Modeling the influence of sea ice on solar heating in the upper ocean during the SHEBA experiment

In the early summer of 1998, a curious situation arose in the surface waters at the SHEBA site. The mixed-layer temperature experienced a strong diurnal cycle superimposed on a steady increase in magnitude relative to freezing. The ocean heat flux at 4 and 8 meters below the ice was nearly phased locked to the diurnal cycle of the mixed-layer temperature but lagged by several hours. Yet by mid-June, the lead fraction was only a few percent, although the surface began melting several weeks earlier and the snow layer was nearly gone. Together these observations suggest enough solar radiation passed through the sea ice to significantly warm the surface waters.

These unique conditions are the focus of our simulations with a coupled sea ice-upper ocean model. The sea-ice component is a column version of the current NCAR Community Climate System Model, which includes an ice-thickness distribution model and advanced thermodynamics with solar absorption and transmission computed in two broad wavelength bands. The upper-ocean component uses a local turbulence closure scheme. We use the SHEBA data set to prescribe atmospheric conditions and the ocean-ice stress and to initialize the ice and ocean models.

Warming of the upper ocean depends on the rate at which heat is lost to the ice through basal melt, and on the rate at which shortwave energy is transmitted through the ice cover. These, in turn, depend on the ice-thickness distribution, ocean-ice stress, and optical properties such as surface albedo and attenuation of shortwave radiation in ice and melt ponds. We compare the accuracy and consistency of the simulated ice-ocean system with observations from SHEBA to test the sea ice model's physics.