Observing the Isotopic Expression of Weak vs. Strong El Niño: A Palmyra Atoll Case Study

Abstract:

The El Niño/Southern Oscillation (ENSO) is the dominant driver of interannual climate variability, yet its sensitivity to external changes in climate remains uncertain. Coral oxygen isotopic records (δ¹⁸O) provide some of the best records of ENSO variability prior to the instrumental era, yet these data are collected from reef environments which experience a variety of dynamical influences during the ENSO cycle. Understanding the translation between ENSO and δ¹⁸O variability has been difficult to date owing to the lack of direct observations of conditions on the reef: here we present a new, comprehensive set of physical oceanographic and geochemical observations collected at Palmyra Atoll (5.9°N, 162.1°W) over 2014-2017, covering both the weak 2014-15 and strong 2015-16 El Niño event. These observations are combined with a data assimilative experiment using the isotope-enabled Regional Ocean Modeling System (isoROMS) to provide large-scale dynamical context. During both El Niño events, seawater δ¹⁸O accounts for nearly half of the change in coral δ¹⁸O at Palmyra, due to a combination of surface flux changes and advective transport of isotopically depleted waters. The magnitude of seawater δ¹⁸O depletion is similar between the two events, but the salinity change is much larger during 2014-15; this suggests that the relationship between salinity and seawater d18O varies substantially across El Niño events. The 2014-15 and 2015-16 patterns are then compared with previous weak and strong El Niño events, showing that different types of events have distinct isotopic signatures which can provide insight into past variation in their respective behavior.