Try this
If the Southern Hemisphere graph depicts, like the Northern one, a percentage variation in ice coverage from the mean given in the graph...
Remember that Antarctica is a continent, and its ice is mostly glacial. A reduction in depth would not necessarily be detectable from space, and a rise in ocean temperature would not effect the land mass. In the Arctic, there's nothing but ocean under the ice. a rise in its temperature has a direct effect.
Keep your eye on the Weddel Sea and the Ross Ice Sheet.