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.

Methods
Physical oceanographic measurements were performed via regular (1 per 6 months) deployments at 5 different sites off the coast of Palmyra. At each site, Seabird SBE37s were installed for the measurement of temperature and salinity. However, biofouling and other instrument failures interfered with the continuous collection of salinity data. In deployments 2015 and later, Seabird SBE56s were deployed at all sites for additional temperature measurements. Aquadopp current meters were deployed at all locations as well, in addition to temperature, these instruments collected data on 3D current velocities at 5m vertical resolution. Aquadopp current data was averaged over the 25m closest to the bottom at each site to minimize issues from near-surface scattering. Daily averages were computed for each velocity component, and data trimmed to eliminate artifacts associated with deployment/retrieval. In situ data were inspected visually, and discontinuities related to noise, deployment/retrieval, and instrument failure manually removed. In the case of salinity, discontinuities related to biofouling were identified through visual inspection, and subjected to a requirement of a temporal derivative less than 0.5 ppt/sample. In ambiguous cases, matching with TAO buoy or satellite information was performed to identify corrupted portions of the time series.

Collection of rain and seawater samples was carried out by the science team during deployment visits, and by Nature Conservancy staff between visits. Seawater samples were collected at the locations of each instrument during deployment and retrieval, using manual bucket sampling from the surface ocean extending as deep as possible (roughly 1-2 feet) to minimize influences from the `skin' layer. Since boat visits to each site were not possible on a continuous basis, weekly sampling by the TNC staff was performed from shore. This was done in three locations: within the main atoll lagoon (the Ripple Wharf, or RW1), on the northern shore of the atoll directly south of the FR7 site (`FR7 Shore'), and on the northwestern shore of the atoll (`Strawn1'). For the two shore locations, samples were obtained by wading out past the breakers to ensure open ocean conditions were being sampled, and again manually retrieving waters from the upper 1-2 feet. Rainwater sampling was performed using a rain collector installed at the existing Palmyra weather station. Rain amounts were recorded every day, and samples collected daily on days where precipitation occurred. After collection, the seawater sample bottles were sealed using Parafilm and rainwater sealed by crimping the top of the bottle to prevent post-collection fractionation, and samples were stored in atoll-based climate-controlled laboratory facilities until shipment to Georgia Tech for processing.