Orbital-scale El Niño/Southern Oscillation-Like Variability During the Last Glacial-Interglacial Cycle

How have the changes in the Earth's orbit have driven glacial-interglacial climate changes? Recently, a new hypothesis has been proposed that the tropical Pacific ocean-atmosphere interactions have a strong influence on global climate changes on an orbital timescale (Cane, 1998; Clement et al., 1999; Lea et al., 2000). Nevertheless, the orbital-scale changes in the tropical interactions are less clear, and their impacts on a global climate have not yet been proven. Our study was aimed at understanding whether or not and how the long-term tropical El Niño-Southern Oscillation (ENSO)-like variability has a global impact on orbital-scale climate changes. We generated continuous records of the alkenone sea surface temperature (SST) at the western and eastern margins of the mid-latitude North Pacific (MD012421 and ODP1014) during the last 145,000 years. We found that the difference between the SSTs of the NE and NW Pacific margins (Δ SST) reflected an orbital-controlled ENSO-like variability, and the Δ SST can be used as an indication of the long-term ENSO. The variation of Δ SST was large and pronounced at the 23-kyr cycle during 0-60 ka (MIS-1 to MIS-3) and 120-145 ka (MIS-5e to MIS-6), which agreed well with the long-term ENSO behavior predicted by the Zebiak-Cane ENSO model (Clement et al., 1999), as regards both the timing and frequency. In contrast, the variation was relatively small and pronounced at the 41-kyr cycle during 60-120 ka (MIS-4 to MIS-5d), which disagreed with the model prediction. Our observation also demonstrated that a strongly El Niño-like SST pattern prevailed in the mid-latitude North Pacific during the last two deglaciations. The synchronous warming of the Antarctica (Petit et al., 1999) and the tropical Pacific (Lea, 2000; Koutavas et al., 2002; Visser et al., 2003) prevailed within these strongly El Niño-like intervals during deglaciations. These findings are concordant with Cane (1998)'s hypothesis that a long-term El Niño must have resulted in global heating. Both meridional heat transport and pCO₂ increases might have been accelerated under a strongly El Niño-like condition during the last two deglaciations. These new findings suggest that a strongly El Niño-like condition was essential to the global warming that induced glacial terminations.