climate model rather than aggregating the results of multiple models. This approach would allow the scientists to factor in the impact of internal variability on the occurrence of La Niñas, which is missing in the present study.

The approach used by the scientists does have its drawbacks. Climate models do not fully represent complex phenomena playing out in the Pacific and Indian oceans, such as La Niñas, which can make untangling natural and human-caused trends a challenge.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500