Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO
Water isotope-enabled coupled atmosphere–ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ 18O coral) variability arising from sea surface temperature (SST) and the isotopic composition of seawater (δ 18O sw). The unforced behaviour of the isotope-enabled HadCM3 coupled general circulation model suggests that the extent to which inter-annual δ 18O sw variability contributes to that in model δ 18O coral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ 18O coral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ 18O sw variability is correlated to that in SST, meaning that local calibrations of the effective local δ 18O coral–SST relationships are likely to be essential. Furthermore, the relationship between δ 18O sw and SST can be non-linear, such that the model interpretation of central and western equatorial Pacific δ 18O coral in the context of a linear dependence on SST alone leads to overestimation (by up to 20%) of the SST anomalies associated with large El Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ 18O sw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ 18O sw relationship.