El Nino lulls lead to harsh floods, fires and droughts: study

The world's biggest climate pattern, the swing between El Ninos and La Ninas in the Pacific, operates with a kind of memory – with periods of low activity followed by stints of extreme events.

A study, led by CSIRO scientist Wenju Cai, found the roughly 20-year relative lull in the so-called El Nino-Southern Oscillation (ENSO) from 1998 will be made up for in future decades by an increase of as much as a third in activity. For Australia, that could mean more extreme droughts, fire seasons and floods.

Two big El Ninos, forming in 1997 and 2015 in the Pacific. The climate patterns swing between periods of lower activity to more active ones, with global consequences.Credit:NASA

Rising greenhouse gases and the resulting global heating amplify this giant self-modulating system, exacerbating the extremes, the research published on Thursday in Nature found.

"If the greenhouse effect now on the El Nino [is to suppress it], in the future the response to that subsequent greenhouse warming will be much more dramatic," Dr Cai told the Herald and The Age. "The [1998-2018] hiatus will cause us more extremes in the future."

The researchers, including Agus Santoso from the University of NSW and Michael McPhaden from the US National Oceanic and Atmospheric Administration, derived their findings by applying the so-called butterfly effect on computer models to see how tiny changes in initial conditions affect eventual El Nino activity.

Those tweaks involved altering temperatures by one hundred of a trillionth of a degree to mimic the effects akin to the metaphoric flapping of a butterfly's wings.

As the Nature paper noted, El Nino and La Nina years typically have major impacts on extreme weather, ecosystems and food production.

El Nino events are marked by rainfall patterns shifting eastwards away from Australia and south-east Asia, with western parts of South America often hit by flooding. La Nina years – including possibly this one by spring, according to Australia’s Bureau of Meteorology – are the reverse, with Australia typically hit by more cyclones and heavy rain.

The self-organising system retains a "memory", Dr Cai said, with effects playing out over a century or more. "What happens now actually can affect the future. El Nino actually remembers its own past," Dr Cai said.

During El Nino years, surface winds take out more heat than usual from the tropical Pacific, lifting temperatures. Those events are often followed by a La Nina, as the system naturally reorganises itself.

The lull, playing out over most of the first two decades of the 21 century, amounted to a reduction of 13 per cent in ENSO activity, Dr Cai said. The modelling indicates that suppressed activity will be countered by a projected increase in ENSO variability by as much as 37 per cent from the 2000-19 level, the paper concludes.

While the system will eventually compensate for the suppressed activity with more intense events, "we don't know which decades" that payback will come, he said.

Dr Cai, who has published previous studies that found most extreme El Nino years will become stronger and more intense by the end of the 21st century with global heating, has also submitted a paper examining how the butterfly effect plays out in the Indian Ocean, another of Australia's main climate influences.

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