The circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean. Hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity, which at least provides the potential for melting and freezing processes. Applying a three-dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier, we found an important parameter controlling the interaction between these two systems. Idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction, because of the sudden decrease in water column thickness. Since the water column thickness and the Coriolis force determine the characteristics for geostrophic flow, separated circulation systems arise for the open ocean and the ice shelf cavity. Only in areas where constant water column thickness and, from the oceanographic point of view, constant f/H contours can be observed across the barrier, an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf. This is only the case at lateral sloping sidewalls or at deep depressions, which can be found, for example, in the southern Weddell Sea. In all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier.¿ 1997 American Geophysical Union