EXPLORING THERMOCLINE INFLUENCE ON ENSO BY RECONSTRUCTING SUBSURFACE CONDITIONS IN THE EEP OVER THE PAST 2500 YEARS USING TRACE ELEMENT ANALYSIS

The El Niño Southern Oscillation (ENSO) influences variability in climate conditions across the globe. It is characterized by sea surface temperature (SST) anomalies in the central and eastern equatorial Pacific Ocean (EEP). Various factors may influence ENSO expression, including global climate, surface conditions, subsurface conditions, or orbital change. It has been hypothesized that the conditions of the eastern tropical Pacific thermocline exert a major influence on ENSO expression. In this project, we investigate the relationship between ENSO and the eastern Pacific thermocline over the past ~2500 years. Thermocline conditions were reconstructed using Mg/Ca ratios in the sub-surface dwelling foraminifera species Neogloboquadrina dutertrei found in an eastern equatorial Pacific sediment core. We analyzed Mg/Ca-inferred temperatures over time to determine how thermocline conditions changed and how these changes may relate to ENSO by comparing our data with an existing sea surface temperature record from the mixed-layer species Globigerinoides ruber. We find variations in subsurface temperatures over the past 2500 years. Differences between surface and subsurface temperatures also show variability with an enhanced vertical gradient at ~1750 CE, and a reduced gradient in the interval prior to 1500 CE. We examine these changes in the context of tropical Pacific variability and ENSO. By determining how thermocline change is related to ENSO variability, we can gain a deeper understanding of its mechanisms and better project potential future expressions of ENSO.