Robust evidence for forced changes in ENSO: from the mid-Holocene to the 21st century
Abstract
The El Niño-Southern Oscillation (ENSO) represents the largest source of year-to-year global climate extremes, as evidenced during the record-breaking 2015/16 event. However, its sensitivity to external climate forcing, whether natural or anthropogenic, is difficult to assess with available records. Here, we extend the paleo-ENSO record through the generation of 16 new fossil coral d18O 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. Monthly-resolved coral d18O records from the heart of the ENSO region, in the central tropical Pacific, track changes in sea surface temperature with high fidelity and have been used to quantify ENSO variability over the last 7,000 years (Cobb et al., 2013). 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 multi-millennial simulations from both statistical and dynamical models 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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A42E..03G
- Keywords:
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- 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 3373 Tropical dynamics;
- ATMOSPHERIC PROCESSESDE: 4215 Climate and interannual variability;
- OCEANOGRAPHY: GENERALDE: 4522 ENSO;
- OCEANOGRAPHY: PHYSICAL