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

The El Niño Southern Oscillation (ENSO) is a climatic phenomenon characterized by anomalies in sea surface temperatures (SST) in the central and eastern equatorial Pacific Ocean (EEP), impacting climate conditions globally. Various factors, such as global background climate state, surface and subsurface conditions, and orbital changes, may influence ENSO expression. It is hypothesized that the state of the thermocline in the eastern tropical Pacific plays a significant role in shaping ENSO expression. In this study, we explore the connection between ENSO and the eastern equatorial Pacific thermocline over the past 2500 years. Thermocline conditions were reconstructed using Mg/Ca ratios in Neogloboquadrina dutertrei, a sub-surface foraminifera species from eastern equatorial Pacific sediment core KNR195-5 MC42C. Subsurface temperatures were compared with an existing sea surface temperature record from the mixed-layer species Globigerinoides ruber to generate records of the surface-subsurface gradient. Our findings reveal fluctuations in subsurface temperatures over the past 2500 years, with the highest recorded subsurface temperature occurring around 1895 CE while the lowest subsurface temperature in our data is observed around 1587 CE. Our record of the surface-subsurface gradient shows variability over the last 2000 years, exhibiting an intensified vertical gradient at approximately 1750 CE and a diminished gradient before 1500 CE. We analyze these variations within the context of tropical Pacific variability and ENSO reconstructions. Initial results suggest a potential relationship between thermocline variability and ENSO variability, with stronger ENSO activity apparent when the surface-subsurface gradient is larger, suggesting a link, but additional data is needed to comprehensively test this hypothesis. Investigating the link between thermocline changes and ENSO variability enhances our understanding of the mechanisms that control ENSO, aiding in predicting potential future ENSO expression under changing climate conditions.