Glacial climate variability and millenial scale oscillations

We present the results of model studies that are able to reproduce some aspects of observed glacial climate variability, and which in particular provide elucidation of the mechanisms of millenial scale warm and cold climate variability. The essential mechansm involves both the presence of sea-ice and an instability in the three-dimensional thermohaline circulation. In particular, we show that during glacial climates the growth of sea-icea over the subpolar gyres will lead to a weakening, and shallowing, of the thermohaline circulation. This circulation is unstable to oscillations on the millennial time-scale, and these have many of the characteristics of so-called Dansgaard-Oeschger cycles. We demonstrate this with an idealized three-dimensional planetery geostrophic ocean model, coupled to a simple atmospheric model, and compare with various reconstructions of the climate record for the North Atlantic region.