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Study reveals how El Nino responds to external forcing factors

The El Nino/Southern Oscillation, known as ELSO, is the world's largest display of annual climate variability, and some experts believe the unusual weather events it brings could intensify under climate change.

Predicting how that might play out requires a fuller understanding of how external forces affect the system.

A new study in the journal Science Advances offers some clues.

"Our study highlights that in fact we do need to go back to get a more clear picture of how the ENSO system is going to change in the future," said co-author Kaustubh Thirumalai, an assistant professor in the UA Department of Geosciences. "The further we go back to different global climatic states when the Earth was different, that is going to give us a better handle on the sensitivity of El Nino to climate forcing."

ENSO is a large system driven by even larger forces, some of which are external, such as orbital variations or greenhouse gasses.

“We are in the driver's seat in terms of changing the external forcing situation for the future with anthropogenic carbon dioxide emissions. And so we really need to get a better handle on how this system changes to external forcing,” said Thirumalai.

To better fathom those relationships, researchers compared climate models to oxygen isotope ratios in corals from the Line Islands, located in the central equatorial Pacific Ocean. Corals are a proven proxy for estimating ancient climate conditions.

"Unfortunately, at present, we do not have observations of the future. So the only way to try and understand where we're going to be headed into is to use climate models and to use these numerical simulations of climate," said Thirumalai.

The results show ENSO intensifying over recent, post-ice-age millennia, possibly driven by precession, the "wobble" in Earth's rotation. 

"There is a trend over the last 6,000 years and particularly over the Holocene that the models and the data are in agreement," said Thirumalai.

ENSO is a coupled interaction in the tropics between unusually warm ocean conditions along the west coast of South America and an atmospheric pressure fluctuation over the Indo-Pacific region. 

Like a spinning top, the Earth gradually changes the direction its axes are facing. Over thousands of years, its poles point to  different stars. This realignment alters the seasonal solar radiation reaching various latitudes, including those that help drive ENSO. 

Those climate and oceanic changes are captured in corals, whose calcite skeletons retain a telltale ratio of oxygen 18 versus oxygen 16. The heavier 018 evaporates less readily than lighter 016 and condenses faster as temperatures cool, so a relative abundance of 018 in the tropics signals cooler conditions when the corals formed. 

More research is needed to clarify these interactions, and Thirumalai hopes to use other proxy evidence to double the time scale covered by this study.  

"We can push it back into the deglaciation. Some of my colleagues on this paper are doing that. They're funded for it by NSF. It's very exciting," he said.

Nicholas Gerbis was a senior field correspondent for KJZZ’s Arizona Science Desk from 2016 to 2024.