Twentieth Century Seawater δ 1 8 O Dynamics and Implications for Coral‐Based Climate Reconstruction

The oxygen isotopic composition of tropical coral skeletons ( δ 18 O) is a crucial source of information on past El Niño/Southern Oscillation behavior. Both temperature and the δ 18 O of the surrounding seawater ( δ 18 O sw ) affect coral δ 18 O; a linear proportionality with δ 18 O sw is often used to infer past salinity variations, but the degree to which dynamical influences on δ 18 O sw may affect that relationship is still unclear. Here we use the isotope‐enabled Regional Ocean Modeling System to investigate the dynamics of δ 18 O sw and salinity variations in different twentieth century climate regimes. The dominant modes of δ 18 O sw 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 δ 18 O sw variations during El Niño and La Niña events. These dynamics lead to alterations in the δ 18 O sw :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 δ 18 O sw excursions during El Niño. At other sites, the δ 18 O sw :salinity relationship has substantial scatter and explains relatively little δ 18 O sw variance. Strikingly, interannual variability can lead to site‐specific δ 18 O sw :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 δ 18 O sw and salinity variability.