Characterizing seawater oxygen isotopic variability in a regional ocean modeling framework: Implications for coral proxy records

Reconstructions of the El Niño-Southern Oscillation (ENSO) are often created using the oxygen isotopic ratio in tropical coral skeletons (δ¹⁸O). However, coral δ¹⁸O can be difficult to interpret quantitatively, as it reflects changes in both temperature and the δ¹⁸O value of seawater. Small-scale (10-100 km) processes affecting local temperature and seawater δ¹⁸O are also poorly quantified and contribute an unknown amount to intercoral δ¹⁸O offsets. A new version of the Regional Ocean Modeling System capable of directly simulating seawater δ¹⁸O (isoROMS) is therefore presented to address these issues. The model is used to simulate δ¹⁸O variations over the 1979-2009 period throughout the Pacific at coarse (O(50 km)) resolution, in addition to 10 km downscaling experiments covering the central equatorial Pacific Line Islands, a preferred site for paleo-ENSO reconstruction from corals. A major impact of downscaling at the Line Islands is the ability to resolve fronts associated with tropical instability waves (TIWs), which generate large excursions in both temperature and seawater δ¹⁸O at Palmyra Atoll (5.9°N, 162.1°W). TIW-related sea surface temperature gradients are smaller at neighboring Christmas Island (1.9°N, 157.5°W), but the interaction of mesoscale features with the steep island topography nonetheless generates cross-island temperature differences of up to 1°C. These nonlinear processes alter the slope of the salinity:seawater δ¹⁸O relationship at Palmyra and Christmas, as well as affect the relation between coral δ¹⁸O and indices of ENSO variability. Consideration of the full physical oceanographic context of reef environments is therefore crucial for improving δ¹⁸O-based ENSO reconstructions.