Local physical oceanographic and oxygen isotopic observations at Palmyra Atoll of the 2014-15 and 2015-16 El Nino events

Paleoclimate reconstructions of El Nino/Southern Oscillation (ENSO)
behavior often relies on oxygen isotopic records from tropical
corals (δ18O). However, few reef-based observations of physical conditions
during El Nino 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 2014-17, along with a
data assimilation product using the isotope-enabled Regional
Ocean Modeling System (isoROMS). Coral δ18O signals
are comparably strong in 2014-15 and 2015-16; notably, over 50%
of the signal is driven by seawater δ18O, not
temperature. If a constant seawater δ18O: salinity was present,
this would imply a comparable salinity anomaly during both
events. However, salinity changes are much larger during 2014-15,
indicating a highly nonstationary relationship. isoROMS then
shows that advection strongly influences δ18O during
both the 2014-15 and 2015-16 El Nino, driving differences in the
salinity/seawater δ18O relationship. This demonstrates
the need for considering ocean dynamics when interpreting
coral δ18O.