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. Plain Language Summary Coral reefs record past El Niño events through changes to their oxygen isotopic composition, but since very few observations exist of local reef conditions during El Niño, the exact effects of El Niño on local δ18O are not well known. We measured oceanographic conditions at Palmyra Atoll during two El Niño events, along with the isotopic compositions of seawater, rainwater, and corals themselves. Salinity changes were much stronger during the smaller 2014–2015 El Niño, but the coral and seawater δ18O changes were comparable—similar coral δ18O changes came about for different reasons during these two events. Ocean dynamics appears to be the dominant factor in coral and seawater δ18O anomalies. This indicates that properly interpreting results from coral δ18O records requires considering how ocean circulation has varied in the past. Key Points A new set of physical oceanographic and oxygen isotopic observations was collected at Palmyra Atoll, spanning two El Niño events Palmyra experienced stronger seawater δ18O changes during the weak 2014–2015 El Niño than during the strong 2015–2016 El Niño Oceanic dynamics strongly affect coral and seawater δ18O, and must be considered when interpreting coral‐based reconstructions