MIXED BOUNDARY-CONDITIONS IN OCEAN GENERAL-CIRCULATION MODELS AND THEIR INFLUENCE ON THE STABILITY OF THE MODELS CONVEYOR BELT

When driven under ''mixed boundary conditions'', coarse resolution oce an general circulation models (OGCMs) generally show a high sensitivit y of the present-day thermohaline circulation against perturbations. W e will show that an alternative formulation of the boundary condition for temperature, a mixture of prescribed heat fluxes and additional re storing of the sea surface temperature to a climatological boundary te mperature with a longer time constant, drastically alters the stabilit y of the modes of the thermohaline circulation. The results from simul ations with the Hamburg large-scale geostrophic OGCM indicate that the stability of the mode of the thermohaline circulation with formation of North Atlantic deepwater increases, if the damping of sea surface t emperature anomalies is reduced, whereas the opposite is true for the mode without North Atlantic deep water formation. It turns out that th e formulation of the temperature boundary condition also affects the v ariability of the model.