Trends in ENSO Amplitude Through the Holocene at the Line Islands, Central Tropical Pacific

Abstract:

El Niño-Southern Oscillation (ENSO) is the strongest mode of interannual variability in the modern climate; however, its sensitivity to external forcing is still debated. We examine ENSO’s response to varying late summer insolation during the Holocene, using a sediment core near the Line Islands, central equatorial Pacific, which is well situated to test contrasting hypotheses of ENSO behavior. Some paleoclimate models predict reduced ENSO during the mid-Holocene, when equatorial late summer insolation was at a maximum, due to negative feedbacks from upwelling. This prediction is borne out by coral- and foraminiferal-based paleoclimate records. However, a compilation of Line Island coral records shows no overall Holocene trend, indicating instead that ENSO is dominated by unforced internal dynamics with little response to external insolation forcing.

Here we present Mg/Ca-derived temperatures from individual planktonic foraminifera analyzed by laser ablation ICP-MS, from seven time intervals during the mid- and early Holocene. Both mixed-layer dwelling G. sacculifer and subsurface-dwelling G. tumida were analyzed to constrain upper water column structure. For each time interval, we generated 70-85 individual temperatures from each species. By analyzing the distribution of temperatures, the dominant modes of variability (including ENSO) can be identified, and distributions from different time intervals can be compared quantitatively. El Niño events strongly affect the warm “tail” of the distribution, whereas changes in seasonality have a small effect on the middle of the distribution, and are readily distinguished from ENSO. Our data show muted El Niño amplitude relative to the coretop in five time intervals 4680-9050 years before present, whereas at 10700 years before present, El Niño amplitude was similar to the coretop. This finding, in conjunction with Holocene temperature records from the eastern equatorial Pacific showing relatively cool mid-Holocene temperatures compared to the early and late Holocene, supports the hypothesis that insolation can control ENSO on orbital timescales via regulation of upwelling in the eastern equatorial Pacific.