Developing a Coral Proxy System Model to Compare Coral and Climate Model Estimates of Changes in Paleo‐ENSO Variability.

Coral records of surface‐ocean conditions extend our knowledge of interannual El Niño–Southern Oscillation (ENSO) variability into the preinstrumental period. That said, the wide range of natural variability within the climate system as well as multiple sources of uncertainties inherent to the coral archive produce challenges for the paleoclimate community to detect forced changes in ENSO using coral geochemical records. We present a new coral proxy system model (PSM) of intermediate complexity, geared toward the evaluation of changes in interannual variance. Our coral PSM adds additional layers of complexity to previously published transfer functions of sensor models that describe how the archive responds to sea surface temperature (SST) and salinity. We use SST and salinity output from the Community Earth System Model Last Millennium Ensemble 850 control to model coral oxygen isotopic ratios and SST derived from Sr/Ca. We present a detailed analysis of our PSM using climate model output for sites in the central and southwest Pacific before extending the analyses to span the broader tropical Pacific. We demonstrate how variable growth rates, analytical and calibration errors, and age model assumptions systematically impact estimates of interannual variance and show that the relative magnitude of the change in interannual variance is location dependent. Importantly, however, we find that even with the added uncertainties in our PSM, corals from many circum‐Pacific locations are broadly able to capture decadal and longer (decadal+) changes in ENSO variability. Our code is publicly available on GitHub to facilitate future comparisons between model output and coral proxy data. Plain Language Summary: Climate scientists use the chemistry of coral skeletons to study past tropical climate conditions. The elemental ratio of strontium to calcium (Sr/Ca) and the oxygen isotopic composition (δ18O) in the coral skeleton are used to reconstruct past sea surface temperature and salinity. Coral Sr/Ca varies in response to changes in sea surface temperature, whereas coral δ18O records both changes in temperature and salinity. Individual corals provide tens to hundreds of years of climate information from the tropical oceans. They are well‐suited for studying variability related to the El Niño–Southern Oscillation (ENSO), a climate phenomenon that impacts global temperature and rainfall patterns every few years. We rely on both climate proxy data and simulations from global climate models to study changes in ENSO variability in the past. Nevertheless, it is difficult to directly compare proxy data with climate model output due to the imperfect nature of how the climate signal is recorded in the coral skeleton. Proxy system models are a tool designed to help bridge the gap between climate information recorded in corals and climate model output. In this study, we develop a coral proxy system model to demonstrate how different processes impact a coral's ability to record changes in ENSO variability. Key Points: We present a new coral proxy system model to facilitate comparison between proxy observations and climate model outputAnalytical and calibration errors, variable growth rates, and age modeling uncertainties all have measurable impacts on interannual varianceThe relative importance of different uncertainties on interannual variance are site dependent [ABSTRACT FROM AUTHOR]