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Abstract
El Niño events, the warm phase of the El Niño–Southern Oscillation (ENSO) phenomenon, amplify climate variability throughout the world. Uncertain climate model predictions limit our ability to assess whether these climatic events could become more extreme under anthropogenic greenhouse warming. Palaeoclimate records provide estimates of past changes, but it is unclear if they can constrain mechanisms underlying future predictions. Here we uncover a mechanism using numerical simulations that drives consistent changes in response to past and future forcings, allowing model validation against palaeoclimate data. The simulated mechanism is consistent with the dynamics of observed extreme El Niño events, which develop when western Pacific warm pool waters expand rapidly eastwards because of strongly coupled ocean currents and winds. These coupled interactions weaken under glacial conditions because of a deeper mixed layer driven by a stronger Walker circulation. The resulting decrease in ENSO variability and extreme El Niño occurrence is supported by a series of tropical Pacific palaeoceanographic records showing reduced glacial temperature variability within key ENSO-sensitive oceanic regions, including new data from the central equatorial Pacific. The model–data agreement on past variability, together with the consistent mechanism across climatic states, supports the prediction of a shallower mixed layer and weaker Walker circulation driving more frequent extreme El Niño genesis under greenhouse warming.
Bio
Dr. Kaustubh Thirumalai is an assistant professor at the Department of Geosciences, University of Arizona. His main research interests are centered on (paleo)climate dynamics, monsoons, ENSO, paleoceanography, and biogeochemistry. Dr. Thirumalai and his research group use a combination of carbonate geochemical measurements, climate simulations, and statistical models to help constrain climatic variability and its impacts across seasonal to million-year timescales. Originally from Bangalore, Dr. Thirumalai holds a chemical engineering B.Tech from the National Institute of Technology at Surathkal, Karnataka, after which he obtained his M.S. and Ph.D. at the University of Texas at Austin. He was then awarded a Presidential Postdoctoral Fellowship at Brown University in 2017, after which he joined the Geosciences faculty at the University of Arizona in 2019. Dr. Thirumalai has authored/co-authored more than 60 peer-reviewed journal articles and is the recipient of the 2022 American Geophysical Union's Nanne Weber Early Career Award.
