A sharp contrast in low cloud fraction and boundary layer structure across the Antarctic sea ice edge during winter is clearly detectable in active satellite retrievals, which provide an unprecedented view of high latitude clouds during winter, as well as in reanalysis products an in situ measurements. During polar winter, sea ice insulates the ocean and prevents heat and moisture transport to the atmosphere, causing the boundary layer to be convective, moist, warm and cloudy over open water, and stable, dry, cold and clear over sea ice. The average surface downward flux of longwave radiation is at least W/m2 larger over regions of open water slightly equatorward of the ice edge than over sea ice slightly poleward of the ice edge. This enhanced greenhouse effect of the boundary layer over open water slows heat loss from the ocean and horizontal expansion of sea ice. Several state-­‐of-­‐the-­‐art global climate models with satellite simulators are compared to observations. Four out of seven fully coupled models simulate cloudier conditions equatorward of the sea ice edge than poleward. Other model biases are also discussed.