The Tropical Pacific Mean State, Annual Cycle and ENSO in Holocene Corals: a Multi-proxy Approach

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The El Nino-Southern Oscillation (ENSO) is the dominant mode of year-to-year climate variability on a global scale, affecting precipitation and temperature patterns across the globe every few years. As a growing number of people around the world are vulnerable to natural disasters under a changing climate, there is an urgent need for improved understanding of how ENSO is likely to change in the future. While the last decade has seen three of the largest El Nino events on record, there is substantial uncertainty regarding how ENSO will change in a warmer world, despite extensive research efforts. Observations are limited, while state-of-the-art climate models suffer from persistent errors. Paleoclimate reconstructions, which extend the short observational record, suggest that ENSO variability around 3,000-5,000 years ago was about half as large as it is today, but the reason for this reduction is unknown. ENSO is closely tied to the long-term state of tropical Pacific climate, and thus its evolution represents an important piece of the puzzle in understanding this period of Earth history. However, changes in the tropical Pacific background state are poorly constrained from the paleoclimate record during this time. This project aims to reconstruct the evolution of temperature and rainfall from the center of the equatorial Pacific on timescales ranging from seasons to millennia over the past 5,000 years. In doing so, the project will provide a way to test predictions from climate models. The project will promote the progress of science, advance science education, and facilitate public engagement in the sciences by supporting a graduate student and several undergraduate students through independent laboratory-based research projects, by supporting an early career scientist in her first role as lead PI, and by supporting two PIs who are actively engaged in science education, climate literacy, and disseminating their research results to the general public and policymakers. More specifically, the project will quantitatively reconstruct central equatorial Pacific sea surface temperature and seawater d18O variability during the past 5,000 years using paired d18O and trace element measurements in Kiritimati fossil coral. Trace element measurements will be paired with an extensive collection of Kiritimati coral d18O records to generate a set of highly- replicated and quantitative reconstructions of the seasonal cycle, ENSO extremes and long-term evolution of central equatorial Pacific climate. Through the project, these coral-based reconstructions will be compared to comprehensive climate model simulations forced with changes in Earth's precessional cycle, utilizing a forward-modeling approach to quantitatively compare the coral data to model output and contextualize the changes observed at Kiritimati to changes across the tropical Pacific and beyond. These reconstructions will yield important insight into the linkages between the background tropical Pacific climate and ENSO that will inform projections of future change. In association with these reconstructions, a coral diagenesis rating tool will be developed to promote a community-wide standard for assessment and reporting of diagenesis in fossil corals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.