Sunlight and sea ice: on the partitioning of solar radiation by a sea ice cover (Invited Presentation)

Understanding the interaction of sunlight with the Arctic sea ice cover is important to a diverse array of sea ice and polar climatology problems. The summer melt cycle of Arctic sea ice is strongly affected by the partitioning of solar radiation among reflection to the atmosphere, absorption in the ice, and transmission to the ocean. On a larger scale, the ice-albedo feedback mechanism has the potential to influence not only polar, but also global climate. Observations from the year-long SHEBA field experiment are combined with a radiative transfer model to compute aggregate-scale estimates of the partitioning of solar radiation. Prior to the onset of melt, the surface consisted primarily of large, snow-covered ice floes with only a few percent covered by open water. In these conditions approximately 80% of the incident solar radiation was reflected, with 17% was absorbed in the snow, and less than 3% was transmitted to the ocean. As summer melt progressed, the surface evolved into a variegated mixture of bare ice, melt ponds, and leads. By the end of summer melt in August, the surface was approximately 60% bare ice, 20% ponds, and 20% open water. The evolution in surface composition had a profound impact on the partitioning of the incident solar radiation reducing the reflected energy to 45%, while increasing the portion absorbed in the ice to 33% and the transmitted to the ocean to 22%. This change in solar partitioning impacted surface, bottom, and lateral melt rates, as well as internal heating of the ice, and heat input to the ocean.