Title: Future increase in extreme El Niño supported by past glacial changes

Authors: Kaustubh Thirumalai

Affiliations: Dept. of Geosciences, University of Arizona, USA

Abstract: El Niño events, the warm phase of the El Niño/Southern Oscillation (ENSO) phenomenon, amplify climate variability throughout the world. Uncertain model predictions limit our ability to assess whether these climatic events could become more extreme in response to greenhouse warming. Paleoclimate records provide estimates of past changes, but it is unclear if they can constrain mechanisms underlying future predictions. Using numerical simulations, we uncover a mechanism driving consistent changes in response to past and future forcings allowing model validation against paleoclimate data. The simulated mechanism is consistent with the dynamics of observed extreme El Niño events, which develop when warm pool waters expand rapidly eastward due to strongly coupled ocean currents and winds. These coupled interactions slow down under glacial conditions due to a deeper mixed layer driven by a stronger Walker circulation. The resulting decrease in ENSO variability and suppressed genesis of extreme El Niño is supported by a series of paleoceanographic reconstructions showing reduced glacial temperature variability in key ENSO-sensitive locations. The model-data agreement for past variability, together with the consistent mechanism across climatic states, supports the prediction of more frequent extreme El Niño under greenhouse warming due to a shallower mixed layer in response to a weaker Walker circulation.

Biography: Dr. Kaustubh Thirumalai is a faculty member at the Department of Geosciences, University of Arizona. His main research interests are centered on (paleo)climate dynamics, 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 degree from the National Institute of Technology 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 65 peer-reviewed journal articles and is the recipient of the 2022 American Geophysical Union's Nanne Weber Early Career Award.