Title: Long-term controls on the strength of El Nino
Abstract: ENSO is the dominant source of inter-annual climate variability, but it is unclear whether it will strengthen or weaken in response to anthropogenic warming. This uncertainty stems from a lack of understanding of ENSO's dependence on the mean climate, and on various positive and negative feedbacks. To constrain these uncertainties, I collected paleo-ENSO data from time periods with different mean climate: the mid- and early Holocene, and the Pliocene (~4,000-12,000 years ago, and ~2.6-5.3 million years ago). My data are based on Mg/Ca measurements of foraminifera (a proxy for temperature) from marine sediment cores in the central and eastern equatorial Pacific. By measuring many individual foraminifera in a sample, I reconstruct the distribution of temperatures. Differences in the warm “tail” of the distribution are attributable to changes in El Niño amplitude. I found that El Niño amplitude was dampened during the mid- and early Holocene, relative to the late Holocene. I also found dampened El Niño amplitude ~5.0 - 3.5 million years ago. By ~3.1 million years ago, ENSO amplitude was similar to the late Holocene and appears to have varied on thousand-year timescales. My findings are consistent with modeling studies: Models with mid- and early Holocene boundary conditions unanimously show dampened ENSO, as do models with Pliocene-like tropical Pacific temperatures. Though modeling studies agree on changes in past ENSO, they disagree on the mechanisms of change, and here the proxy data (on both ENSO and mean climate) provide key constraints for model validation. The dampening mechanism best supported by proxy data, and which provides a unified explanation of my findings from all time periods, is that a deeper thermocline in the mid- and early Holocene and in the early Pliocene weakened the upwelling and thermocline feedbacks, thus weakening ENSO. This work highlights the importance of the thermocline, which should help guide modeling efforts to predict ENSO’s response to anthropogenic change.