Cosmos
Cosmos is a quarterly science magazine. We aim to inspire curiosity in ‘The Science of Everything’ and make the world of science accessible to everyone.
By Cosmos
By Wenju Cai and Guojian Wang, CSIRO and Ocean University of China.
Melbourne: Reports of a possible ‘super El Niño’ this year might have been greatly exaggerated.
Media headlines may be saying the world could be set to face a ‘super El Niño’ this year, but in reality the likelihood of that is low.
After a triple-dip La Niña, in early July the World Meteorological Organization declared an El Niño is underway, and Australia’s Bureau of Meteorology followed suit this week.
Looking to history as a guide, this makes the likelihood of a so-called ‘super El Niño’ this year low. What scientists refer to as extreme El Niños such as the 1997 and 2015 events tend not to follow consecutive La Niña events.
Since 1950, there have been five three-year La Niña events — 2020-2022, 1998-2000, 1983-1985, 1973-1975 and 1954-1956.
None of them were followed by an extreme El Niño, and only one was followed by a strong El Niño in 1957.
All of this may be bad news for newspaper headline writers, but it’s probably better for the rest of us.
During the neutral phase of the El Niño–Southern Oscillation, westward-blowing trade winds pile up warm sea surface temperature water in the western Pacific Ocean, and drive the upwelling of cold, subsurface water in the east along the equator and off the west coast of South America, forming a cold tongue extending to central equatorial Pacific.
Warm and moist air masses rise high into the atmosphere — referred to as atmospheric deep convection — over this western Pacific warm pool, producing rainbands or convergence zones over the western Pacific.
But during an El Niño, these trade winds weaken and warm sea surface temperature anomalies develop in the eastern equatorial Pacific.
El Niños can vary in strength, from weaker ones with generally small effects, to strong ones. The strongest of them are sometimes called extreme El Niños. The last extreme El Niño was in 2015.
Scientifically, when an extreme El Niño occurs, the equatorial eastern Pacific anomalies are particularly large, meaning the sea surface temperature of the water is much warmer than normal.
All the convergence zones congregate in the equatorial eastern Pacific too, generating a massive reorganisation of the atmospheric circulation — for example, the centre of the western Pacific convection moves approximately 18,000km to the east.
This reorganisation also leads to devastating extreme weather events, like severe thunderstorms and tropical cyclones. For example, during the 1997 El Niño, extreme tropical cyclones killed many in the Cook Islands.
During past extreme El Niño events, severe droughts and wildfires have occurred in western Pacific regions, including Australia such as the Ash Wednesday bushfires in February 1983. There have also been catastrophic floods in the eastern equatorial region of Ecuador and northern Peru.
The South Pacific convergence zone shifted equatorward, spurring floods and droughts in south Pacific Island countries and inducing extreme cyclones to regions normally not affected by such events.
Other impacts have included floods in the southwest US, the disappearance of marine life, and the decimation of the native bird population in the Galapagos Islands because of absence of upwelling water that otherwise brings nutrients to the surface.
The associated global economic losses of such events amounts to several trillions of dollars each time.
The intensity of an El Niño event can be measured in two ways.
One definition uses the equatorial eastern Pacific sea surface temperature anomalies.
However not everyone uses this definition in the same way. Some people define an extreme El Niño as when the sea surface temperature is at least 2 or 2.5 degrees Celsius warmer than average.
A stricter definition of an extreme El Niño is when the average of the warm anomalies over the peak season of December, January and February is within the top five percent of all seasonal average anomalies or more than two standard deviations of the long-term time series.
The 1982, 1997, and 2015 El Niño events meet this definition, all with peak sea surface temperature anomaly averages between 2 to 3 degrees Celsius.
Whereas the strong El Niño event in 1957 was only in the top 20 to top five percent of seasonal average anomalies.
Another way of measuring the intensity of an El Niño event is to use the rainfall over the equatorial eastern Pacific. This reflects the impact arising from the shift of atmospheric convection and convergence zones to the equatorial eastern Pacific.
An extreme El Niño defined this way is when the equatorial eastern Pacific rainfall averaged over the peak season of December, January and February is more than 5mm per day.
The eastern equatorial Pacific is normally cold and dry, receiving approximately 1mm of rainfall per day. But the arrival of the convergence zones brings dramatic increases in rainfall, as much as 11mm per day during the 1997 extreme El Niño event.
The BoM calls it but it’s not a super El Niño
One advantage of the rainfall definition is that it helps scientists assess how extreme El Niños may change under global warming.
For example, under global warming, the equatorial Pacific and particularly the eastern equatorial Pacific, is projected to warm faster than regions away from the equator.
The warming differential means that it is easier for the atmospheric convection centre and convergence zones to shift to the eastern equatorial Pacific.
Although this year is unlikely to be an extreme El Niño year, other research shows that rare extreme El Niño events are projected to occur more often under global warming.
Steps taken to limit greenhouse gas emissions will help stabilise El Niño-Southern Oscillation-associated economic and social risks in the centuries ahead.
Dr Wenju Cai is a Chief Research Scientist at CSIRO and a visiting scientist at Ocean University of China. He specialises in research into global climate variability and change, including conceptual nonlinear frameworks for extreme El Niño and La Niña, and the Indian Ocean Dipole.
Dr Guojian Wang is a senior research scientist at CSIRO and a visiting scientist at Ocean University of China. His research interests include the mechanisms of extreme El Niño–Southern Oscillation and extreme positive Indian Ocean Dipole, their impacts on regional and global climate, and their response to greenhouse warming.
The authors’ research referred to in this article was supported by the Earth Systems and Climate Change Hub of the Australian Government’s National Environmental Science Program.
Originally published under Creative Commons by 360info™.