Intercomparison of 20th century tropical climate model hindcasts and coral δ18O data using a forward proxy system model

When coupled to climate model (CGCM) output, forward models provide a means to assess CGCM output through direct comparison with the available proxy observations. Here we model reef coral oxygen isotopic composition (δ18O) as a function of sea-surface temperature (SST) and sea-surface salinity (SSS), with SSS acting as a linear proxy for the isotopic composition of seawater. When driven with historical SST and SSS data over the 1958-1990 period, the forward model is able to capture the spatial pattern and temporal evolution of the El Niño-Southern Oscillation (ENSO) and trend observed in a sparse observational network of 23 Indo-Pacific coral δ18O records. The SST and SSS components of the model are both necessary to capture the full magnitude of the coral trend. The δ18O trend simulated from historical SST and SSS data is equivalent to or smaller than the observed δ18O trend. For the 1890-1990 period, δ18O records simulated from AR4 CGCMs generally display greater overall interannual variance and a weaker 20th century warming/freshening trend than is observed in corals. The discrepancies between observed and CGCM-simulated δ18O trends may arise in part from unmodeled physiological controls on δ18O or from sampling-network induced biases. On the other hand, the tropical Pacific may be more sensitive to radiative changes during the 20th century than is simulated by the AR4 generation of CGCMs.