Expanded Composite Coral Indices of South Pacific Convergence Zone Oceanographic Variability Since 1848 CE

Abstract In the South Pacific Convergence Zone (SPCZ), interannual to interdecadal oceanic and atmospheric variability is especially pronounced. The El Niño Southern Oscillation and the Interdecadal Pacific Oscillation significantly influence the SPCZ diagonal axis and salinity front. Regional coral‐based paleo‐environmental reconstructions extend the relatively short and discontinuous instrumental sea surface temperature (SST) and sea surface salinity (SSS) record to elucidate past variability. We present monthly resolved, composite indices of coral skeletal δ 18 O, Sr/Ca, and calculated δ 18 O sw utilizing coral geochemical time‐series from Rotuma, Fiji, Tonga, and Rarotonga, dubbed the SPCZ coral (SPCZ c ) indices. The new indices build upon previous efforts to describe variability and trends in the SPCZ region with expanded coverage due to a new northwestern coral addition from the SPCZ fresh pool. The increased spatial and temporal resolution of the new indices allows for higher‐fidelity sub‐annual reconstructions of SST and SSS in the region dating back to 1848. The results confirm the secular warming trend of 1°C in the SPCZ region and show a 0.4 S p freshening starting in the 1880s. The SPCZ c δ 18 O sw is the first regional reconstruction of δ 18 O sw and provides valuable insights into past SSS variability, including elucidating responses to El Niño and La Niña events, as well as identifying past SPCZ zonal events. The SPCZ c δ 18 O sw reconstruction extends the instrumental records by ∼100 years. The SPCZ c indices prove the utility of the compositing approach in describing regional oceanographic variability with the increased signal‐to‐noise ratio of the obtained coral climatic data.