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Abstract The El Niño Southern Oscillation (ENSO) is highly dependent on coupled atmosphere-ocean interactions and feedbacks, suggesting a tight relationship between ENSO strength and background climate conditions. However, the extent to which background climate state determines ENSO behavior remains in question. Here we present reconstructions of total variability and El Niño amplitude from individual foraminifera distributions at discrete time intervals over the past ~285,000 years across varying atmospheric CO₂levels, global ice volume and sea level, and orbital insolation forcing. Our results show a strong correlation between eastern tropical Pacific Ocean mixed-layer thickness and both El Niño amplitude and central Pacific variability. This ENSO-thermocline relationship implicates upwelling feedbacks as the major factor controlling ENSO strength on millennial time scales. The primacy of the upwelling feedback in shaping ENSO behavior across many different background states suggests accurate quantification and modeling of this feedback is essential for predicting ENSO’s behavior under future climate conditions. 

Abstract El Niño–Southern Oscillation (ENSO) dominates interannual climate variability; thus, understanding its response to climate forcing is critical. ENSO's sensitivity to changing insolation is poorly understood, due to contrasting interpretations of Holocene proxy records. Some records show dampened ENSO during the early to mid‐Holocene, consistent with insolation forcing of ENSO amplitude, but other records emphasize decadal‐centennial fluctuations in ENSO strength, with no clear trend. To clarify Holocene ENSO behavior, we collected proxy data spanning the last ~12 kyr and find relatively low El Niño amplitude during the early to mid‐Holocene. Our data, together with published work, indicate both a long‐term trend in ENSO strength due to June insolation forcing and high‐amplitude decadal‐centennial fluctuations; both behaviors are shown in models. The best supported mechanism for insolation‐driven dampening of ENSO is weakening of the upwelling feedback by insolation‐forced warming/deepening of thermocline source waters. Elucidating the thermocline's role will help predict future ENSO change.