The seasonal location of the Antarctic sea-ice edge is determined by the interplay between the dynamic and the thermodynamic forcing at the ocean–ice–atmosphere interface. Satellite observations indicate large interannual variability of the maximum ice extent, with lesser variability at minimum extent. Recent observations have shown regional-scale (order of 100 km) deviations in the ice edge, which may persist from days to months. Here we investigate the relative contributions of the processes impacting on the ice edge with a numerical model. Results of the ice-edge location from an 11 year simulation of the ice model have been verified against satellite observations from 1998 to 2008. Importantly, we found that the relative contributions to the ice-edge advance/retreat change as the ice edge moves through meridional regions during the seasons, characterized by their thermodynamic properties. In autumn the advance of the sea-ice edge is primarily due to dynamic processes with significant thermodynamic contributions at certain places and times. Southerly winds cause northward ice advection but also induce thermodynamic growth of new ice, which occurs south of the 15% ice concentration contour. As the ice moves into warmer water it melts and pushes the ocean mixed-layer freezing point contour northwards. Sustained large pools of cold air north of the ice edge will cool the ocean and reduce, sometimes to near zero, the melt needed to cool the ocean to the freezing point. This speeds the northward progress of the ice edge. However, the average freezing point air temperature contour is located near the ice edge. Near maximum ice extent the volume of northwardly advecting sea ice is approximately equal to the volume of local ice melt. This maximum extent occupies a quasi-stationary location that regionally varies by up to 2.5° latitude from year to year. This is a nonstatic balance, and the instantaneous location of the ice edge varies depending on the strength and sign of ice advection to the north and the local melt rate. As winter transitions into spring the shortwave radiation increases and the sea ice melts from both the top and the base, while summer is characterized by intensified melt at the top. Basal melt occurs under nearly all the sea ice in the Southern Ocean in summer. This results in broad bands of thin ice, especially in the Weddell and Ross Seas, and in high freshwater fluxes.