Response of the sea ice seasonal cycle to climate change

The Northern Hemisphere sea ice cover has diminished rapidly in recent years and is projected to continue to diminish in the future. The year-to-year retreat is faster in summer than winter, which has been identified as one of the most striking features of satellite observations as well as of IPCC AR4 model projections. This is typically understood to imply that the sea ice cover is most sensitive to climate forcing in summertime, and previous studies have explained this by calling on factors such as the surface albedo feedback. However, in the Southern Hemisphere it is the wintertime sea ice cover that retreats fastest in IPCC AR4 model projections. Here, we address the seasonal structure of observed and simulated sea ice retreat in both hemispheres. We propose a physical theory that primarily involves the shape of coastlines and eddy heat fluxes in the atmosphere, with vertical heat conduction in sea ice and surface albedo also playing a role. We demonstrate that the theory accurately describes a wide range of climates simulated with a hierarchy of models ranging from an idealized atmosphere-sea ice model to a state-of-the-art coupled GCM.