Extreme La Nina events set to increase
In Australia, La Nina is associated with flooding(Source: Conan Whitehouse/User Submitted)
Wild girl Extreme La Niña events that affect weather conditions on both sides of the Pacific will almost double in frequency as the climate warms, a new study shows.
This increase is likely to drive a corresponding increase in devastating weather conditions including floods in the Asia Pacific region, more severe and frequent west Pacific cyclones, droughts in the southwestern United States, and hurricanes in the Atlantic.
In Australia, La Niña is associated with flooding, and was linked to the Queensland floods in 2011 that left at least 38 people dead, affected about 70 towns, saw the evacuation of thousands of residents and hit the economy by about $30 billion.
The finding, by an international team including Australian researchers, is published today in Nature Climate Change.
Lead author Dr Wenju Cai, chief scientist at Australia's CSIRO Oceans and Atmosphere Flagship, says their work shows La Niña events will occur every 13 years compared with a past frequency of one every 23 years.
During typical La Niña events, the central-to-eastern equatorial Pacific is colder than normal, inhibiting formation of rain- producing clouds there, but enhancing atmospheric convection and rainfall in the western equatorial Pacific.
An extreme La Niña is defined as a cooling in the central Pacific that is greater than 1.5 degrees Celsius cooler than normal average temperatures.
This increase is driven by increased land warming relative to the ocean and an increased frequency of extreme El Niño events, Cai says.
While Cai says extreme La Niña events tend to occur after an extreme El Niño because the El Niño events counter-intuitively aid the cooling process in the central Pacific.
"In an El Niño event the heat in the upper ocean tends to release to the upper atmosphere [so that] the cooler water at the ocean's sub surface is more easily brought to the surface and [therefore it is] easier to generate cooling in the central Pacific."
The work follows on from a study by the same team last year, also published in Nature Climate Change, which showed a doubling in "super" El Niño events.
Wenju says this latest study begins to fill the gap in understanding what will happen to El Niño's counterpart La Niña.
"This issue of how La Niña/El Niño will respond to climate change has been challenging scientists for the past 20 years," he says.
To assess the potential future pathway for La Niña the team used 21 global climate models that were able to simulate extreme La Niña events.
The study covered 200 years from 1900-2005 and then 2006-2099 and used historical data in the first period and then the Intergovernmental Panel on Climate Change forecasts on greenhouse emissions growth for the latter period.
The study resulted in 2100 years of virtual climate with only four of the 21 models not predicting an increase, says Cai.
"The intermodel consensus is very strong," he says.
The team also finds approximately 75 per cent of the extreme La Niñas will occur immediately following an extreme El Niño event.
The implication of this is that weather patterns will switch between extremes of wet and dry.
Devastating impacts
Although the study does not look at the economic impact of the future changes, the authors note that in 1998 a number of extreme weather events occurred that were "in part linked to the developing 1998 -1999 La Niña event".
They point out:
- Southwestern United States experienced one of the most severe droughts in history;
- Venezuela endured flash flooding and landslides that killed 25,000 to 50,000 people;
- In China, river floods and storms led to the death of thousands and displaced more than 200 million people;
- Bangladesh experienced one of the most destructive flooding events in modern history, with more than 50 per cent of the country's land area flooded, leading to severe food shortages and the spread of waterborne epidemic diseases, affecting more than 30 million people; and
- 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 in Honduras and Nicaragua.
In an accompanying commentary, Dr Antonietta Capotondi, of the Cooperative Institute for Research in Environmental Studies, University of Colorado, says the study highlights that "events associated with devastating impacts on the world climate and economies will become more frequent, but they may shortly follow other devastating effects of somewhat opposite sign".
She says it is a warning call that "the possibility of more frequent devastating La Niña events must be seriously considered as we prepare to face the consequences of global warming".
