DRY, damaging extreme El Niño weather events will be twice as common in a warming world.
Australia is set to suffer at least twice as many extreme El Niño weather events by 2050, even if international efforts to limiting the rise in global mean temperature to 1.5° Celsius are successful.
That’s the unhappy news from new CSIRO research, published today in Nature Climate Change, which projected the the number of extreme El Niño events would grow from four every 100 years to 10 events every 100 years.
The 2015 Paris climate agreement set a warming reduction target for 195 United Nations signatory countries (except the US, which withdrew this year). It aimed to curb carbon emissions to a level where the rise in mean global temperature are restricted to 1.5°C.
But even if efforts to stabilise warming are successful, and the temperature is stabilised at 1.5°C above the current average, extreme El Niños will continue to grow in frequency, reaching 14 every 100 years by 2150.
Long term drying, aggravated by extreme El Niños, will make droughts more intense, last longer and be harder to break
Model of confidence
Director of the Centre for Southern Hemisphere Oceans Research and report co-author Dr Wenju Cai said the projections were made aggregated over a combination of 13 climate models.
“Looking at the data using a multi-model interpretation, our confidence in the projected (El Niño frequency) exceeds 99 per cent confidence,” Dr Cai said.
The strength of future El Niños is not expected to increase, but Dr Cai warned their increased frequency would compound with global warming’s drying effect on eastern and southern Australia.
“There is no evidence that their intensity will get stronger, just more often,” he said.
“But there will be a long term decline in rainfall for eastern Australia, an aggregate drying, and extreme El Niños which will make the impact worse.”
Dr Cai said there is an established process that warming will have a drying effect on southern Australia, reducing precipitation from September to November, and shifting rain systems towards the pole, making them more likely to miss the continent.
“That trend of long term drying, aggravated by extreme El Niños, will make droughts more intense, last longer and be harder to break.”
Northern Australia, with periodic heavy rain at times, is “harder to model”, Dr Cai said.
“More frequent El Niños will have a drying effect, but when it rains in a warming world, it rains harder, because a hotter atmosphere can hold more water.”
Dr Cai said the research results were unexpected.
“It was very surprising. We had thought that when the global mean temperature stabilised, the risk of climate change would stabilise too. That’s because extreme climate events have to be driven by temperature increases.”
But unfortunately, extreme El Niño events will continue to grow after the temperature levels out.
“We thought after emissions were stabilised the eastern equatorial Pacific Ocean would not warm as fast as the western Pacific. But it turns out (the eastern Pacific) will continue to warm faster, because after stabilisation of emission, the winds tend to intensify and blow more heat into eastern Pacific.
CSIRO’s results indicate that warming to 1.5°C will not increase the frequency of La Niña events, which are the converse of an El Niños where cooler seas on the eastern Pacific bring increased rain to eastern Australia.
El Niño explained
El Niños bring hot, dry air to eastern Australia and typically cause lower rainfall across eastern and northern Australia. Other countries can also suffer devastating impacts, such as weak monsoon in India and drought in China.
In Australia, El Niños are associated with higher day-time temperatures, more clear nights and increased frost risk, fewer tropical cyclones and a higher fire danger in the south east.
El Niño forms when the Pacific Ocean weather cycle that drives the easterly trade winds breaks down. This occurs when the eastern Pacific, along the west coast of south America, heats up and winds that usually blow cooling surface water from the chilly Peruvian coastline don’t pack their usual punch.
Under neutral conditions, in non-El Niño years, the sea surface temperature in the eastern Pacific is tempered by cold water upwelling to the surface. This cycle replaces the sun-heated surface water and fuels the trade winds, which blow into the central Pacific and toward Australia.
But as the trade winds break up, the central and eastern Pacific heats up and high pressure systems form over eastern Australia bringing hot, dry air.
Meanwhile, evaporation rises in the warmer central and eastern Pacific bringing more rain to the west coast of South America.
Classification of Extreme El Niño events depends on how far the rainband is pushed eastwards across towards South America. In extreme cases it moves more than 15,000 kilometres to the east.
The Bureau of Meteorology advises that El Niño’s shift on rainfall, away from the western Pacific, usually reduces winter–spring rainfall, particularly in eastern and northern Australia.
Nine of the ten driest winter–spring periods occurred during El Niño years.
In the Murray–Darling Basin, winter–spring rainfall averaged over all El Niño events since 1900 was 28% lower than the long-term average, with the severe droughts of 1982, 1994, 2002 and 2006 all associated with El Niño.