Solar heating of the Arctic mixed layer

Data from the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX) are used to examine energy exchange between the Arctic mixed layer and the ice pack. Conductivity‐temperature‐depth profiles from four drifting stations reveal significant heat storage in the upper 50 m of the water column during summer, with mixed layer temperature elevation above freezing δ T reaching as high as 0.4°C. Combining δ T with turbulent friction velocity obtained from local ice motion provides an estimate of heat flux from the ocean to the ice F w which was found to be strongly seasonal, with maximum values reaching 40–60 W m −2 in August. The annual average value of F w was 5.1 W m −2 , about half again as large as oceanic heat flux inferred from bottom ablation measurements in undeformed ice at the central station. Solar heat input to the upper ocean through open leads and thin ice, estimated using an ice thickness distribution model, totaled about 150 MJ m −2 , in general agreement with integrated values of F w . Results indicate that oceanic heat flux to the ice in the central Arctic is derived mainly from shortwave radiation entering the ocean through the ice pack, rather than from diffusion of warm water from below. Indeed, during the AIDJEX project the mixed layer appears to have contributed 15–20 MJ m −2 of heat to the upper pycnocline. During the summer, F w was found to vary by as much as 10–30 W m −2 over separations of 100 to 200 km and thus represents an important term in the surface heat budget not controlled by purely local deformation and thermodynamics.