Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO

Abstract. Water isotope-enabled coupled atmosphere–ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ¹⁸O_coral) variability arising from sea surface temperature (SST) and the isotopic composition of seawater (δ¹⁸O_sw). The unforced behaviour of the isotope-enabled HadCM3 coupled general circulation model suggests that the extent to which inter-annual δ¹⁸O_sw variability contributes to that in model δ¹⁸O_coral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ¹⁸O_coral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ¹⁸O_sw variability is correlated to that in SST, meaning that local calibrations of the effective local δ¹⁸O_coral–SST relationships are likely to be essential. Furthermore, the relationship between δ¹⁸O_sw and SST can be non-linear, such that the model interpretation of central and western equatorial Pacific δ¹⁸O_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 δ¹⁸O_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–δ¹⁸O_sw relationship.