Sea ice forms as a porous matrix consisting of pure ice crystals in equilibrium with brine. We present an experimental and theoretical study that reveals that the brine expelled by growing sea ice initially remains trapped within the crystalline interstices but that drainage into the underlying water is ultimately triggered abruptly due to the onset of buoyancy-driven convection. The onset is quantified by a critical porous-medium Rayleigh number, and we present an empirical marginal stability diagram that defines the transition.¿ 1997 American Geophysical Union