Collaborative Research: Reconstructing Climate Linkages Across the Tropical Oceans Over the Last Millennium

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Year-to-year changes in the temperatures of the tropical oceans impact climate and weather around the world. The clearest example of this is the El Niño-Southern Oscillation (ENSO) system in the tropical Pacific. Warming and cooling patterns related to ENSO have major effects on droughts, floods, and tropical storms. The temperature of the tropical Atlantic and Indian also affects weather in the same types of ways. Not only that, the three ocean basins can interact with each other since the atmosphere connects them all. For example, warming in the tropical Atlantic tends to lead to wind patterns that then cool off the tropical Pacific. This means that to predict how climate variations may change in the future, one must understand the connections between the tropical oceans. However, those relationships are known to change over long timescales (multiple decades), and temperature records are short and scarce in the tropics. As a result, this problem is difficult to study with traditional observations (e.g. thermometers). Instead, this project will produce records of temperature using the geochemistry of long-lived corals. By comparing data from corals in the Pacific, Atlantic, and Indian oceans, this research will show how these oceans have interacted over the past several centuries. Comparison of the 20th century with previous centuries will reveal the influence of recent warming. This work explores whether these patterns appear in the climate models used to understand future climate change, and what physical processes are involved. Several different model groups will be used for comparison. Students and postdocs will also participate in this research. Finally, the project will develop training materials to make data and model results more accessible to those without a highly technical background. Interannual to multidecadal variability in the tropical oceans drives climate extremes that impact natural and human systems around the world. The equatorial Pacific ENSO system is particularly important, and recent work highlights that Atlantic and Indian ocean variability impact the recurrence and intensity of ENSO extremes. Multiple mechanisms drive these interactions, which are affected by natural and anthropogenic forcing. Yet the relative importance of each mechanism remains unknown, as does the time history of the inter-basin teleconnections. Closing this knowledge gap is fundamental to understanding how inter-basin interactions are influenced by external forcing and improving the accuracy of future model projections in the tropics and beyond. This study will create new paleoclimate records of sea surface temperature (SST) using the elemental and isotopic composition of coral skeletal material, in cores from centuries-old colonies in the Tropical North Atlantic (TNA) and Western Indian Ocean (WIO). These data will be combined with published data to reconstruct pre-instrumental SST for these regions and assess inter-basin interactions. The study will also explore the relationship of these patterns to external forcing. The project will evaluate the degree to which interbasin interactions are manifest across relevant model simulations. Insights from coral proxies will be explored with the ensemble of isotope-enabled Last Millennium Ensemble simulations: the study will analyze existing iCESM output, including ‘single forcing’ experiments to diagnose individual external influences on climate and isotopic variability, and perform targeted sensitivity experiments isolating the isotopic signatures associated with particular dynamical mechanisms.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.