The Evolution of the El Niño-Southern Oscillation and Tropical Pacific Climate Across the Last Deglaciation

The El Niño – Southern Oscillation (ENSO) is the largest interannual component of Earth’s climate system, capable of exerting significant influence over global climate patterns that affect communities around the globe. Nevertheless, a comprehensive understanding of the ENSO system and its relationship to tropical Pacific climate dynamics remains unclear. Although new paleoceanographic proxies have shown promise in in their ability to constrain past ENSO change, little is known about how ENSO varied in response to millennial-scale climate events over the last 25,000 years. Here, I present new records of tropical Pacific mean state and ENSO variability over the last 25,000 years reconstructed from a high-resolution sediment core recovered from the Eastern Equatorial Pacific (EEP) cold tongue (MV1014-02-17JC, 0°10.8\u27 S, 85°52.0\u27 W, 2846 m water depth). Mean state changes are reconstructed by measuring Mg/Ca ratios in the thermocline-dwelling foraminifera Neogloboquadrina dutertrei. In addition, I performed Individual Foraminiferal Analysis (IFA) on single N. dutertrei shells from seven targeted time slices in order to estimate thermocline temperature variability under different climate regimes across the last deglaciation. I combine these results with Monte Carlo simulations of hypothetical ENSO change and robust, nonparametric statistics to reconstruct “snapshots” of past ENSO activity from the Holocene, the Last Glacial Maximum (LGM), and the abrupt climate events of the last deglaciation. Taken together, the results indicate that Modern/Late Holocene ENSO activity is more variable than at any other time in the last 25,000 years. I speculate that a combination of orbital forcing, changes in upper ocean thermal stratification, trade wind strength, and the mean position of the Intertropical Convergence Zone (ITCZ) acted to suppress past ENSO variability. These results emphasize the dynamic relationship between tropical Pacific climate and the ENSO and underscore the need for more detailed studies under variable background conditions in order to predict future ENSO evolution in an anthropogenically warming world