Coupling of the hemispheres in observations and simulations of glacial climate change

We combine reconstructions, climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres. A lead-lag analysis of synchronised proxy records indicates that temperature changes in Greenland preceded changes of the opposite sign in Antarctica by 400-500 yr. A composite record of the Dansgaard-Oeschger events shows that rapid warming (cooling) in Greenland was followed by a slow cooling (warming) phase in Antarctica. The amplitudes, rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of North Atlantic Deep Water is perturbed. The simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the Southern Ocean. Our climate model simulations also show that increased deep water formation in the North Atlantic leads to a reduction of the Antarctic Circumpolar Current through diminishing meridional density gradients in the Southern Ocean. We construct a simple conceptual model of interhemispheric Dansgaard-Oeschger oscillations. This model explains major features of the recorded temperature changes in Antarctica as well as the general shape of the north south phase relation found in the observations including a broad peak of positive correlations for a lead of Antarctica over Greenland by 1000-2000 yr. The existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism, contrary to previous suggestions. Our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability. (C) 2002 Elsevier Science Ltd. All rights reserved.