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 circumpolar continental shelf equal to ~75% of the annual mean net precipitation in coastal seas. 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 caverns is absent, sea ice is thinner, shelf waters are warmer and saltier, and the Southern Ocean deep basins are flushed by denser waters.