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, last week the World Meteorological Organization declared an El Niño is underway, although Australia’s Bureau of Meteorology El Niño–Southern Oscillation Outlook remains at El Niño Alert.
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 is 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.
Many scientists, like those at the United State’s National Oceanic and Atmospheric Administration, define an El Niño partly as when these sea surface temperature anomalies are 0.5 degrees Celsius above average, whereas Australia’s Bureau of Meteorology looks for 0.8 degrees Celsius above average.
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.
Dr Wenju Cai and Dr Guojian Wang are research scientists at CSIRO, an Australian government agency responsible for scientific research, and visiting scientists at Ocean University of China. Cai specialises in research into global climate variability and change, including conceptual non-linear frameworks for extreme El Niño and La Niña, and the Indian Ocean Dipole. Guo’s 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.
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