A coral ensemble record of the El Niño southern oscillation over the mid-to-late holocene

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Grothe, Pamela R.
Cobb, Kim M.
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The El Niño-Southern Oscillation (ENSO) represents the largest source of year-to-year global climate extremes. However, its sensitivity to external climate forcing, whether natural or anthropogenic, is difficult to assess with available records. Paleoclimate reconstructions from the central tropical Pacific provide much-needed targets for climate models that are used to simulate future projections of ENSO variability under enhanced greenhouse emissions. Coral oxygen isotopes track variations in sea surface temperature and sea surface salinity, which is largely driven by ENSO. To date, paleo records rely on rare but decades-long fossil coral sequences that date to the last 7,000 years [Cobb et al., 2013]. This study turns to using abundant but shorter sequences (7-20yrs-long) of fossil coral rubble samples to produce a more statistically robust reconstruction of ENSO from the central tropical Pacific through the last 7,000 years. In Chapter 1, I provide relevant background information on ENSO in order to place this dissertation into broader scientific context. This includes the basics on ENSO dynamics and the different spatial patterns of El Niño events. I also explain the different proxies that have evolved our understanding of ENSO throughout the Holocene. Lastly, I mention where we currently are in understanding future projections of ENSO behavior under greenhouse gas emissions. In Chapter 2, I address one of the largest challenges in using a large number of fossil corals for paleoclimate reconstruction, which involves dating hundreds of coral samples cheaply and quickly. I compared the coral dating results from a rapid radiocarbon (14C) dating method, developed at the University of California Irvine, to high precision uranium-thorium (U/Th) dates, considered the gold standard of coral dating. The rapid 14C dating method allows for ~300 samples to be analyzed per week at $40/sample versus ~30 samples per week at $500/sample for U/Th dating. My results demonstrate the utility of employing 14C dating to screen large numbers of corals, followed by more limited U/Th dating on samples chosen for paleoclimate reconstruction. In addition, my extensive fossil coral dating has provided a map of fossil coral age distributions across Christmas Island in the central tropical Pacific. In Chapter 3, we extend the paleo-ENSO record through the generation of 16 new fossil coral δ18O timeseries, averaging 15yrs each, for a total of 233 years of data that greatly augment the available paleo-ENSO archive. Combining this new dataset with published data, we quantify the differences in natural variations in ENSO from the early mid-Holocene to present. In this study, we document a significant increase in recent ENSO variance as compared to the last 7,000 years, implying a role for greenhouse gases in driving an intensification of ENSO. We also find a significant reduction in ENSO variance of roughly 20% from 3,000-5,000yr before present, relative to the preceding and subsequent intervals of data. The causes of the late mid-Holocene reduction in ENSO variance may be linked to the influence of fall and/or spring equatorial insolation forcing, which perturbs the seasonal cycle at the critical growth and decay phases of ENSO extremes, respectively. In distinguishing between natural variability and forced changes in ENSO, we assess the significance of our results using a variety of different null hypotheses that includes output from both a statistical and dynamical model of ENSO variability. Our findings imply that ENSO is sensitive to external forcing, both natural and anthropogenic, although the precise mechanisms for such responses require further study. Our results imply that anthropogenic climate change likely contributed to the record-breaking 2015/2016 El Niño event, and that future ENSO variance is unlikely to decrease under continued greenhouse forcing. In Chapter 4, I conclude the major work presented in this study and highlight the next steps in this research towards understanding how mean climate has changed throughout the Holocene and how it affects ENSO variability.
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