Contrasting Central Equatorial Pacific Oxygen Isotopic Signatures of the 2014/2015 and 2015/2016 El Niño Events

Paleoclimate reconstructions of El Niño/Southern Oscillation (ENSO) behavior often rely on oxygen isotopic records from tropical corals (δ¹⁸O). However, few reef-based observations of physical conditions during El Niño events exist, limiting our ability to interpret coral δ¹⁸O. Here we present physical and geochemical measurements from Palmyra Atoll (5.9°N, 162.1°W) from 2014–2017, along with a data assimilation product using the isotope-enabled Regional Ocean Modeling System (isoROMS). Coral δ¹⁸O signals are comparably strong in 2014–2015 and 2015–2016; notably, over 50% of the signal is driven by seawater δ¹⁸O, not temperature. If a constant seawater δ¹⁸O:salinity relationship were present, this would imply a comparable salinity anomaly during both events. However, salinity changes are much larger during 2014–2015, indicating a highly nonstationary relationship. isoROMS then shows that advection strongly influences δ¹⁸O during both the 2014–2015 and 2015–2016 El Niño, driving differences in the salinity/seawater δ¹⁸O relationship. This demonstrates the need for considering ocean dynamics when interpreting coral δ¹⁸O.