El Nino Events To Become 'Stronger' And More Intense, Study Finds

Fairfax - Peter Hannam

Big El Nino events will increase in intensity and frequency as the planet warms, worsening their related extreme weather impacts, new Australian-led research has found.
The study used 17 climate models to examine how sea surface temperatures by this century's end will change assuming greenhouse gas emission increases remain on their current trajectory.

Extreme El Ninos, such as 1997-98 (left) and 2015-16, are likely to become more common in the future because of climate change. Credit: NASA

The team found a 15 per cent increase in temperature variance in a key region of the Pacific, which translates into 50 per cent more extreme El Ninos, said Dr Wenju Cai, a senior climate scientist at the CSIRO and lead author of the paper published on Thursday in Nature.
"They are stronger and more frequent," Dr Cai said, adding the likelihood of intense El Nino events as measured by sea-surface temperatures will increase from about one every 15 years now to every 10 years on average during this century.
The paper's release comes as delegates from almost 200 nations are meeting in the Polish city of Katowice to hammer out a rulebook for greenhouse gas reductions based on pledges made in the 2015 Paris Agreement.
The results add to a string of recent studies pointing to accelerating impacts of climate change, such as the loss of almost all the thick, multi-year Arctic ice, and the discovery by NASA that glaciers in a large region of eastern Antarctica have "woken up" and are melting.

Stratifying oceans
The Nature researchers described the El Ninos - and their reverse, La Ninas - as "the dominant and most consequential climate variation on Earth".
Conditions are primed in the Pacific for an El Nino, with sea-surface temperatures in the eastern equatorial Pacific at about the threshold level that such events can begin, the Bureau of Meteorology says.
All it needs is for the atmosphere to "couple" with the ocean, creating a process that reinforces the up-welling of warm waters as normally easterly tradewinds stall or reverse.
According to projections, the Pacific Ocean, and most others, become more stratified, meaning more heat and therefore energy stays near the surface rather than gets mixed to lower depths.
"Under climate change, the upper levels of the eastern equatorial Pacific [a centre of El Nino formation] are warming faster than the sub-surface waters," Dr Cai said. "In the future, this coupling of the ocean and atmosphere will be stronger."
Big El Ninos of recent decades include 1982-83, 1997-98 and 2015-16. During these events, western parts of the Americas tend to receive unusually heavy rain, while droughts and wildfires are common in western Pacific regions, including Australia.
Mass coral bleaching is also a feature in the Pacific and beyond, the paper noted.
The paper concludes: "With this projected increase, we should expect extreme weather events associated with the [eastern Pacific El Nino], with important implications for 21st century climate, extreme weather and ecosystems."

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