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

摘要

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 affirms 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 in the central and western equatorial Pacific is non-linear, such that the interpretation of model Δ18O_coral in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Ni?o 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.