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), characterized by sea surface temperature (SST) anomalies in the central and eastern equatorial Pacific Ocean (EEP), influences variability in climate conditions across the globe. Varying factors may exhibit influence on 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. Here, we investigate the relationship between ENSO and the eastern equatorial 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 reconstruct subsurface temperatures and compare 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. Subsurface temperatures around 1895 CE appear to be at their highest, an average of 18.9℃. The lowest subsurface temperature in our data occurs around 1587 CE, with a subsurface temperature of 15.7℃. Using an existing sea surface temperature record, we calculate the difference between surface and subsurface temperatures. Differences between surface and subsurface temperatures also show variability through the last 2000 years 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. Initial results suggest that variability in the thermocline and variability in ENSO may be related, however more data is required to fully test this hypothesis. 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.