Ocean oxygen isotope constraints on mechanisms for millennial-scale climate variability - art. no. PA1014

摘要

Millennial-scale climate variability pervades the last several million years of Earth history with largest amplitudes during moderate glacial conditions. However, the processes behind such variability remain unclear. Here we present results from a simplified, coupled climate model that includes an explicit treatment of the atmosphere-ocean cycle of oxygen-18 isotope. The model exhibits a relatively strong North Atlantic overturning circulation for present day and last glacial maximum conditions. Self-sustained, millennial-scale oscillations with strong and weak overturning states are found for moderate glacial conditions. These oscillations reproduce observed features like ''sawtooth'' structure, Northern-Southern Hemisphere asynchrony, sensitivity to climate state, and oxygen isotope signals in the ocean. Amplitudes, structures, and phasing of observed millennial-scale variability in high-resolution, benthic oxygen isotope records from the North Atlantic Ocean are consistent with large heating/cooling cycles in the ocean interior, as found in the model. Glacial meltwater, changes in solar irradiance and high-frequency climate variability affect the timing, period, and persistence of the model climate cycles. However, the basic dynamics, structure, amplitude, and timescale of variability are due to internal, self-sustained oscillations in the model ocean-atmosphere system.