Dansgaard–Oeschger Oscillations in a Coupled Atmosphere–Ocean Climate Model

Abstract

A reduced model of the global thermohaline circulation has been asynchronously coupled to a simple energy balance climate model in order to investigate the natural variability of the overturning circulation that may have been characteristic of late glacial conditions. Previous analyses with the ocean-only component of the model have suggested that the nature of the internal variability of the thermohaline circulation was a strong function of the surface boundary conditions on temperature and salinity flux. When the boundary conditions were altered from those corresponding to modern conditions to those appropriate to full glacial conditions, then the internal variability was shown to be radically transformed. Under modern conditions the ocean-only version of the model delivered an overturning circulation that was only weakly time dependent with a characteristic period of centuries. Under full glacial boundary conditions, however, the circulation became strongly time dependent with a characteristic period of millennia. This timescale corresponds to that of the Dansgaard?Oeschger oscillation, which is a prominent feature of the proxy climate records that have been derived on the basis of oxygen isotope data from Summit, Greenland, ice cores by the GRIP and GISP2 collaborations. By coupling the reduced model of the global thermohaline circulation to a simple model of the atmosphere, the authors are able to address the issue of whether the amplitude of the millennium timescale oscillation of atmospheric temperature matches that inferred on the basis of the ice core records. The results reported herein demonstrate that model and observations agree extremely well in this regard. The authors construe this to provide strong further support for the idea that the Dansgaard?Oeschger oscillation is a natural mode of internal variability that would exist even with time-independent boundary conditions appropriate to the full glacial state.