Twentieth Century Seawater δ18O Dynamics and Implications for Coral‐Based Climate Reconstruction

Abstract

AbstractThe oxygen isotopic composition of tropical coral skeletons (δ18O) is a crucial source of information on past El Niño/Southern Oscillation behavior. Both temperature and the δ18O of the surrounding seawater (δ18Osw) affect coral δ18O; a linear proportionality with δ18Osw is often used to infer past salinity variations, but the degree to which dynamical influences on δ18Osw may affect that relationship is still unclear. Here we use the isotope‐enabled Regional Ocean Modeling System to investigate the dynamics of δ18Osw and salinity variations in different twentieth century climate regimes. The dominant modes of δ18Osw variability are the background trend and eastern/central Pacific El Niño, similar to salinity; likewise, budget analysis reveals a strong impact of ocean dynamics (both advection and vertical mixing/diffusion) on δ18Osw variations during El Niño and La Niña events. These dynamics lead to alterations in the δ18Osw:salinity relationship across the Pacific: the linear approximation is most accurate near the eastern edge of the western Pacific warm pool but with nonlinearities due to large δ18Osw excursions during El Niño. At other sites, the δ18Osw:salinity relationship has substantial scatter and explains relatively little δ18Osw variance. Strikingly, interannual variability can lead to site‐specific δ18Osw:salinity regression coefficient changes of up to 0.4‰/psu on multiyear time scales. This calls the reliability of coral‐based estimates of El Niño/Southern Oscillation extremes into question and highlights the need for increased dynamical understanding as well as sustained in situ observations of δ18Osw and salinity variability.