Résumé : Convective dynamics developing below growing sea ice are studied experimentally by freezing salt water from above in a quasi-two-dimensional Hele-Shaw cell. Observations of the convective processes are made with Schlieren and direct imaging systems, allowing visualization both under and within the growing ice. Buoyancy-driven flows are seen to develop under the ice layer via two different mechanisms: On one hand, brine diffuses out from the ice layer creating a denser boundary layer of enhanced salinity, which triggers boundary layer convection resulting in small-scale interfacial fingers. On the other hand, internal flow within brine drainage channels inside the ice is observed flushing out longer-scale convective streamers at given locations at the ice-water interface. Streamers descend in the bulk aqueous layer faster and for longer distances than fingers. Simulations confirm that, despite nonlinear interactions between fingers and streamers, the different speeds observed can be correlated to different density differences between the interfacial or internal rejection and the underlying bulk salt water. Estimates of relative mass fluxes through the interface by the two mechanisms suggest that, when streamers are active, the mass of salt rejected through the streamer pathway can be larger than the one expelled through the finger pathway. However, as fingers are maintained throughout the ice growth while the rejection from brine channels features an intermittent "on-off" behavior, there are certain periods of time when the mass flux of the two mechanisms is similar, but also some time intervals during which the flux due to interfacial short fingers becomes dominant.