An approximation of Antarctica's rocky and icy coastline normally forms the southern boundary in global climate models. Such a configuration neglects extensive ice shelf areas where ocean-ice interaction initiates a net freshwater flux to the continental shelf at least equal to coastal precepitation. The results of a numerical model for the Southern Ocean using two contrasting configurations with and without caverns beneath major Antarctic ice shelves are compared. They show that the freshwater flux due to deep basal melting significantly stabilizes the shelf water column in front of an ice shelf as well as downstream due to advection by the coastal current. If the freshwater from the cavern is absent, sea ice is thinner, shelf waters are warmer and saltier, and Southern Ocean deep water export is stronger.