The El Niño Southern Oscillation (ENSO) has been implicated in large-scale climate shifts of the past millennia, but paleoclimate records from the dynamically vital eastern equatorial Pacific Ocean (EEP) spanning the past millennium are sparse. This has limited our understanding of tropical Pacific dynamics, leaving questions regarding the source of ENSO variability unanswered. Here I seek to address some of these questions regarding relationship between ENSO and tropical Pacific mean state, and the relationship between the tropical Pacific and large-scale climate. Mean EEP sea surface temperature (SST) for the past millennium was reconstructed using Mg/Ca ratios in Globigerinoides ruber from a sub-centennially resolved sediment core collected near the Galápagos Islands. ENSO activity for the past ~1000 years was reconstructed using oxygen isotope ratios (δ18Oc) from individual G. ruber foraminifera from the same sediments. For the first time, a "Mid-Millennial Shift" is identified at ~1500 CE when the tropical Pacific switched from a "La Niña-like" mean state with a strong zonal SST gradient and dampened ENSO to an "El Niño-like" mean state with weak zonal gradient and amplified ENSO. The ~350-year periods preceding and following ~1500 CE represent fundamentally different ocean-atmosphere circulation states in terms of both tropical Pacific mean state and variability. The Mid-Millennial Shift coincided with the deepest Little Ice Age cooling and a southward shift of the Intertropical Convergence Zone. This research provides context to the ENSO modulations of the past millennium by extending the SST record back to ~2700 years before present. This record reveals a long-term cooling trend of -0.22 ℃/ky, similar to Northern Hemisphere temperature trends suggesting a common origin, likely insolation forcing. Warm SST, within error of modern, is observed during the peak Medieval Climate Anomaly (~900-1150 CE), contradicting the prevailing La Niña-like paradigm. Much of the past millennium is characterized by an out-of-phase EEP, which is attributed to dynamical adjustments consistent with the "dynamical ocean thermostat" mechanism. Reconstruction of the zonal SST gradient from existing western Pacific SST records supports this hypothesis. I propose that the long-term pattern of EEP SST is a response to high-latitude forcing, and propose a mechanism for the communication of this signal via the shallow overturning circulation. A combination of dynamical and thermodynamic mechanisms is invoked to explain the region's complex SST history. The overall record suggests ENSO modulations are coupled to tropical Pacific mean state, and this paired system participates in extra-tropical climate processes.

Additional exploration of the foraminiferal record reveals a decrease of 0.3‰ in the stable carbon isotope ratios (δ13C) of modern individual G. ruber compared to pre-industrial specimens. I attribute this to the δ13C-Suess effect, a result of the emission of isotopically depleted carbon into the atmosphere from fossil fuels. Based on changes in the δ13C-δ18Oc relationship and differences between G. ruber morphotypes, I propose that upwelling in the EEP limits the penetration of the atmospheric δ13C signal in this region. Investigation of Mg/Ca and δ18Oc from individual specimens of the thermocline-dwelling foraminifer Neogloboquadrina dutertrei suggest the depth habitat of this species is consistent with the deep thermocline, but its utility as a thermocline proxy may be limited by the inability to record strong El Niño events.