How Declining Sea Ice Concentration Leads to Greater Arctic Warming and Establishes Its Seasonal Pattern

Models in the CMIP5 RCP8.5 simulations project diminishing sea ice concentration and a warming of the Arctic surface that seasonally is large during fall and winter, and much smaller during summer. Focusing on surface skin temperature, we find the increase in heat capacity of the Arctic surface, due to the conversion of the Arctic surface from sea ice to water, shifts the seasonal cycle and slows the seasonal cooling and warming during fall/winter and spring/summer, respectively. The reduction of the seasonal cooling rate amplifies Arctic warming during fall/winter, leading to a maximum in late fall or early winter. While the reduction of the seasonal warming rate suppresses Arctic warming during spring/summer, leading to a minimum in summer. The slowing of the seasonal warming during spring and summer, due to the heat capacity increase, is countered by the greater absorption of solar radiation at the surface, which is also a result of the reduction in sea ice concentration. Moreover, the increase of solar absorption reduces the sea ice concentration further, which translates to an additional reduction of the seasonal cooling rate during fall and winter. The reduction of the seasonal cooling rate thus outpaces the reduction of the seasonal warming rate, such that CMIP5 models with a greater reduction in sea ice concentration exhibit stronger Arctic warming. The large intermodel surface warming spread in the Arctic is thus principally a result of the intermodel spread in sea ice concentration change.