NOAA/WDS Paleoclimatology - Central Equatorial Pacific NINO3.4 850 Year SST Reconstructions

Reducing the uncertainties surrounding the impacts of anthropogenic climate change requires vetting general circulation models (GCMs) against long records of past natural climate variability. This is particularly challenging in the tropical Pacific, where short, sparse instrumental data preclude GCM validation on multidecadal-to-centennial timescales. In this two-part paper, we apply two statistical methodologies to a network of accurately dated tropical climate records to reconstruct sea-surface temperature (SST) variability in the NINO3.4 region over the past millennium. While part 1 described the methods and established their validity and limitations, this paper presents several reconstructions of NINO3.4, analyzes their sensitivity to procedural choices and input data, and compares them to climate forcing timeseries and previously published tropical Pacific SST reconstructions.

Our reconstructions show remarkably similar behavior at decadal to multidecadal scales, but diverge markedly on centennial scales. The amplitude of centennial variability in each reconstruction scales with the magnitude of the 1860-1995 trend in the target dataset's NINO3.4 index, with ERSSTv3>HadSST2>Kaplan -these discrepancies constitute a major source of uncertainty in reconstructing pre-instrumental NINO3.4 SST. Despite inevitable variance losses, the reconstructed multidecadal variability exceeds that simulated by a state-of-the-art GCM (forced and unforced) over the past millennium, while reconstructed centennial variability is incompatible with constant boundary conditions. Wavelet coherence analysis reveals a robust anti-phasing between solar forcing and NINO3.4 SST on bicentennial timescales, but not on shorter timescales. Implications for GCM representations of the tropical Pacific climate response to radiative forcing are then discussed.