Climate and ocean-only models have shown that the ocean will respond abruptly to significant perturbations in surface forcing. Centennial-scale oscillation is a characteristic of circulation in large semi-enclosed ocean basins such as the Arctic, whereas millennial-scale adjustment to changes in surface forcing has been found in the global ocean component of climate models. We show that the millennial time-scale in climate models is likely to be intrinsic to the ocean through its presence in an ocean-only model. The strength of the thermohaline circulation is shown to be very sensitive to the magnitude of ice albedo and, to a lesser extent, perturbation in the surface freshwater flux. Modelled glacial ocean circulation, in contrast to present-day simulations, requires an enhanced freshwater flux over the northern Atlantic, even in its non-Heinrich state, to obtain realistic overturning in the North Atlantic. Copyright © 2001 John Wiley & Sons, Ltd.