Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool.

Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18O c) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18O pseudo) were calculated from gridded instrumental observations and compared to δ18O c , followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18O c was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18O sw), a proxy for SSS. The Sr/Ca, and δ18O sw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18O sw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOL SST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18O sw anomalies from both sites produces an SSS index (KOSWOL SSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOL SST and KOSWOL SSS , are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age. • Sr/Ca and δ18O sw from the WPWP corals show significant calibrations to SST and SSS. • The calibrations show significant relationships at multi-timescales (dry, wet, annual). • Combining dual-site SST and SSS reconstructions record spatiotemporal variability. • The zonal SST and SSS gradients record past fingerprints of ENSO. [ABSTRACT FROM AUTHOR]