Quantifying errors in coral-based ENSO estimates: toward improved forward modeling of d18O

The oxygen isotopic ratio (<em>δ</em>¹⁸O) in tropical Pacific coral skeletons reflects past El Niño–Southern Oscillation (ENSO) variability, but the <em>δ</em>¹⁸O-ENSO relationship is poorly quantified. Uncertainties arise when constructing <em>δ</em>¹⁸O data sets, combining records from different sites, and converting between <em>δ</em>¹⁸O and sea surface temperature (SST) and salinity (SSS). Here we use seasonally resolved <em>δ</em>¹⁸O from 1958 to 1985 at 15 tropical Pacific sites to estimate these errors and evaluate possible improvements. Observational uncertainties from Kiritimati, New Caledonia, and Rarotonga are 0.12–0.14‰, leading to errors of 8–25% on the typical <em>δ</em>¹⁸O variance. Multicoral syntheses using five to seven sites capture the principal components (PCs) well, but site selection dramatically influences ENSO spatial structure: Using sites in the eastern Pacific, western Pacific warm pool, and South Pacific Convergence Zone (SPCZ) captures “eastern Pacific-type” variability, while “Central Pacific-type” events are best observed by combining sites in the warm pool and SPCZ. The major obstacle to quantitative ENSO estimation is the <em>δ</em>¹⁸O/climate conversion, demonstrated by the large errors on both <em>δ</em>¹⁸O variance and the amplitude of the first principal component resulting from the use of commonly employed bivariate formulae to relate SST and SSS to <em>δ</em>¹⁸O. Errors likely arise from either the instrumental data used for pseudoproxy calibration or influences from other processes (<em>δ</em>¹⁸O advection/atmospheric fractionation, etc.). At some sites, modeling seasonal changes to these influences reduces conversion errors by up to 20%. This indicates that understanding of past ENSO dynamics using coral <em>δ</em>¹⁸O could be greatly advanced by improving <em>δ</em>¹⁸O forward models.