The role of sea ice in establishing the seasonal Arctic warming pattern

In response to a positive CO2 forcing, the seasonal Arctic warming pattern is characterized by an early winter maximum and a summer minimum. While robust, our fundamental understanding of the seasonal expression of Arctic surface warming remains incomplete. Our analysis explores the relationship between the seasonal cycle of surface heating rate changes and the seasonal structure of Arctic warming in modern climate models. Consistent across all models, we find that the background summer-to-winter surface cooling rate and winter-to-summer surface heating rate slows over sea ice regions in response to increased CO2. The slowing of the background summer-to-winter surface cooling rate leads to an early winter Arctic warming maximum, whereby regions and models with a greater slowing also produce a greater winter warming peak. By decomposing the contributions to the background seasonal heating rate change, we find that reductions in sea ice cover and thickness are primarily responsible for the changes. The winter warming peak results from the loss of sea ice cover, which transitions the Arctic surface from a lower thermal inertia surface (sea ice) to a higher thermal inertia surface (ice-free ocean) that slows the seasonal cooling rate. The seasonal cooling rate in autumn is further slowed by the thinning of sea ice, which allows for a greater conductance of heat from the ocean through the sea ice to the surface. These results offer an alternate perspective of the seasonality of Arctic warming, whereby the changing thermal inertia of the Arctic surface is an important aspect of the seasonality, complementary to other perspectives.