Seasonal-scale features and forcing of the El Niño-Southern Oscillation 4,400-4,200 years ago
Abstract
The El Niño-Southern Oscillation (ENSO) is the main driver of Earth's interannual climate variability, yet predicting its response to global warming remains problematic. The brevity of the instrumental record and lack of quantitative data on ENSO behaviour under different climate states contribute to this uncertainty. Here we present a continuous monthly-resolved Porites coral microatoll δ18O record spanning 175 years collected from Kiritimati (Christmas) Island in the NINO3.4 region of the central equatorial Pacific, which quantifies ENSO variability ~4,300 years ago when climate forcing was markedly different from today. The ~4.3 kyBP δ18O record shows a 78% reduction in ENSO variance and a stronger annual cycle that persisted for at least 175 years. The power spectrum of the interannual time series shows a peak at 2.5 years and peaks between 6 and 8 years, indicating that ENSO events were less frequent than at present. Cluster analysis, which identifies El Niño and La Niña events, shows that these events occur as only weak-moderate δ18O anomalies at ~4.3 kyBP, with a distinct absence of moderate-strong δ18O anomalies that characterise the present day. These results provide substantive support for the idea that ENSO amplitude was reduced during the middle Holocene. A critical aspect of the ~4.3 kyBP δ18O record is that the 175 years of growth contain sufficient El Niño and La Niña events to produce an accurate picture of the average seasonal evolution of an event ~4,300 years ago. Season-specific analysis shows that El Niño events were damped during the September-November growth phase, and their peak delayed relative to the climatological year. Furthermore, the annual cycle in the ~4.3 kyBP δ18O record is amplified. Together, these results are best explained by ITCZ-related strengthening of annual cycle processes in the east Pacific, in response to orbital forcing during the middle Holocene. A stronger east Pacific annual cycle at ~4.3 kyBP would result in strengthened zonal trade winds and, as a result, it would be harder to initiate El Niño events in the boreal autumn and early winter, leading to reduced ENSO variability. Our findings show that ENSO is capable of a strong, systematic response to external forcing and provide a distinctive baseline upon which to explore ENSO under changing global climates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP24A..03M
- Keywords:
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- 1616 GLOBAL CHANGE / Climate variability;
- 4522 OCEANOGRAPHY: PHYSICAL / ENSO;
- 4916 PALEOCEANOGRAPHY / Corals;
- 4922 PALEOCEANOGRAPHY / El Nino