The El Niño-Southern Oscillation, which is characterized by irregular alternations between anomalously warm (El Niño) and cold (La Niña) conditions, was present at least 250 million years in the past, and was often of greater magnitude, according to a new modeling study.
The El Niño-Southern Oscillation, originating in the central and eastern equatorial Pacific, is a defining mode of interannual climate variability with profound impact on global climate and ecosystems. Image credit: Li et al., doi: 10.1073/pnas.2404758121.
Climate scientists study El Niño, a giant patch of unusually warm water on either side of the equator in the eastern Pacific Ocean, because it can alter the jet stream, drying out the U.S. northwest while soaking the southwest with unusual rains.
Its counterpart, the cool blob La Niña, can push the jet stream north, drying out the southwestern U.S., while also causing drought in East Africa and making the monsoon season of South Asia more intense.
“In each experiment, we see active El Niño Southern Oscillation (ENSO), and it’s almost all stronger than what we have now, some way stronger, some slightly stronger,” said Duke University’s Dr. Shineng Hu.
Dr. Hu and colleagues used the same climate modeling tool used by the Intergovernmental Panel on Climate Change (IPCC) to try to project climate change into the future, except they ran it backwards to see the deep past.
The simulation is so computationally intense that the researchers couldn’t model each year continuously from 250 million years ago. Instead they did 10-million-year ‘slices’ — 26 of them.
“The model experiments were influenced by different boundary conditions, like different land-sea distribution (with the continents in different places), different solar radiation, different carbon dioxide,” Dr. Hu said.
Each simulation ran for thousands of model years for robust results and took months to complete.
“At times in the past, the solar radiation reaching Earth was about 2% lower than it is today, but the planet-warming carbon dioxide was much more abundant, making the atmosphere and oceans way warmer than present,” Dr. Hu said.
In the Mezozoic period, 250 million years ago, South America was the middle part of the supercontinent Pangea, and the oscillation occurred in the Panthalassic Ocean to its west.
The current study shows that the two most important variables in the magnitude of the ENSO historically appear to be the thermal structure of the ocean and ‘atmospheric noise’ of ocean surface winds.
“Previous studies have focused on ocean temperatures mostly, but paid less attention to the surface winds that seem so important in this study,” Dr. Hu said.
“So part of the point of our study is that, besides ocean thermal structure, we need to pay attention to atmospheric noise as well and to understand how those winds are going to change.”
“Atmospheric noise — the winds — can act just like a random kick to this pendulum.”
“We found both factors to be important when we want to understand why the El Niño was way stronger than what we have now.”
“If we want to have a more reliable future projection, we need to understand past climates first.”
The study was published this week in the Proceedings of the National Academy of Sciences.
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Xiang Li et al. 2024. Persistently active El Niño-Southern Oscillation since the Mesozoic. PNAS 121 (45): e2404758121; doi: 10.1073/pnas.2404758121