On the control of the position of the winter sea ice edge by the Antarctic Circumpolar Current

Abstract. The Antarctic Circumpolar Current (ACC) is often considered a natural barrier for the northward expansion of the Antarctic sea ice, but the underlying processes remain little explored. Here, we focus on the main fronts of the ACC – as a measure of the current system's path – to study how they may control the mean state of sea ice. We find that the latitude of all ACC fronts as a function of longitude shows a correlation above 0.85 with the climatological mean latitude of the winter sea ice edge, indicating a strong link across all sectors of the Southern Ocean. Among the ACC fronts, the Polar Front is identified as the best indicator for studying the ACC's influence on sea ice, as it marks a distinct transition in upper-ocean water mass properties and is consistently found north of the sea ice edge. The distance between the Polar Front and the sea ice edge decreases when the Polar Front lies farther south, due to the presence of warmer waters at higher latitudes. These warmer waters enable efficient heat transport toward the ice edge and constitute a barrier to sea ice expansion, via two mechanisms in particular. First, mesoscale ocean eddies generated downstream of large topographic barriers transport heat poleward. Second, warmer oceanic surface waters near the front heat the atmosphere above, which then carries this heat poleward towards the ice, especially in regions with more southward-directed winds. Since the Polar Front's path is largely shaped by topographic barriers, these results indicate why the position of the winter sea ice edge is strongly constrained, under current conditions, by bathymetry.