Early summer heating of the upper ocean in the vicinity of SHEBA

Summertime solar heating of the upper ocean, and subsequent basal melting, are important factors in the ice mass and energy balance, as well as the seasonal evolution of the Arctic mixed layer. Despite relatively high ice concentrations near the SHEBA site in June, we observed a steady increase in elevation of mixed-layer temperature above freezing, indicating that heat loss to basal melting could not keep pace with insolation. Transmission of solar energy through the ice cover, particularly when melt ponds are present, is not well known, hence ocean heating during a time of large ice fraction is of special interest.

We focus on a particular period in June, (1998 year days 167-171, just prior to summer solstice), when there were continuous records of mean properties and turbulent fluxes from two instrument clusters in the upper 10 m, and extensive coverage by the SHEBA profiling CTD. Over the four days, the mixed layer was moderately turbulent, with little stratification in the upper 15-20 m. We observed continuous warming, along with a superimposed diurnal signal, with temperature maxima lagging maximum solar angle by 4-6 h. A diurnal cycle was also present in turbulent heat flux measured 4.2 and 8.2 m below the interface. At midday, turbulent heat flux was downward, reaching values < -10 W m^-2 at 8.2 m. At low sun angles, upward turbulent heat flux at 4.2 m reached 5 W m^-2, as heat was extracted by melting.

Main features of the observed behavior have been simulated using a simple one-dimensional model, initialized with T/S structure at time 167.0 and forced by surface stress obtained from observations. The enthalpy balance at the ice/ocean interface determines heat and buoyancy fluxes there, while insolation is treated as an exponentially distributed source term in the heat equation, with total strength calculated as a fraction of downwelling shortwave flux measured at the ice surface. To account for the observed warming and turbulent heat flux with plausible model parameters, preliminary results imply that 8-9% of the incoming surface shortwave radiation reached the water column.